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A SURVEY
OF RBCENT
LITERATURE
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Consumer Protection and Environmental Health Service
-------
THE CLIMATE OF CITIES:
A SURVEY OF RECENT LITERATURE
by
James T. Peterson
Division of Meteorology
U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Consumer Protection and Environmental Health Service
National Air Pollution Control Administration
Raleigh, North Carolina
October 1969
-------
The author is a meteorologist assigned to the National Air Pollution
Control Administration by the Air Resources Laboratory, Environ-
mental Science Services Administration.
The AP series of reports is issued by the National Air Pollution Con-
trol Administration to report the results of scientific and engineering
studies, and information of general interest in the field of air pollu-
tion. Information reported in this series includes coverage of NAPCA
intramural activities and of cooperative studies conducted in conjunc-
tion with state and local agencies, research institutes, and industrial
organizations. Copies of AP reports may be obtained upon request,
as supplies permit, from the Office of Technical Information and
Publications, National Air Pollution Control Administration, U.S.
Department of Health, Education, and Welfare, 1033 Wade Avenue,
Raleigh, North Carolina 27605.
National Air Pollution Control Administration Publication No. AP-59
For sab by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402 - Price 55 cents
-------
ABSTRACT
The climate of a city differs from that of the rural areas surround-
ing it, and an increasing amount of scientific research is devoted to
comparative studies of urban and rural climates. This report is a
survey of the literature on city climatology, •with emphasis on that
written since the series of articles published by Dr. H. Landsberg,
from 1956 to 1962. Those meteorological aspects of urban climate
that have been most frequently investigated are discussed herein; they
are temperature, humidity, visibility, radiation, wind, and precipita-
tion.
KEY WORDS: urban climatology, climate of cities, urban-rural cli-
mate differences, literature survey, temperature, pre-
cipitation, humidity, visibility, solar radiation, wind
speed and direction.
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CONTENTS
SUMMARY 1
INTRODUCTION 5
TEMPERATURE 7
Nighttime Differences 7
Daytime Differences 9
Annual Differences 10
Effects' of City Size 11
Vertical Temperature Profile Differences 12
Importance of Meteorological Factors 14
Causes of the Heat Island 15
HUMIDITY 17
VISIBILITY 21
Atmospheric Particulates 21
Urban-rural Visibility Contrasts 22
Effects of Air Pollution Control 22
RADIATION 25
WIND 27
Wind Speed 27
Wind direction 29
PRECIPITATION 31
The Effects of Cities 31
The Important Urban Factors 34
Other Precipitation Elements 35
SUGGESTED FUTURE RESEARCH 37
REFERENCES 39
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THE CLIMATE OF CITIES:
A SURVEY OF RECENT LITERATURE
SUMMARY
This review of urban-rural comparative studies discusses many
aspects in •which the climate of a city differs from that of the "natural"
rural areas surrounding it. The several papers by Landsberg, which
have served as the standard review of urban climate, provide the basis
for this summary of city-country meteorological differences; his re-
sults, which were presented in tabular form in his articles of 1960 and
19&2, are summarized in Table 1. Research in the decade since
Landsberg's work has concurred with almost all the values in his table;
however, additional aspects of urban climatology have been studied,
and more detailed knowledge is now available.
Table 1. CLIMATIC CHANGES PRODUCED BY CITIES
(AFTER LANDSBERG, 1962)
Element
Comparison with rural environs
Temperature
Annual mean
Winter minima
Relative humidity
Annual mean
Winter
Summer
Dust particles
Cloudiness
Clouds
Fog, winter
Fog, summer
Radiation
Total on horizontal surface
Ultraviolet, winter
Ultraviolet, summer
Wind speed
Annual mean
Extreme gusts
Calms
Precipitation
Amounts
Days with < 0-2 inch
1.0 to 1.5 °F higher
2-0 to 3.0 °F higher
6 % lower
2% lower
8 % lower
10 times more
5 to 10% more
100% more
30% more
15 to 80% less
30% less
5 % less
20 to 30% lower
10 to 20% lower
5 to 20 % more
5 to 10% more
10% more
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Recent studies of urban-rural temperature differences have essen-
tially supported the results of Landsberg's summary. However, the.
emphasis of these papers has now shifted to such considerations as the
effect of city size on heat island magnitude. Small cities and even
small building complexes have been shown to create nocturnal heat is-
lands, and urban-rural differences in temperature have been shown to
depend strongly on local microclimatic conditions. In studies of the
vertical distribution of temperature over a city, such phenomena as
multiple elevated inversions and a downwind urban heat plume have
been observed. Finally, additional research on the determination of
meteorological parameters important for heat island formation has
shown that the size of urban-rural temperature differences is highly
correlated with the suburban low-level temperature lapse rate.
Although the recent investigations of relative humidity agree with
those of Landsberg, such studies now consider absolute humidity as
•well. Chandler (1965, 1967) adequately summarizes the results of
such papers. The relative humidity of towns has been shown to be
almost always lower than that of adjacent rural areas, whereas urban-
rural differences of absolute humidity show no such clear-cut relation-
ship. Although a weak, general tendency for drier air is apparent
within the city, the frequencies of occurrence of urban vapor pressures
above and below those of the country are almost equal.
Landsberg's conclusions on dust particle concentrations and the
occurrence of fog also are generally valid. However, some recent
evidence suggests that the frequency of very dense fog in the city cen-
ter is less than that in outlying regions, possibly because of •warmer
urban temperatures . Other studies have shown that visibilities are
improving in some cities, probably as a result of local efforts at air
pollution abatement.
Recent solar radiation investigations have substantiated Lands-
berg's results on this subject. Additional papers have been devoted to
urban-rural contrasts, and such aspects as vertical variation of the
attenuation of solar radiation and hours of bright sunshine over a city
have also been studied.
A modification of Landsberg's results for wind speed may be indi-
cated by the results of a recent study by Chandler (1965). His limited
data sample showed that "when regional -wind speeds were relatively low,
speeds over London were higher than those over the countryside, and
that fewer calms occurred over the city than over the country. The
critical value of wind speed (which determined whether urban or rural
winds were faster) usually was between 4 and 5 ro • s , though it
varied with time of day and season. New work on direction of wind
flow has indicated that when conditions are conducive to heat island
formation the wind flow converges toward the city.
Precipitation research has been greatly expanded since the publi-
cation of Landsberg's articles. Precipitation patterns around several
cities "with dense rain gauge networks have been studied, and small
CLIMATE OF CITIES
-------
increases were usually found downwind of the cities. Changnon's
paper (1968) on the La Porte, Indiana, anomaly, -which recorded large
increases of precipitation and of days with thunderstorms and hail,
has indicated the size of the possible precipitation increase that can
result from man's activities. A new research effort has been directed
at determining the effects of the many dust particles, produced by a
city, on the development and growth of clouds. Such studies hope to
show whether dust particles, which serve as condensation and ice nuclei,
act to inhibit or promote rainfall and in what concentrations they are
most effective.
Summary
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INTRODUCTION
As metropolitan areas expand, they exert a growing influence on
their climate. An increasing amount of scientific literature is being
devoted to analyses of data on urban climates, often comparing urban
data with data from nearby rural areas to show the differences between
"natural" conditions and those influenced by man. These studies also
contribute to such areas as the effects of urban environment on health,
the influence of meteorological parameters on urban diffusion, and the
possible global climatological consequences of increased atmospheric
pollution.
The standard review of urban climate consists of the paper by Dr.
H. Landsberg (1956) and his supplementary articles in I960 and 1962.
The purpose of this report is to review the recent literature in this
field, primarily that in English, to note the areas of agreement and
difference with Landsberg's earlier summaries and to point out the
aspects of urban climatology for which more detailed information is
now available. This survey concentrates on the most frequently dis-
cussed aspects of city-country climatic differences: temperature,
humidity, visibility, radiation, wind, and precipitation. Also included
is a discussion on urban particulate concentrations. A review of urban
concentrations of various gaseous pollutants has recently been published
by Tebbens (1968) and thus will not be discussed in this report.
