LAKE MICHIGAN STUDIES
Special Report Number LM8
LAKE TEMPERATURES
April 1963
U. S. DEPARTMENT OF HEALTH, EKJCAIXCM, Aim WELFARE
Public Health Service
Division of Water Supply and Pollution Control
Great lakes-Illinois River Basins Project
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TABLE OF CONTENTS
Page
INTRODUCTION 1
Purpose 1
Period of Study 1
Definitions 1
Previous Studies 2
METHOD OF STUDY k
Instruments h
Cruises 5
RESULTS 6
Fall, 1961 6
Winter, 1961-62 6
Spring, 1962 7
Summer, 1962 8
Oscillations of the Thermocline 8
Relationship to Previous Studies 9
SIGNIFICANCE OF RESULTS 10
REFERENCES
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TABLES
Page
1 Degree of Accuracy k
2 Schedule of Cruises 5
3 Representative Temperature Profiles 12
FIGURES
1 Station Locations
2 Density of Fresh Water
3 The Bathythermograph
k Temperatures in Fall, 196*1
5 Temperatures in Winter, 1961-62
6 Temperatures in Spring, 1962
7 Temperatures in Summer, 1962
8 The Inertial Wave
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INTRODUCTION
Purpose
Knowledge of temperatures within the waters of a lake and
variations in water temperature, from place to place and from time
to time, yields valuable insight into such questions as density
stratification, extent and effectiveness of mixing, and consequent
variations in water quality.
This paper presents the results of temperature observations
in Lake Michigan, a review of and comparison with recorded previous
studies, and the conclusions which may be drawn concerning temperature
regimes and the fate of pollutants discharged into the Lake.
Period of Study
Field observations of temperature changes in Lake Michigan
began in September 1961, and continued on an intermittent basis
during the winter, spring, summer, and fall of 1962. Temperature
profiles were made throughout the lake at the sampling station sites
(Figure 1, and Figure 1 of Special Report Number LM2). Temperature
measurements were made in the deeper portions of the lake during
the winter of 1961-62.
Definitions
Stratification in a lake means that its waters are divided
into layers having identifiable differences in temperature, density,
or other characteristics with rather sharply defined boundaries or
zones of transition between layers. Thus, a lake in which the
temperature was either constant or varied uniformly from top to
bottom would not be thermally stratified. A deep lake in the
temperate zone usually stratifies, however, especially during the
summer period. Very shallow lakes rarely stratify, due to constant
mixing from top to bottom by wind action. However, during prolonged
calm periods in mid-summer, even shallow lakes will stratify for
short periods of time. A typically stratified lake is divided into
three layers: the top layer, called the epilimnion; the bottom
layer, called the hypolimnion, and a zone of rapid temperature change
called the thermocline. The thermocline is normally defined as any
abrupt change in temperature which would indicate that there are two
vertically separated masses of water. There may also be: secondary
thermoclines, where more than one exists; winter thermoclines, where
colder but less dense water lies over warmer but denser water; and
pseudo or false thermoclines, sometimes produced by unusual local
conditions.
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In Lake Michigan the epilimnion varies from a few feet in
thickness in late spring or early summer to over 200 feet in late
fall. The thermocline normally ranges about 20 feet thick but can
be over 50 feet in thickness or as little as two feet (during storm
periods, as shown by studies in other lakes). The hypolimnion
encompasses all the water below the thermocline.
An overturn is a descriptive term denoting vertical mixing
or circulation from top to bottom of the entire lake. If the lake
is shallow a complete overturn may occur. Lakes which are extremely
deep or sheltered from the wind may only experience a partial or
incomplete overturn. An overturn occurs when the lake is isothermal
and therefore of the same density. According to Welch the thermal
resistance is at a minimum and relatively light winds could cause
complete circulation (l). Most lakes in the temperate zone have an
overturn in the spring and fall. In Lake Michigan a fall overturn
occurs when the lake begins to cool, and is characterized by the
sinking and mixing of cold, dense, water from the surface, displacing
the warmer and lighter water below. Cooling continues until the lake
reaches the temperature of maximum density and the water mass offers
little resistance to mixing from the wind energy transferred to late
fall storms. Figure 2 shows the temperature-density curve for
fresh water. In some deep lakes, such as Lake Michigan, the bottom
portion of the lake remains permanently at the temperature of
maximum density. (The temperature of maximum density of water varies
with pressure and therefore with depth, being about k°C at the surface
and decreasing about 0.06°C per 100 feet of depth.) In Lake Michigan,
it appears that the bottom portion of the northern basin remains at
the temperature of maximum density throughout the year. This zone
of constant temperature was found to extend from the 600-foot level
downward during the period of observations. The level probably varies
from year to year depending on the severity of the winter. A spring
overturn occurs in Lake Michigan when the surface water temperature
rises to 4°C and the denser surface water sinks through the less dense
layers below.
