United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory N\ V4 '
Research Triangle Park IMC 27711 -^ "
Research and Development
EPA/600/S3-89/025 Aug. 1989
&ERA Project Summary
A Climatology of
Temperature and Precipitation
Variability in the United States
Brian K. Eder, Lawrence E. Truppi, and Peter L. Finkelstein
This summary examines the
seasonal and annual variance and
standardized range for temperature
and the seasonal and annual
coefficient of variation and
normalized standardized range for
precipitation on a climatic division
level for the contiguous United States
for the period 1895 to 1985.
Examination of the temperature
variance revealed a continentality
phenomenon in which the largest
variance occurs in the upper midwest
section of the country, while the
smallest variance is generally found
in coastal regions along the west
coast, the Gulf coast, and south-
eastern states. The winter season
displayed roughly twice the amount
of seasonal variance as did spring
and roughly four times that of sum-
mer or autumn. Analysis of the stan-
dardized temperature range supports
the continentality phenomenon; how-
ever, the transitional seasons, spring
and autumn, displayed the largest
amount of within-season variability,
with winter and summer displaying
the least amount.
Examination of the coefficient of
variation for precipitation depicted a
propensity for the largest seasonal
and annual variation to occur over
the southwestern states from Texas
to California. Conversely, the smal-
lest coefficient of variations were
found over the northeastern sections
of the country from New England into
the mid-Atlantic and Great Lakes
states. Analysis of the seasonal and
annual standardized precipitation
range reveals that the pattern mimics
the coefficient of variation patterns
but exhibits less of a gradient,
resulting in a smoother pattern.
Areas of greater than normal
seasonal and annual precipitation
ranges include the southwestern
states from Texas to California, while
areas of less than normal ranges
include the northeastern and Ohio
River Valley states.
This Project Summary was
developed by EPA's Atmospheric
Research and Exposure Assessment
Laboratory, Research Triangle Park,
NC, to announce key findings of the
research project that Is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
Despite the increasing interest shown
by the scientific community in climate
and its interactions with the evolution of
ecosystem structures, there continues to
be a lack of a consensus among
climatologists and ecologists concerning
the future of global climate and its pos-
sible impact upon ecosystems. Policy
makers and planners need plausible des-
criptions of possible long-term changes
of such ecologically important variables
as temperature, precipitation, evapora-
tion, and soil moisture conditions on all
spatial and temporal scales.
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Such descriptions may be found with
climatic scenarios, which are sets of
solutions either derived empirically from
observational data (paleoclimatic or
instrumental analogues), or from Global
Climate Models (GCMs), often in the
form of seasonal maps showing the
range of conditions or possible variances
that may occur in the future.
Although research has begun in EPA's
Atmospheric Research and Exposure
Assessment Laboratory, the develop-
ment of climatic scenarios that have real
utility for ecological impact assessment
is still rudimentary. Subsequently, this
development must be supported by an
enhanced understanding of the climatic
sensitivities of a broad range of
ecological activities and of the detailed
nature of recent and past climatic
patterns and their variability. Two such
variables which should receive a
concentration of research efforts are
temperature and precipitation. From
these two measured variables, numerous
derived parameters relevant to local eco-
systems, such as surface moisture
stress, duration of rainless periods, and
length of growing season, can be
calculated. The development and evolu-
tion of ecosystems are as sensitive to the
ranges and variances of temperature and
precipitation as they are to mean
conditions. Because of this, ecosystems
evolving in regions that have exhibited
little variance in temperature and pre-
cipitation over the years are likely to be
more sensitive to climatic changes that
those ecosystems which evolved in
regions exhibiting larger variability.
Therefore, a need exists to not only
delineate these regions of differing
variance, but to also establish monitoring
networks within both types of regions,
which may provide an understanding of
potential ecological responses toward
future climatic change.
Though the delineation of such regions
may seem to be trivial, little, if any,
literature concerning the subject is
available. This summary therefore
represents an initial effort toward the
fulfillment of the requirements mentioned
above through the delineation of areas of
the country which experience differing
amounts of temperature and precipitation
variability. This is accomplished through
the examination of the variance and
standardized range of temperature data
and the coefficient of variation and
standardized range of precipitation data
across the contiguous United States, on
a climatic division level, from the period
1895 through 1985. Establishment of
monitoring networks within these
delineated regions will help provide a
new understanding of key ecosystem
processes, as well as their responses to
possible climatic change, which should
therefore enhance their treatment in
GCM based scenarios as well as pave
their way for their representation in
observationally based scenarios.
Data
The monthly temperature and precip-
itation data employed m this analysis
were obtained from the National Climatic
Data Center (NCDC) located in Asheville,
NC. These data, which cover the period
1895 to 1985, are collected on a climatic
division basis, where each climatic
division is designed to represent regions
within a state that are climatically
homogeneous or consistent. Within the
contiguous United States, there are 344
such divisions, the areal coverage of
which, can vary tremendously, with the
largest divisions generally found in the
western states and the smallest found in
the east.
Stations used in calculating the
divisional monthly averages of tempera-
ture (measured to the nearest tenths in
degrees F) and the monthly totals of
precipitation (measured to the nearest
hundredths in inches) include all first
order stations and those cooperative
stations which have maintained
consistent records.
Methodology
The seasonal and annual variability of
both temperature and precipitation were
examined in order to better understand
the variability of climate within the
contiguous United States. For tempera-
ture data this consisted of examining the
variance from season to season, and by
examining the standardized range within
seasons over the United States.