Previous summaries of literature on urban climates include an
extensive article by Kratzer in 1937 (revised in 1956 and translated
into English), containing references to 533 •works, and a bibliography
by Brooks (1952) listing 249 references. Books by Geiger (1965),on
microclimatology and Chandler (1965) on the climate of London, from
which several examples are included herein, also refer to a number of
other urban studies. Chandler is currently compiling a bibliography on
urban climate under the auspices of the World Meteorological Organi-
zation (WMO); this bibliography will reference well over 1000 articles.
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TEMPERATURE
Of all the urban-rural meteorological differences, those of air
temperature are probably the most documented. That the center of a
city is warmer than its environs, forming a "heat island, " has been
known for more than a hundred years and continues to receive consid-
erable attention in the literature. Many aspects of a heat island have
been studied, such as possible reasons for its occurrence; diurnal,
weekly, and seasonal variations; relation to city size; and dependence
on topography. This section reviews some of these characteristics of
urban-rural air temperature relationships.
NIGHTTIME DIFFERENCES
The fact that a city is warmer than its environs is seen most
readily in a comparison of daily minimum temperatures. As Lands -
berg (1956) pointed out, such comparisons often show temperature
differences of 10 °F and occasionally differences as great as 20 °F.
However, since nocturnal temperatures are dependent on topography,
a fraction of these differences, sometimes a large fraction, ca.n often
be ascribed to terrain features.
Numerous measurements of urban heat islands have been made,
frequently by use of automobiles to obtain many observations within a
short time period. An example of a London temperature survey asso-
ciated with clear skies, light winds, and anticyclonic conditions is
presented in Figure 1 (Chandler, 1965). This figure shows certain
features common to most heat islands. The temperature anomalies
are generally related to urban morphology. The highest temperatures
are associated with the densely built-up area near the city center;
moreover, the degree of warming diminishes slowly, outward from the
city's heart, through the suburbs and then decreases markedly at the
city periphery. The effect of topography is also evident in this example.
Urban warming is reduced along the Thames River, in the smaller non-
urbanized valleys, and near the city's higher elevations.
Steep temperature gradients at a city's edge have been measured
during clear, calm conditions at Hamilton, Ontario (Oke and Hannell,
1968), * and Montreal, Quebec (Oke, 1968). These investigators found
temperature changes of 3. 8 and 4. 0 °C- km" , respectively, which they
regarded as typical values for moderate to large cities.
*Several papers from an international conference on urban climatology
at Brussels, Belgium, sponsored by the WMO, in October 1968, are
mentioned in this review; although they are not generally available now,
their publication as a proceedings is planned.
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MINIMUM
TEMPERATURE
14 MAY 1959
Figure 1. Minimum temperature distribution in London, May 14, 1959, in °C with °F in
brackets (from Chandler, 1965).
Some recent studies have indicated that the mean annual minimum
temperature of a large city may be as much as 4 °F higher than that of
surrounding rural areas. Chandler (1963, 1966) reported on two
studies of London, •which showed differences of 3. 4 and 4. 0 °F in mean
minimum temperatures at urban and rural sites. The first study "was
based on data from 1921 to 1950 for several stations in and around
London; the second compared 1959 data for one downtown and one rural
location and applied a correction for the difference in elevation at the
stations. In another study, Woollum (1964) and Woollum and Canfield
(1968) presented data for several stations in the vicinity of Washington,
D. C. , for a ZO-year period; mean minimum temperatures for each
season were approximately 4 °F higher in downtown areas than in
outlying regions.
Although the city heat island as indicated by minimum temperatures
can be readily detected year-around, the investigations in London by
Chandler (1963, 1966) and in Reading, England, by Parry (1966) indi-
CLIMATE OF CITIES
-------
cated that the greatest tempe.ratu.re differences occur in summer or
early autumn. Woollum (1964) also found that the mean differences
between the warmest and coldest stations of his network were greatest
in fall and summer, but that the greatest extreme differences between
these stations occurred in winter (see also Landsberg, 1956).
DAYTIME DIFFERENCES
The heat island of a city can be detected during the day, but much
less readily than during the evening. The slight daytime temperature
differences observed are often difficult to distinguish from those due
to the effects of topography. In some instances daytime city tempera-
tures may even be lower than those of the suburbs. For example,
Landsberg (1956) presented 1 year of data from city and airport obser-
vations in Lincoln, Nebraska, a. location essentially free from compli-
cating terrain factors. Daily maxima in the cold season showed little
difference between the two sites. During the warm season, however,
the airport was more frequently warmer than the downtown site. Such
results are not the rule, however, and Landsberg also points out cases
of daily maxima that are higher in the city. Similar examples have
been given by Chandler (1963, 1966); his data showed that the annual
average maximum temperatures of London were 0. 6 and 1.1 °C higher
than those in the outlying areas. Munn et al. (1969) also readily de-
tected a daytime heat island at Toronto, Canada, using daily maximum
temperatures.
A recent report by the Stanford Research Institute (Ludwig, 1967;
Ludwig and Kealoha, 1968) presents perhaps the most comprehensive
documentation of urban daytime temperatures to date. The authors
made about twelve auto traverses each at San Jose, California;
Albuquerque, New Mexico; and New Orleans, Louisiana, during day-
times in the summer of 1966. Although the temperature anomalies
resulting from topographical influences in these cities were greater
than those from the heat island, the downtown areas were approxi-
mately 0.5 °C warmer than the suburbs despite the effects of topog-
raphy.
In the summer of 1967 SRI extended the study to Dallas, Ft. Worth,
and Denton, Texas (population 35, 000), where they made 20, 4, and 2
surveys, respectively. When the 1966 and 1967 data were combined
with data from three surveys each at Minneapolis, Minnesota, and
Winnipeg, Canada (Stanford U. Aerosol Laboratory, 1953a, 1953d,
1953e), * and one at London,*-the investigators found that for these 67
cases the city's warmest part near the downtown area averaged 1.2 °C
above the typical areas of its environs, "with a standard deviation of
about 1. 0 °C. This average value is higher than that observed by
other studies, and Ludwig and Kealoha pointed out two possible reasons
why investigations of the daytime heat island may underestimate its
*These reports are part of a series (1952, 1953a, 1953b, 1953c, 1953d,
1953e) which were recently declassified (1963) and describe a number
of detailed observational surveys of urban temperature and diffusion.
Temperature
-------
magnitude. First, they noted that at ground level the highest tempera-
tures of a city do not occur in the central area of tall buildings but
rather near that part of the downtown area with "densely packed three-
to five- story buildings and parking lots. " Second, they noted that
temperatures observed at suburban airports were higher than those of
a true rural environment, either grass-covered or forested.
The diurnal and seasonal heat island variations discussed above ?.
are illustrated in Figure 2, taken from Mitchell (1962). This figure
shows hour-by-hour monthly averages of temperature at an urban and
a suburban site in Vienna, Austria. The temperature of the city is
higher at night during both February and July, but this urban-rural
difference is greater in July. In the daytime, however, the city-rural
temperature differences are small during July and consistently small
and positive during winter.
' SCHOTTENSTIFT
(URBAN STATION)
HOME WARTE
(SUBURBAN STATION)
6
A.M.
12 2
NOON
6
P.M.
Figure 2. Diurnal variation of temperature in Vienna for February and July for both an
urban and suburban station (from Mitchell, 1962).
10
CLIMATE OF CITIES
-------
ANNUAL DIFFERENCES
The average annual temperatures of a city and its environs, cal-
culated from the daily maxima and minima, also reflect the presence
of the urban heat island. Table 2 (Landsberg, I960) lists the average
annual urban-rural temperature differences for several large cities.
To this can be added the average value for London of 1. 3 °C based on
the two studies by Chandler (1963, 1966). Although Woollum and
Canfield (1968} do not state a specific number for the mean annual
urban-rural temperature differences of Washington, D. C. , their
recent data for that city indicate that it should be at least 1.0 °C,
after consideration of the elevation changes of more than 200 feet
within the area.
Table 2. ANNUAL MEAN URBAN-RURAL TEMPERATURE
DIFFERENCES OF CITIES, °C
Chicago
Washington
Los Angeles
Paris
0.6
0.6
0.7
0.7
Moscow
Philadelphia
Berlin
New York
0.7
0.8
1.0
1.1
EFFECTS OF CITY SIZE
Several urban studies have considered the effect of city size on the
magnitude of the heat island. For example, Mitchell (1961, 1962)
showed that during this century most major U. S. metropolitan areas
have been both expanding and warming. While the temperature increase
may be partly due to global climatic conditions, the amount of warming
is well correlated with city growth rate. Dronia (1967) compared tern- >
perature trends from 67 paired locations around the world, each pair
representing an urban and a rural site, usually separated by several
hundred kilometers. He found that in the first five decades of this
century the urban areas warmed by 0. 24 °C more than the rural loca-
tions. Similarly, Lawrence (1968) noted that from the late 1940!s to
early 1960's mean daily minimum temperatures at the Manchester,
England, airport increased by about 2.0 °F relative to nearby rural
stations as the urban area expanded beyond the airport. Landsberg's
paper (I960) gave 30 years of data for Los Angeles and San Diego which
showed that as the difference in population between those cities increased
so did the difference between their mean temperatures.
The relation between city size and urban-rural temperature differ-
ence is not linear, however; sizeable nocturnal temperature contrasts
have been measured even in relatively small cities. For example, in
more than 20 surveys of Palo Alto, California (population 33, 000),
Duckworth and Sandberg (1954) found that the maximum temperature
difference of the survey area was 4 to 6 °F. Hutcheon et al. (1967)
measured the temperature distribution in Corvallis, Oregon (population
21, 000), on two occasions and noted a. definite heat island, with maxi-
mum temperature differences of 13 and 10 °F. Sekiguti (1964) observed
a heat island in Ina, Japan (population 12, 000). Finally, a heat island
Temperature 11
-------
effect resulting even from a small, isolated building complex after
sunset has been detected (Landsberg, 1968). In contrast, during the
two daytime surveys at Denton, Texas, Ludwig and Kealoha (1968)
reported no appreciable difference in the maximum temperatures at
the center of town and at its outskirts.
Although general relationships have been developed between heat
island magnitude and some parameter representing city size, be it
area, population, or building density, Chandler (1964, 1966, 1967b)
has emphasized that the heat island magnitude at a given location often
depends strongly on the local microclimatic conditions. He noted
(1968) that data from several English towns showed that the strength
of the local heat island •was strongly dependent upon the density of urban
development very near the observation point, sometimes within a
circle as small as 500 meters radius. During nights with strong heat
islands, the correlation between the heat island and building density
•was usually greater than 0.9.
VERTICAL TEMPERATURE PROFILE DIFFERENCES
Investigators need detailed knowledge about the vertical distribu-
tion of temperature near urban areas to accurately determine the dis-
persion of pollutants. In two recent investigations, one over New York
City (Davidson, 1967; Bornstein, 1968) and one over Cincinnati (Clarke,
1969) helicopters have been used to measure the three-dimensional,
nocturnal temperature patterns over a city. Besides recording multiple
elevated inversions over New York City, Davidson and Bornstein ob-
served that a ground-based inversion was present over the outlying
areas, while over the city temperatures were generally higher than.
those over the countryside from the surface up to about 300 meters.
At heights around 400 meters temperatures over the city were gener-
ally lower than those of the surrounding area. This-finding is similar
to those of Duckwork and Sandberg (1954), who noted a "crossover" of
the urban and rural temperatures on about half of their wiresonde data.
Clarke has studied vertical temperature distributions in Cincinnati
both upwind and downwind of the city. Figure 3 shows an example from
one of his surveys. On clear evenings, "with light consistent surface
winds, he found a. strong surface-based inversion upwind of the urban
area. Over the build-up region, lapse conditions occurred in the
lowest 200 feet, while downwind of the urban area a strong inversion
was again observed at the surface. Above this inversion, weak lapse
conditions prevailed, which Clarke interpreted as the downwind effect
of the city. This "urban heat plume'1 was detectable by vertical tem-
perature measurements for several miles in the lee of the city.
Other authors have investigated the vertical distribution of night-
time urban temperatures with tower-mounted instruments. DeMarrais
(1961) compared lapse rates in Louisville between 60 and 524 feet with
typical rural profiles. During the warm half of the year, while sur-
face inversions were regularly encountered in the country, nearly 60
percent of the urban observations showed a weak lapse rate and another
12 CLIMATE OF CITIES
-------
§
2600
2400
2200
2000
r 1800
• '1600f
UOflL
1200
1000
800
600
400
200 L
13 JUNE 1967
DOWNTOWN CINCINNATI
2600
2400
2200
2000
1800 3
1600 5.
1400 J
1200 r
1000 S
800
600
400
200
Figure 3. Cross section of temperature (°F) over metropolitan Cincinnati about 1 hour before sunrise on June 13, 1967. The heavy
solid line indicates the top of the urban boundary layer and the dashed lines indicate a temperature discontinuity with
less stable air above. Wind flow was from left to right (from Clarke, 1969).
-------
15 percent showed weak lapse conditions above a super-adiabatic lapse
rate. Munn and Stewart (1967) instrumented towers at 20 and 200 feet
in central Montreal, suburban Ottawa, and a rural location near Sarnia,
Canada. They noted that inversions occurred more frequently and
were stronger over the country than over the city. Both DeMarrais,
and Munn and Stewart, however, found little difference between rural
and urban daytime temperature profiles.
In another study of the vertical distribution of temperature, Hosier
(1961) compiled statistics on the frequency of inversions based below
500 feet above station elevation for selected United States localities
and discussed the dependence of these data on season, cloud cover,
wind speed, time of day, and geographic location. Since he used
radiosonde data, -which are usually taken at an airport on the outskirts
of a. city, his results are generally representative of suburban locations
and thus underestimate the inversion frequencies of rural sites and
overestimate those of cities.
IMPORTANCE OF METEOROLOGICAL FACTORS
In addition to city size the magnitude of the urban heat island has
been shown to depend upon various meteorological parameters. An
early study of this type was made by Sundborg (1950), who investigated
the relation between the temperature difference between Uppsala,
Sweden, and its rural surroundings and meteorological variables meas-
ured at the edge of the city. He derived two equations by regression
analysis for daytime and nighttime conditions at Uppsala, based on
more than 200 sets of data. Correlation coefficients between observed
and calculated temperatures -were 0. 49 and 0. 66 for day and night,
respectively. At night, wind speed and cloud cover were the most
important variables for determining the heat island magnitude.
Chandler (1965) made a similar study based on temperature dif-
ferences of daily maxima and minima at city and country sites for
London and meteorological data observed at London airport. The mul-
tiple correlation coefficients for the four equations are 0. 608, 0. 563,
0. 286, and 0. 114 for nighttime (summer and winter) and daytime
(summer and winter), respectively, an indication that the equations
are much better estimators for nocturnal than for daytime conditions.
The magnitude of the heat island was shown to depend on wind speed
and cloud amount at night, whereas no meteorological variables -were
found to be particularly significant during the day.