Standardization of the temperature range
allows for direct comparison between
individual climatic divisions and the
country as a whole.
Due to the tremendous range in normal
precipitation exhibited over the United
States, a different approach was
necessary for the seasonal and annual
precipitation analysis. Rather than take
the variance, which would be biased
towards areas of high precipitation, the
coefficient of variation was examined
which "normalizes" the variance.
Similarly, calculation of the standardized
range also considered this extreme
variability in precipitation and was
therefore normalized.
Results
Examination of the annual temperatun
variance revealed several interesting
features. Most notable of these feature
was the tendency for the largest vanano
to occur in the upper midwest portions c
the country, especially in North am
South Dakota and eastern Montana
where the annual temperature varianci
exceeds 3° F. A trend toward decreasm<
annual variance is exhibited as chmatii
divisions approach coastal regions. Thr
pattern was depicted especially wel
along the west coast from Washmgtoi
and Oregon to California, and again alone
the Gulf coast and southeastern states
where the annual temperature variance
reached a minimum of less than 0.5° 01
the southern Florida peninsula.
The standardized temperature rang<
maps exhibited, in a somewhat differen
manner the same contmentality as seei
with the variance figures Consistent wit!
the annual variance map, the larges
annual standardized ranges occurred ir
the upper midwest, especially in th<
states of North and South Dakota anc
Minnesota. A trend toward decreasing
ranges was found near the coastal areas
especially along the Pacific Coast state:
and the Gulf Coast states.
Unlike the seasonal variance map:
which depicted winter as the seasor
having the most variance, the seasona
standardized range maps depict the
transitional seasons, spring and autumr
as exhibiting the most variability withir
their seasons. This phenomenon is no
unexpected since the range of monthly
temperature would be greater during the
transitional seasons than during winter 01
summer.
Examination of the annual precipitatior
coefficient of variation (%) also revealec
several interesting features. Unlike the
temperature analysis, which indicated £
north-south gradient, the precipitatior
analysis depicts somewhat of ar
eastwest gradient. This is supported by
the propensity for the largest coefficient
of variation to occur over the south-
western states from Texas to California
where the values exceeds 25.9%, while
the smallest variation generally occurs
over the eastern sections of the country
from the mid-Atlantic and Great Lake
States into New England, where values
are less than 14.0%. The maps
depicting the coefficient of variation for
seasonal precipitation are, with only a few
exceptions, similar to the annual map
Most notable of these exceptions is the
extension or shift of high variations
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into the lower midwestern states during
the winter season, and into the Pacific
coast states during the summer season.
Examination of the annual standardized
precipitation range, revealed patterns
similar to those of the precipitation
coefficient of variation. The
southwestern states from New Mexico to
California tend to have larger annual
ranges when compared to the rest of the
country. Another area exhibiting annual
ranges which are greater than "normal"
is found in the upper midwest from North
and South Dakota into Montana. Areas
exhibiting smaller than "normal" annual
ranges include the New England and
Appalachian Mountain states. The
standardized seasonal ranges of precip-
itation again somewhat mimic the annual
map; the patterns, however, tend to be
somewhat flatter, indicating less within
seasonal variability. Areas of greater
than "normal" precipitation ranges
include the southwestern states from
Texas to California, while the eastern
states, especially those in New England
and the Ohio River Valley, tend to exhibit
less than "normal" ranges.
Conclusions
Because there continues to be no
consensus among climatologists and
ecologists concerning climate change
and its possible impact upon
ecosystems, the development of climatic
scenarios will be necessary in order to
assist scientists in evaluating possible
adverse effects of climatic change on the
ecology. Unfortunately, the development
of such scenarios as a utility in assessing
this impact is still somewhat in a
rudimentary stage, and therefore must
be supported by an enhanced under-
standing of recent and past climatic
patterns and their variability. In an initial
attempt to assist in this understanding,
this summary has examined the seasonal
and annual variance and standardized
range for temperature and the seasonal
and annual coefficient of variation and
normalized standardized range for pre-
cipitation, on a climatic division level for
the contiguous United States for the per-
iod 1895 to 1985.
Examination of the temperature
variance and standardized range re-
vealed a contmentality phenomenon in
which the largest variance occurred in the
upper midwest section of the country,
while the smallest variance were
generally found in coastal regions along
the west coast, the Gulf coast and
southeastern states.
Examination of the coefficient of
variation and standardized range for
precipitation depicted a propensity for the
largest seasonal and annual variation to
occur over the southwestern states from
Texas to California. Conversely, the
smallest variation was found over the
northeastern sections of the country from
New England into the mid-Atlantic and
Great Lakes states.
Successful climate scenarios, whether
derived from climate models or analogue
techniques, should duplicate the patterns
produced in this summary as well as the
simple mean patterns. Present models
are, for the most part, unable to do this.
The design of ecological monitoring net-
works, both for base line stations, which
require some climatic stability, and for
stations where a range of climatic condi-
tions is required should also be cognizant
of the information developed in this and
similar studies.
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The EPA authors Brian K. Eder, Lawrence E. Truppl, and Peter L Finkelstein are
with the Atmospheric Research and Exposure Assessment Laboratory, Research
Triangle Park, NC 27711.
Brian K. Eder is the EPA Project Officer (see below).
The complete report entitled "A Climatology of Temperature and Precipitation
Variability In The United States," (Order No. PS 89-765 9301 AS; cost: $13.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-89/025
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