Ludwig and Kealoha (1968) estimated the magnitude of a city's heat
island by using the near-surface temperature lapse rate, which was
usually measured in the environs of the city or at a radiosonde facility
of a nearby city. They found this single variable to be highly corre-
lated with the heat island magnitude and thus provided a simple, accu-
rate method for predicting urban-rural temperature differences. These
investigators compiled data from 78 nocturnal heat island surveys from
a dozen cities and estimated the heat island magnitude by subtracting a
typical rural temperature from the highest temperature in the city's
14 CLIMATE OF CITIES
-------
center. Examples of their results, stratified by city population, are
as follows:
AT =1.3 6.78,7 population < 500, 000 (1)
AT =1.7 7.247 500, 000 to 2 million (2)
AT = 2. 6 14.8 Y > 2 million, (3')
where the lapse rate, Y , is the temperature change with pressure
("C-mb ~ ),i. e. , a surface-based inversion is represented by negative
Y . Correlation coefficients between AT and Y for the three cases are
-0. 95, -0. 80, and -0. 87, and the root mean square errors are +0. 66,
+_1.0, and +0. 96° C, respectively. Thus, the resulting equations of
this example will usually predict AT to within +2. 0° C.
Another meteorological parameter that influences heat island
development is wind speed. When the regional wind speed is above
a critical value, a heat island cannot be detected. Table 3, taken
from Oke and Hannell (1968), summarizes several reports on the rela-
tion between city population (P) and this critical wind speed (U). Log
P and U are highly correlated (0. 97), and their relationship is des-
cribed by the regression equation:
('4)
U
-11.6 + 3. 4 log P
Table 3. CRITICAL WIND SPEEDS FOR ELIMINATION OF THE HEAT
ISLAND EFFECT IN VARIOUS CITIES
City
London, England
Montreal, Canada
Bremen, Germany
Hamilton, Canada
Reading, England
Kumagaya, Japan
Palo Alto, California
Author
Chandler (1962a)
Oke, et al.a
Mey
Oke, et al.a
Parry (1956)
Kawamura
Duckworth, et al.
Year of
survey
1959-61
1967-68
1933
1965-66
1951-58
1956-57
1951-52
Population
8,500,000
2,000,000
400,000
300,000
120,000
50,000
33,000
Critical
wind speed.
m • s"l
12
11
8
6-8
4-7
5
3-5
aUnpublished
The authors used this equation to estimate the smallest-sized city that
would form a. heat island. When U O, equation (4) yields a popula-
tion of about 2500. Although they had no data to test this estimate, they
recognized that the scatter of their data points increased at the smaller
populations and that sometimes even small building complexes produced
measurable heat island effects.
The location of the highest city temperatures also depends on local
meteorology. Munn et al. (1969) noted that at Toronto, Canada, the
position of the daytime heat island was strongly influenced by the
Temperature
15
-------
regional and lake breeze windflow patterns of the area and was often
displaced downwind of the city center.
CAUSES OF THE HEAT ISLAND
It is generally accepted that two primary processes are involved
in the formation of an urban heat island, both of which are seasonally
dependent (see, for example, Mitchell, 1962). First, in summer the
tall buildings, pavement, and concrete of the inner city absorb and
store larger amounts of solar radiation (because of their geometry and
high thermal admittance) than do the vegetation and soil typical of rural
areas. In addition, much less of this energy is used for evaporation in
the city than in the country because of the large amount of run-off of
precipitation from streets and buildings. At night, while both the city
and countryside cool by radiative losses, the urban man -made construc-
tion material gradually gives off the additional heat accumulated during
the day, keeping urban air warmer than that of the outlying areas.
In winter a different process dominates. Since the sun angle at
mid-latitudes is low and lesser amounts of solar radiation reach the
earth, man-made energy becomes a significant addition to the solar
energy naturally received. Artificial heat results from: combustion
for home heating, power generation, industry, transportation, and
human and animal metabolism. This energy reaches and warms the
urban atmosphere directly or indirectly, by passing through imper-
fectly insulated homes and buildings. This process is most effective
when light winds and poor dispersion prevail.
Many authors have investigated the magnitude of man-made energy
in metropolitan areas. In two often-cited older studies for Berlin and
Vienna (see Kratzer, 1956), the annual heat produced artificially in the
built-up area equalled 1/3 (for Berlin) and 1/6 to 1/4 (for Vienna) of
that received from solar radiation. More recently, Garnett and Bach
(1965) estimated the annual average man-made heat from Sheffield,
England (population 500,000), to be approximately 1/3 of the net all-
wave radiation available at the ground. "Bornstein (1968) reported
results from a similar study of densely built-up Manhattan, New York
City. During the winter the amount of heat produced from combustion
alone was 2-1/2 times greater than that of the solar energy reaching
the ground, but during the summer this factor dropped to 1/6.
In addition to the two seasonal primary causes of heat islands,
other factors are important year-around. The "blanket" of pollutants
over a city, including particulates, "water vapor, and carbon dioxide,
absorbs part of the upward-directed thermal radiation emitted at the
surface. Part of this radiation is re-emitted downward and retained
by the ground; another part warms the ambient air, a process that
tends to increase the low-level stability over the city, enhancing the
probability of higher pollutant concentrations. Thus, airborne pollu-
tants not only cause a more intense heat island but alter the vertical
temperature structure in a way that hinders their dispersion.
16 CLIMATE OF CITIES
-------
Reduced wind speed within an urban area, a result of the surface
roughness of the city, also affects the heat island. The lower wind
speeds decrease the city's ventilation, inhibiting both the movement of
cooler outside air over the city center and the evaporation processes
within the city.
Tag (1968) used a numerical model of the energy balance at the
atmosphere-ground interface to investigate the relative importance of
albedo, soil moisture content, soil diffusivity, and soil heat capacity on
urban-rural temperature differences. The author found that during
the day the lower city values of albedo and soil moisture caused higher
urban temperatures, whereas the higher diffusivity and heat capacity
of the city surface counteracted this tendency. At night, however, the
relative warmth of the city was primarily the result of the higher urban
values of soil diffusivity and heat capacity.
Temperature 17
-------
HUMIDITY
Even though little research on humidity has been done, the consen-
sus of urban climatologists is that the average relative humidity in
towns is several percent lower than that of nearby rural areas whereas
the average absolute humidity is only slightly lower in built-up regions.
The main reason to expect differences in the humidity of urban and i
rural areas is that the evaporation rate in a city is lower than that in
the country because of the markedly different surfaces. The country-
side is covered with vegetation, which retains rainfall, •whereas the
floor of a city is coated with concrete, asphalt, and other impervious
materials that cause rapid run-off of precipitation. Although the city's
low evaporation rates result from the shortage of available "water and
the lack of vegetation for evapotranspiration, some moisture is added
to urban atmospheres by the many combustion sources.
Variations of relative humidity within metropolitan areas resemble
those of temperature, since the spatial temperature changes of a city
are significantly greater than those of vapor pressure. Thus, because
of the heat island, relative humidities in a city are lower than in the
suburbs and outlying districts. The humidity differences are greatest
at night and in summer, corresponding to the time of greatest heat
island intensity (Chandler, 1967a). Other studies have yielded similar
findings. Sasakura (1965) reporting on 1 year of data from Tokyo
showed that the mean relative humidity in the city center "was 5 percent
lower than that in the suburbs, a value that concurs with Landsberg's
average figure. Chandler (1965) also found a 5 percent difference in
the relative humidity of a downtown and a rural site near London. In
other work (1962b, 1967a) he has presented nocturnal relative humidity
profiles across London and maps of spatial distribution for Leicester,
England. These show the dependence of relative humidity variations
on the form of the city's heat island, which in turn depends upon the
density of the built-up complex. Typical humidities of 90 to 100 per-
cent were noted in rural areas during conditions favorable to heat
island formation, whereas in the heart of the city humidity values were
approximately 70 to 80 percent. Because of the temperature depend-
ence, when the magnitude of the heat island was small, the urban-rural
humidity differences were also small.
Although Chandler (1965) found that the mean annual vapor pressure
in London was slightly lower (0. 2 mb) than that at a nearby rural loca-
tion, he (1962b, 1967a) frequently observed that at night the urban
absolute humidity was higher than in the outlying regions. Furthermore,
variations of humidity within the city often directly corresponded to
building density, especially when the meteorological conditions "were
conducive to heat island formation. Typically under these conditions
19
-------
urban vapor pressures were about 1.5 to 2.0 mb higher than those in
the country. The corresponding relative humidities were about 80 and
90 percent for the city and country, respectively. Chandler attributed
these higher urban absolute humidities to the low rate of diffusion of
air near ground level between tall city buildings during nights with
light winds. This air with its high daytime moisture was trapped in
the city canyons and remained there into the evening, keeping the
absolute humidity .high.
The summer surveys at Dallas by Ludwig and Kealoha (1968)
recorded slightly lower values of absolute humidity within Dallas than
outside it, with the greatest differences measured in the afternoon.
During the morning hours the observed patterns were not well developed
and could not be related to the urban complex.
20 CLIMATE OF CITIES
-------
VISIBILITY
The atmosphere of metropolitan areas is usually characterized by
increased concentrations of pollutants, which cause a difference be-
tween the visibilities of urban and rural regions. In this section, the
effect of cities on atmospheric particulates is discussed first, followed
by examples of visibility contrasts between city and country, with em-
phasis on the effects of fog. Readers interested in further information
on these subjects should consult Robinson (1968), who presents a com-
prehensive review of practical and theoretical aspects of visibility, or
the summary by Holzworth (1962), which also discusses visibility
trends and their analysis.
ATMOSPHERIC PARTICULATES
A consequence of metropolitan areas is increased concentrations
of atmospheric particles, such as smoke and combustion products.
Landsberg (I960) stated that on the average the number of particulates
present over urban areas is 10 times greater than that over rural
environs. A more recent study by Horvath (1967), who measured the
number and size distribution of submicron particles over Vienna and
a nearby mountainous region, concurred with this figure. Although the
particulate concentrations in Vienna varied considerably, often because
of differing meteorological conditions, the general shapes of the size
distribution curves of both the urban and rural samples were similar.
Summers (1966) also showed that the number of particles in an
urban atmosphere depends on industrial activity. In central Montreal
the soiling index (Hemeon et al. , 1953) was approximately 20 percent
lower on Saturday and Sunday than on weekdays. Furthermore, in
midwinter, the time of maximum heating requirements, this index was
2 to 3 times that of midsummer. Similar weekly and annual variations
were also found by Weisman et al. (1969) for Hamilton, Ontario. In
addition to industrial sources, urban particulate concentrations depend
on meteorological conditions, particularly ventilation. In a summary
of the relation between smoke density over Montreal and various
weather factors, Summers (1962) showed that winter snow cover was
particularly important in that it produced low-level atmospheric sta-
bility and frequent temperature inversions.
As a result of air pollution and the associated high aerosol concen-
trations, visibilities are lower and occurrences of fog are higher in a
city than outside the metropolitan area. Fog is more frequent within
urban regions because many atmospheric particulates are hygroscopic
(Byers, 1965). Thus water vapor readily condenses on them and forms
small water droplets, the ingredients of fog. An analogous example
21
-------
•was described by Buma (I960), who analyzed visibility data at Leeu-
warden on the Netherland coast. For similar relative humidities the
visibility was much lower when the wind was from the continent ("with
high concentrations of condensation nuclei) than when the •wind was from
the sea (-with low concentrations of nuclei). An example of the relation-
ship between air pollution and visibility has been given by Georgii and
Hoffman (1966), who showed that for two German cities low visibilities
and high concentrations of SO-, were highly correlated when low wind
speeds and low-level inversions prevailed. McNulty (1968) pointed out
that between 1949 and I960 the occurrence of haze as an obstruction to
visibility at New York City increased markedly as a result of increased
air pollution.
URBAN-RURAL VISIBILITY CONTRASTS
As part of his general summary Landsberg (1956) presented visi-
bility data from Detroit Municipal Airport (6 miles from downtown) and
the Wayne County Airport (17 miles from town). During conditions con-
ducive to the formation of city smogs ("winds of 5 miles per hour or less)
visibilities less than 1 mile were observed an average of 149 hours per
year at the Municipal Airport but only 89 hours at the rural site. The
cause of these low visibilities "was listed as smoke in 49 of the observa-
tions at the Municipal Airport but in only 5 at the County Airport; most
of the occurrences of low visibility were during the late fall and "winter.
Landsberg (I960) summarized urban-rural fog differences by noting
that metropolitan areas had 100 percent more fog in winter and 30 per-
cent more in summer. In another study, Smith (1961) compiled the
number of occurrences of visibilities less than 6 1/4 miles for locations
throughout England in the afternoon, the time of least likelihood of fog
(Figure 4). He found that industrial areas reported low visibilities on
two to three times more days than did the rural areas.
Although fog generally occurs more frequently in metropolitan
areas, this is not true for very dense fog. Chandler (1965) attributed
the high frequencies of fog within a city to atmospheric pollution and
relatively low "wind speeds, but the extra warmth of a. city often prevents
the thickest nocturnal fogs from reaching the densities reported^ in the
outlying districts. Table 4, presented by Chandler from data of Shellard
(1959), shows the estimated hours per year of various density fogs in
the vicinity of London, based on four observations per day. The high
frequency of fog and the low frequency of very dense fog in the city cen-
ter are evident. These same general relationships were also detected
by Brazell (1964) using similar London data.
EFFECTS OF AIR POLLUTION CONTROL
A few recent reports indicate that the visibility in many locations
has improved during the last two decades. The better visibilities of
major U.S. cities have been associated with local efforts at air pollu-
tion abatement and substitution of oil and gas for soft coal in produc-
tion of heat (see Holzworth, 1962; Beebe, 1967). Brazell (1964),
Wiggett (r9~64), and Freeman (1968) have suggested that London's
improved visibility may be due to enforcement of the air pollution ordi-
22 CLIMATE OF CITIES
-------
50
Figure 4. Average number of days per year with afternoon visibilities less than 6V4
miles in England and Wales (from Smith, 1961).
Table 4. FOG FREQUENCIES IN LONDON
Kingsway (central)
Kew (inner suburbs)
London Airport (outer suburbs)
Southeast England (mean of 7 stations)
Hours per year with visibility less than
40 m
19
79
46
20
200 m
126
213
209
177
400 m
230
365
304
261
1000 m
940
633
562
494
nances of 1954 and 1956. Similarly, Atkins (1968) and Corfield and
Newton (1968) found that visibility has also improved near other English
cities as a result of air pollution legislation. In another study of Lon-
don, Commins and Waller (1967) compiled data showing that the
Visibility
23
-------
particulate coijtent of that city's atmosphere has decreased. Measure-
ments from downtown London showed that the average smoke concen-
tration from 1959 to 1964 was 32 percent lower than that from 1954 to
1959.
Visibility is not improving at all United States cities. A study by
Green and Battan (1967) has shown that from 1949 to 1965 the frequency
of occurrence of poor visibility at Tucson, Arizonia, definitely in-
creased and was significantly correlated with that city's population.
24 CLIMATE OF CITIES
-------
RADIATION
The blanket of particulates over most large cities causes the solar
energy that reaches an urban complex to be significantly less than that
observed in rural areas. The particles are most effective as attenua-
tors of radiation when the sun angle is low, since the path length of the
radiation passing through the particulate material is dependent on sun
elevation. Thus, for a given amount of particulates, solar radiation
will be reduced by the largest fraction at high-latitude cities and during
winter. L/andsberg (I960) summarized the average annual effect of
cities on the solar radiation they received as follows: the average
annual total (direct plus diffuse) solar radiation received on a horizontal
surface is decreased by 15 to 20 percent, and the ultraviolet (short
wavelength) radiation is decreased by 30 percent in -winter and by 5 per-
cent in summer.
De Boer (1966) based a. recent study on this topic on 2 years of
global solar radiation measurements at six stations in and around
Rotterdam, Netherlands. The study showed that the city center re-
ceived 3 to 6 percent less radiation than the urban fringe and 13 to 17
percent less than the country. Chandler (1965), also reporting on the
solar energy values in the heart of smoky urban areas, observed that
from November to March solar radiation at several British cities was
25 to 55 percent less than in nearby rural areas. In addition, the cen-
tral part of London annually received about 270 hours less of bright
sunshine than did the surrounding countryside because of the high con-
centration of atmospheric particulates.
Further emphasizing the dependence of the transfer of solar ra-
diation on the air's smoke content, the study of Mateer (1961) showed
that the average annual energy received in Toronto, Canada, was 2. 8
percent greater on Sunday than during the remainder of the week.
Moreover, the Sunday increase during the heating season, October
through April, was 6. 0 percent but was only 0. 8 percent in all other
months.
The investigations of atmospheric turbidity by McCormick and
Baulch (1962) and McCormick and Kurfis (1966), which were based on
aircraft measurements of the intensity of solar radiation, provided
data on the variation of solar energy with height over Cincinnati.
These authors observed that pollutants over the city, which often had
a. layered structure and were dependent on the vertical temperature
profile, significantly reduced the amount of solar energy that reached
the city surface. In addition, they discussed changes of the vertical
variation of turbidity (or solar radiation) from morning to afternoon,
from day to day, and from clean air to polluted air. Roach (1961) and
25
-------
Sheppard (1958) have also studied attenuation of solar radiation by at-
mospheric dust particles. They concurred that most radiation scat-
tered by these particles is directed forward and thus attenuation of
total solar radiation is primarily due to absorption. Roach estimated
that over "heavily polluted areas" absorption of solar energy by the
particles was of sufficient magnitude to cause atmospheric heating
rates in excess of 10° C per day. A discussion of optical properties
of smoke particles is presented in a report by Conner and Hodkinson
(1967).
The introduction of smoke controls in London during the mid-1950's
has afforded an opportunity to check the radiation-smoke relation.
Monteith (1966) summarized data on particulate concentration and solar
energy at Kingsway (central London) for the years 1957 to 1963. During
this time smoke density decreased by 10 jug-m while total solar
radiation increased by about 1 percent. The average smoke concentra-
tion of 80^.g-m~ at Kew (inner suburbs) represents an energy decrease
of about 8 percent, and in the center of town, where smoke concentra-
tions, average 200 to 300/ig-m"^, the income of solar radiation is
about 20 to 30 percent less than that in nearby rural areas. Similarly,
Jenkins (1969) reported that the frequency of bright sunshine in London
also increased in recent years after implementation of the air pollution
laws. During the period 1958 to 1967, the average number of hours of
bright sunshine from November through January was 50 percent greater
than that observed from 1931 to I960.
Measurements of ultraviolet radiation in downtown Los Angeles
and on Mt. Wilson (Nader, .1967) showed its dependence on the cleanli-
ness of the atmosphere. Attenuation of ultraviolet radiation by the
lowest 5350 feet of the atmosphere averaged 14 percent on no-smog
days; when smog was present, attenuation increased to a. maximum of
58 percent. Reduced values of ultraviolet radiation in Los Angeles
were also measured by Stair (1966). He presented an example in which
the effect of smog was to decrease the amount of ultraviolet radiation
received at the ground by 50 percent, and he also noted that on "extremely
smoggy days" the decrease may be 90 percent or more.
26 CLIMATE OF CITIES
-------
WIND
The flow of wind over an urban area differs in several aspects from
that over the surrounding countryside. Two features that represent
deviations from the regional wind flow patterns are the differences in
wind speeds in city and country and the convergence of low-level •wind
over a city. These differences occur because the surface of a built-
up city is much rougher than that of rural terrain--exerting increased
frictional drag on air flowing over it--and because the heat island of a
city causes horizontal thermal gradients, especially near the city
periphery. The excess heat and friction also produce more turbulence
over the urban area. These general ideas •were discussed by Lands-
berg (1956, I960); he stated that the annual mean surface wind speed
over a city was ZO to 30 percent lower than that over the nearby country-
side, that the speed of extreme gusts was 10 to 20 percent lower, and
that calms were 5 to ZO percent more frequent. Since then several
investigations have refined and expanded the studies summarized by
Landsberg. Readers interested in a comprehensive and detailed review
of wind flow over a city are directed to a recent paper by Munn (1968).
A difficulty in estimating urban-rural wind differences is selecting
representative sites within the city from which to take measurements.
Most observations of urban wind flow have been taken either from the
roofs of downtown buildings, usually several stories high, or from
parks or open spaces, •whereas very few data have been obtained at
street level in the city center, that place where most human activity
occurs. However, these conventional measurements are generally
representative of the gross wind flow patterns over a city, and as long
as their limitations are recognized they can be useful for urban-rural
comparisons.
WIND SPEED
Although recent reports have concurred that the average wind
speed within a. city is lower than that over nearby rural areas (Fred-
erick, 1964; Munn and Stewart, 1967; Graham, 1968), the study reported
by Chandler (1965) for London shows significant variation from this
general rule. Although his analysis is based on only Z years of data,
it indicates that differences in urban and rural wind speeds depend on
time of day, season, and wind speed magnitude. Some of these rela-
tions are brought out in Table 5 (taken from Chandler, 1965), which
summarizes the mean wind speeds at London Airport (on the fringe of
the city) and indicates their excess over the values recorded at Kings-
way (in central London). The data show that when the regional wind
speeds are light (typically at night) the speeds in downtown London are
higher than those at the airport, •whereas when "wind speeds are
Z7
-------
relatively high, higher speeds are recorded at the airport. This is evi-
dent in comparison of the daytime and nighttime wind speeds given in
Table 5.
Table 5. AVERAGE WIND SPEEDS AT LONDON AIRPORT AND DIFFERENCES
FROM THOSE AT KINGSWAY, m • s-1
December - February
March - May
June - August
September - November
Year
0100 GMT
Mean speed
3.5
2.2
2.0
2.1
2.2
Excess speed
-0.4
-0.1
-0.6
-0.2
-0.3
1300 GMT
Mean speed
3.1
3.1
2.7
2.6
2.9
Excess speed
0.4
1.2
0.7
0.6
0.7
Chandler (1965) attributes this diurnal variation of urban influence
to the diurnal differences of regional wind speed and atmospheric sta-
bility. At night when surface winds are relatively calm, the stability
is much greater in the country, where inversions are common, than
in the metropolitan area, where lapse conditions may prevail. This
relative instability in the city, combined with the greater surface
roughness, enhances turbulence and allows the faster moving winds
above the urban area to reach the surface more frequently;' thus at
night the average city "wind speeds tend to be greater than those of the
country. During the day, however, with faster regional winds, the
frictional effect of the rough city surface dominates the turbulence
effect, and lower wind speeds are observed within the built-up area.
The critical value of wind speed that determines whether the urban
winds will be faster or slower than those of the country is highest dur-
ing summer nights and during both day and night in •winter (about 5. 0 to
5.5 nrs ). These are times of relatively high atmospheric stability.
The lowest values of critical speed (about 3. 5 m- s ) occur during
summer and fall days. Moreover, the magnitude of the decrease in
urban wind speeds is greatest during spring days and least during
spring nights.
Chandler (1965) summarized the wind speed statistics for London .by
noting that the annual average urban and suburban speeds were lower
than those of the outlying regions but only by about 5 percent. However,
the annual differences between wind speeds in the center of London and
in the outlying areas were somewhat greater. For these locations,
when speeds were more than 1. 5 m- s~l the average difference was
about 13 percent, whereas for speeds greater than 7. 9 m- s-1 the mean
difference was only 7 percent. In summer, little difference between
average city and rural wind speeds was evident. Light regional winds
(1. 5 m- s"l or less), which increase in speed over the city, occur more
frequently in summer and compensate for the occurrences of strong
rural winds, which decrease over the city. Finally, in contrast to the
earlier figures reported by Landsberg, the London data showed that
fewer calms and light winds occurred in the city center than in rural
regions.
28
CLIMATE OF CITIES
-------
WIND DIRECTION
Past research on the direction of wind flow over urban areas has
been primarily concerned with detecting and measuring a surface flow
in toward the urban complex. It has been surmised for some time that
if a city is warmer than its environs the warm city air should rise and
be displaced by cooler rural air. However, this inflow is weak and
occurs only in conjunction with well-developed heat islands, which in
turn are dependent on certain meteorological conditions. Since direct
measurements of the inflow require a coordinated set of accurate
observations, few such investigations have been made.
Pooler (1963) analyzed wind records from the Louisville local air
pollution study and determined that there was indeed a surface inflow
of air toward the city. This inflow was the dominant feature of the
surface wind flow pattern "when the regional winds were -weak, a situa-
tion that could result, for example, from a "weak large-scale pressure
gradient. Georgii (1968) reported on urban wind measuremens in
Frankfort/Main, Germany. During clear, calm nights an inflow toward
the city center was detected, "with a convergent wind velocity of up to
2 to 4 m-s . He also noted that when the large-scale surface geostroph-
ic wind speed reached 3 to 4 m- s" a local city circulation was prevented
from becoming established, although the increased roughness of the
city was still affecting the wind regime.
Measurements of the wind flow around an oil refinery in the
Netherlands have been reported by Schmidt (1963) and by Schmidt and
Boer (1963). Although the area involved (4 km ) was much smaller
than that of a moderate-sized city and the heat produced by the refin-
ery per unit area was considerably greater than that of a. city, the gen-
eral relationships are of interest. A cyclonic circulation occurred
around the area, with convergence toward the center. Furthermore,
ascending air was detected over the center of the area, •with descending
currents over the surroundings. The greatest vertical velocities mea-
sured in the vicinity of the maximum heat production were about 15
r^m • c ~" •*•
Vertical wind velocities were obtained from low-level tetroon
flights over New York City by Hass et al. (1967) and Angell et al. (1968).
They observed an upward flow over densely built-up Manhattan Island
and downward motion over the adjacent Hudson and East Rivers. They
ascribed this flow pattern to the urban heat island, to the barrier effect
of the tall buildings, and to the relatively cool river water. Low-level
convergence over a city has also been detected by Okita (I960, 1965),
who applied two novel techniques. For one study he utilized the fact
that rime ice formed on the windward side of tree trunks and the thick-
ness of the ice was proportional to wind speed. For the other, (Figure
5), he observed the smoke plumes from household chimneys. By ob-
serving these features around the periphery of Asahikawa, Japan, he
deduced the local wind flow patterns and determined that when the
large-scale wind flow was weak there was a convergence over the city.
Wind 29
-------
Figure 5. The direction of wind flow around Asahikawa, Japan, February 26, 1956, as
deduced from formation of rime ice on tree branches (from Okita, 1960).
As a final consideration of urban "wind flow, Chandler's observa-
tions (1960, 1961) on the periphery of London and Leicester are of
interest. Chandler noted that when a "well-developed nocturnal heat
island formed with a strong temperature gradient near the edge of the
built-up area, "winds flowed inward toward the city center, but the flow
was not steady. Rather, movement of cold air from the country to-
ward the city occurred as pulsations, the strongest winds occurring
•when the temperature gradient -was strongest.
30
CLIMATE OF CITIES
-------
PRECIPITATION
A city also influences the occurrence and amount of precipitation
in its vicinity. For several reasons an urban complex might be
expected to increase precipitation. Combustion sources add to the
amount of water vapor in the atmosphere, higher temperatures inten-
sify thermal convection, greater surface roughness increases mechani-
cal turbulence, and the urban atmosphere contains greater concentra-
tions of condensation and ice nuclei. Since no continuous, quantitative
measurements of these various parameters have been made in conjunc-
tion with the few urban precipitation studies, the relative significance
of these factors is not easy to establish. Landsberg (1956, 1958) gave
several European examples of urban-rural comparisons and concluded
that the amount of precipitation over a city is about 10 percent greater
than that over nearby country areas. More recent studies have shown
that his conclusion may be an oversimplification and that the greatest
positive anomalies occur downwind of the city center.
THE EFFECTS OF CITIES
The effects of cities on precipitation are difficult to determine for
several reasons. First, very few rural areas remain undistrubed
from their natural state. Second, there is a lack of rain gauges in
metropolitan areas, especially instruments with long-term records
and uniform exposure throughout the record. Third, many cities are
associated with bodies of water or hilly terrain, and these also affect
the patterns of precipitation. Finally, the natural variability of rain-
fall, particularly the summer showers of mid-America, further com-
plicates the analysis of urban-rural precipitation differences. An
example of the difficulty of rainfall analysis is the study of Spar and
Ronberg (1968). They observed that the record from Central Park,
New York City, showed a significant decreasing trend of precipitation
of 0. 3 inch per year from 1927 to 1965. This trend was not substan-
tiated by data from other nearby sites; at Battery Place, the nearest
station, a small rainfall increase was observed during the period.
A very striking example of the effect of the Chicago urban region
on local precipitation has been documented by Changnon (1968a). He
showed that at La Porte, Indiana, some 30 miles downwind of a large
industrial complex between Chicago and Gary, Indiana, the amount of
precipitation and the number of days with thunderstorms and hail have
increased markedly since 1925. Furthermore, the year-to-year vari-
ation of precipitation at La Porte agrees generally with data on the
production of steel and number of smoke-haze days at Chicago (Figure 6),
31
-------
1940
5 YEAR OF 5-YEAR PERIOD
Figure 6. Five-year moving totals of precipitation at several Indiana stations and
smoKe-haze days at Chicago (from.Changnon, 1968a).
During the period from 1951 to 1965, the positive anomalies at La
Porte "were 31 percent for precipitation, 38 percent for thunderstorm
days, 246 percent for hail days, and 34 percent for days with precipi-
tation _> 0. 25 inch. These results are summarized in Table 6.
Changnon concluded that these observed differences -were a real effect
of the industrial area and that they represent the general size of preci-
pitation increase that is possible as a result of man's activities. How-
ever, because of the effect of Lake Michigan and its associated lake
breeze in channeling the pollutants around the south end of the lake
toward the La Porte area and inhibiting dispersion, the large differences
detected in this study probably are not representative of American
cities in general.
Changnon (1961, 1962, 1968b, 1969) has summarized precipitation
data for several other midwestern cities and detected positive increases,
but not nearly as pronounced as those at La Porte. In St. Louis, Chicago,
Champaign-Urbana, Illinois, and Tulsa, Oklahoma, the city precipita-
tion was 5 to 8 percent higher than the average of nearby rural stations.
At St. Louis and Champaign-Urbana the rainfall maxima were downwind
32
CLIMATE OF CITIES
-------
Table 6. SUMMARY OF URBAN AREA INCREASES IN PRECIPITATION AND RELATED
CONDITIONS (EXPRESSED AS A PERCENT OF RURAL VALUES)
Annual precipitation
Warmer half-year precipitation
Colder half-year precipitation
Rain days > 0.01 or 0.1 inch
Annual
Warmer half-year
Colder half-year
Rain days ? 0.25 or 0.5 inch
Annual
Warmer half-year
Colder half-year
Annual number of thunderstorm days
Summer number of thunderstorm days
Chicago
5
4
6
6
8
4
5
7
0
6
13
La Porte
31
30
33
0
0
0
34
54
5
38
63
St. Louis
7
a
a
a
a
a
a
a
a
11
21
Tulsa
8
5
11
a
a
a
a
a
a
a
a
Champaign-Urbana
5
4
8
7
3
10
5
9
0
7
17
aData not sufficient for comparison.
of the urban center. The increase of 5 percent at Chicago proper rep-
resents a relative maximum over the city and is distinct from the
higher precipitation rates downwind of Chicago at La Porte. Spatial
data were not available for Tulsa. Table 6 (from Changnon, 1968b)
shows that the percentage precipitation increase is greater during the
cold season and that increases in the warm season result from more
days of moderate rain and thunderstorms.
Authors do not totally agree on the distribution of precipitation
over cities. The rainfall minimum over the Missouri half of metro-
politan St. Louis detected by Changnon (1968b) was also noted by Feig
(1968), who observed that a. map of isohyets of annual precipitation
over the eastern United States showed that minimum precipitation
areas occur around most cities having no obvious geographic influences.
The annual precipitation pattern at Washington, D. C. (Woollum and
Canfield, 1968), also shows low values downtown along the Potomac
River, with the greatest amounts of precipitation on the north to
northwest side of town and with relative maxima in both the eastern and
western suburbs (Figure 7). On the other hand, Dettwiller (1968) has
indicated that the effect Of a city is to increase precipitation. He
showed that from 1953 to 1967 the average rainfall in Paris was 31 per-
cent greater on weekdays than on Saturdays and Sundays.
Another reason for believing that more precipitation falls over
metropolitan districts is that heavy thundershowers sometimes occur
over an area that roughly coincides with the urban complex (Staff, 1964;
Chandler, 1965; Atkinson, 1968). However, it is always difficult to
determine whether a thundershower was a natural event or whether the
city actually influenced it in any way.
Precipitation
-------
WASHINGTON METROPOLITAN AREA
Figure 7. Mean annual precipitation (inches) over the Washington, D. C., metropolitan
area (from Woollum and Canfield, 1968).
THE IMPORTANT URBAN FACTORS
Although it is hard to determine the relative importance of the
several urban factors that affect precipitation, the extensive studies by
Changnon (1968b) led him to the following conclusions about the city-
precipitation relationship. Two factors were probably most effective
in enhancing precipitation at La Porte — high concentrations of ice
nuclei from nearby steel mills and added heat from local industrial
sources. Because of the high frequency of nocturnal thunderstorms
and hailstorms at La Porte, he concluded that the thermal and frictional
effects were probably the most significant. In addition, the precipita-
tion maximum associated with Champaign-Urbana, an area -with little
industry and a minimum of nuclei sources, also indicated that the ther-
mal and frictional factors could produce significant differences.
Many recent studies have concentrated on measuring condensation
and ice nuclei and determining their possible influence on cloud develop-
ment and precipitation. Measurements showing that cities are an
important source of nuclei were reported by Mee (1968). Typical
34
CLIMATE OF CITIES
-------
concentrations of cloud droplets and condensation nuclei at the convec-
tive cloud base near Puerto Rico were 50 cm" in the clean air over
the ocean, about 200 cm~^ over the unpolluted countryside, and from
1000 to 1500 cm~3 immediately downwind of San Juan. Abnormally
high concentrations were detected for at least 100 miles downwind of
the city. In another study, Squires (1966) pointed out that measure-
ments of condensation nuclei over Denver showed that the concentrations
of these nuclei produced by human activities "were similar to the nat-
ural concentration there. Moreover, Telford (I960), Langer (1968),
and Langer et al. (1967) observed that industrial areas, and in partic-
ular steel mills, are good sources of ice nuclei. Finally, Schaefer
(1966, 1968a) and others (Morgan, 1967; Morgan and Allee, 1968;
Hogan, 1967) have shown that lead particles in ordinary auto exhaust
form effective ice nuclei when combined with iodine vapor.
Even though cities are generally recognized as good sources of
nuclei, the net effect of such nuclei cannot be definitely determined.
For example, a few ice nuclei added to supercooled clouds may enhance
rainfall, a principle used by commercial cloud seeders. On the other
hand, rain that falls from warm clouds is dependent on a number of
large drops within a cloud so that coalescence may be effective. If
large numbers of condensation nuclei are introduced into a warm cloud,
many small drops will form and thus rainfall may be inhibited (Gunn
and Phillips, 1957; Squires and Twomey, I960).
Many examples in the literature show that rainfall can be artifi-
cially increased, whereas only a. few show decreases. Fleuck (1968)
statistically analyzed results of the Missouri seeding experiments by
the University of Chicago group and concluded that seeding of clouds
in that area suppressed precipitation. In addition, reports of an inves-
tigation of the trend of rainfall in eastern Australia (Warner and Twomey,
1967; Warner, 1968) concluded that as the amount of smoke from burn-
ing sugar cane in the area increased during the past 50 years, rainfall
correspondingly decreased by 25 percent. Therefore, an accurate
determination of the effect of a city on precipitation in its neighborhood
requires knowledge not only of the number and type of nuclei being
introduced by the city but also of such factors as the concentration of
natural nuclei and vertical temperature profiles over the city.
Recent findings have further documented Landsberg's statement
that it is becoming increasingly difficult to find undisturbed rural
areas with which to compare cities to determine urban-rural meteoro-
logical differences. Both direct (Schaefer, 1968b) and indirect (Gunn,
1964; McCormick-and Ludwig, 1967; Peterson and Bryson, 1968;
Volz, 1968) measurements have shown that the particulate content of
the atmosphere, even in remote areas, is increasing as a result of
greater human activity.
OTHER PRECIPITATION ELEMENTS
Two other precipitation elements of interest are hail and snowfall.
Changnon's study (1968b) of the La Porte anomaly found that from 1951
Precipitation 35
-------
to 1965 the number of hail days was 246 percent greater than that of
surrounding stations. Results from similar investigations at other
midwestern cities were not definitive. Landsberg (1956) cited a few
instances in which snowfall over an urban area was lighter than that at
nearby rural locations, presumably because of higher temperatures
over the cities. Potter (1961) found similar results for Toronto and
Montreal, Canaida.
36 CLIMATE OF CITIES
-------
SUGGESTED FUTURE RESEARCH
Although this review indicates that a large volume of research has
been conducted in urban climatology, several areas warrant emphasis
in the future. Among the following suggestions for further study, many
were brought out at the recent international conference on the climate
of cities at Brussels.
Few studies have been made of the vertical structure of the urban
atmosphere, which includes such features as the variation •with height
of temperature, wind flow, pollutants, and radiation. More such
information is needed for a better understanding of the transport and
diffusion of pollutants over metropolitan areas. Likewise, additional
urban wind measurements are needed at a variety of sites with dif-
ferent exposures, to delineate the fine structure of wind flow through-
out a city.
Several aspects of urban-rural radiation differences should be
further investigated. In several studies of the effects of a polluted
atmosphere on ultraviolet radiation in Los Angeles, the effects were
found to be significant. Observations from other cities are now
needed to show the amount of attenuation of ultraviolet radiation at dif-
ferent locations. Similarly, studies should be conducted to determine
the net effect of a polluted city atmosphere on the total radiation bal-
ance of a. city, including visible and thermal infrared wavelengths,
since net radiation comprises a major fraction of a city's energy
budget. In particular, urban-rural differences of infrared radiation
should be measured to determine whether reduced values of solar
radiation at the city surface resulting from atmospheric pollution are
compensated for by an increase in infrared energy.
A cause-and-effect relationship between a city and precipitation
has not yet been found. Although several factors are believed to be
important, the role of anthropogenically produced dust particles is of
special interest. The influence of these particles on the physics of
precipitation and the optimum concentration for modifying precipita-
tion have not yet been determined. Also, investigations should be
undertaken to ascertain whether city-induced precipitation anomalies
of a size similar to that at La Porte, Indiana, are widespread or wheth-
er this example is primarily the result of local influences, such as
Lake Michigan.
Further research should be directed toward determing exactly
upon which parameters urban-rural temperature differences depend.
37
-------
For example, the relative importance of the type and density of local
buildings and gross city size on the temperature at a given city loca-
tion has yet to be definitely established. Similarly, the local cooling
produced by parks and greenbelts and the extent of this cooling into
nearby neighborhoods should be measured. Such information would
be useful in city planning and land use studies, for example.
Finally, these two questions should be studied: How far downwind
does a city influence climate, and to what extend does a city in the
tropics modify its climate? The former question has implications in
larger-scale studies of inadvertent "weather modification; the latter is
important since nearly all research in urban climatology has been done
at cities in temperate climates.
38 CLIMATE OF CITIES
-------
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