Previous Studies
Five important studies on the temperatures of Lake Michigan
have been published. In addition, hundreds of observations are being
taken every day at water intakes by the plant operators. The bulk
of this data normally is not published and not readily accessible
for general use. Several thousands of observations have been made
over the past 15 to 20 years by research groups or other interested
agencies for application to other problems, such as biological
studies. The U.S. Navy made observations during World War II in its
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submarine tests in Lake Michigan (2). The Great Lakes Research
Institute of the University of Michigan and the U.S. Bureau of
Commercial Fisheries at Ann Arbor, Michigan have collected and filed
several thousand temperature soundings.
The five principal published studies on Lake Michigan are:
Van Oosten, Church, Millar and Ayers, et al. (3)(k](5)(6)(7)
Van Oosten carried out most of his work in 1930-32 but the data was
not published until 1960 (3). The work by Church in the 19^0!s is
probably the most comprehensive published to date, covering all
seasons of the year (*0(5). Millar's studies were for the surface
waters of the lake and utilized the temperature recordings from
ships' intakes. The study does not include the mid-winter period (6),
Ayers et al. presented detailed temperature profiles for various
sections of the lake during four synoptic cruises in the summer of
1955 (?)• Van Oosten lists several of the minor published studies
on temperature in Lake Michigan.
Although many studies of temperature have been made in Lake
Michigan there has been a paucity of data for the winter period and
specifically from the deeper parts of the lake.
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METHOD OF STUDY
Instruments
The present investigation by the Great Lakes-Illinois River
Basins Project utilizes a variety of instruments. The bathythermo-
graph, reversing thermometer, hand thermometers and the temperature
recorder are all being used. The bathythermograph (BT) (Figure 3)
was invented and first described by Spilhaus in 1937 (8). The
instrument was not generally available until the end of World War II,
and even then the cost was still prohibitive for its general usage.
A description of its operation and capabilities has been published (9)-
The most accurate of all thermometers is the reversing thermometer,
often called a deep-sea thermometer. A detailed description and
specifications have been reported by Welch (10). A hand thermometer,
of the armored type, is used for calibration of the BT. The
temperature recorder, developed at Woods Hole Oceanographic
Institution, has been designed for long periods of recording,
unattended, and at great depths (ll).
In general, the instruments have the following ranges of
accuracy:
Table 1
Degree of Accuracy of Instruments
Instrument Range in °C
Hand Thermometer j- 1.0
Temperature Recorder +_ 0.25
Bathythermograph +_ 0.1
Reversing Thermometer +0.01
The BT is useful in obtaining a complete temperature profile,
taking a few minutes of time even in 900 feet of water. The reversing
thermometer can get accurate temperatures at one depth (such as a
sampling depth) in a period of three or four minutes. A series of
these instruments are frequently used on a single line. The
temperature recorder can be placed at a specific depth and set to
record the temperature every 30 minutes on a strip-chart for
periods as long as six months. These recorders are mounted in
conjunction with current meters.
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Cruises
Since April 1962 the Project has conducted sampling cruises
on Lake Michigan. These cruises are listed in Table 2. Prior to
April 1962, temperature data was collected from several types of
vessels,, including those operated by the Project.
Vessel
PHS
PHS
USCG-261
USCG-Woodbine
USCG-401
USCG-^0'
USCG-ljO'
USCG-it-01
USCG-36'
USCG-6V
USCG-64 '
USCG-Mesquite
R/V Kaho
R/V Kaho
R/V Cisco
R/V Kaho
R/V Cisco
R/V Kaho
R/V Cisco
R/V Cisco
R/V Cisco
R/V Fitzgerald
R/V Kaho
R/V Kaho
Table 2
Schedule of Cruises
Dates
9/27/61
10/6/61
10/11/61
10/21/61
10/2 V6l
11/3/61
11/8/61
11/15/61
11/21/61
11/29/61
12/21/61
1/25/62
2/20/62
3/1/62
3/20-22/62
- 5/T/62
4/26/62
6/5-18/62
6/20/62
7/17-30/62
8/29-9/9/62
10/10-22/62
10/18-11/30/62
10/28-11/7/62
11/28-12/6/62
Operating Area
South Basin
North Basin
it
II
II
II
II
North & South Basins
North Basin
North & South Basins
North Basin
North & South Basins
South Basin
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RESULTS
Fall, 1961
Studies during the fall of 1961 were made only in the
southern basin. This period includes data from the latter part
of September through November, based on 75 temperature soundings;
a representative number are shown in Table 3; 1 to 12. The
inshore areas with depths to 65 feet were nearly isothermal, with
surface temperatures of 15.5°C and still 15.0°C at 65 feet. The
thermocline appeared sharply defined at depths up to 80 feet and
less distinct at depths of 150 feet or more. With the advance
of colder weather, the thermocline receded to greater depths and
disappeared completely between November l6 and 20. Figure k
shows the typical changes found in the fall of the year.
Temperature soundings were selected to show representative
portions of the southern basin.
Winter, 1961-62
The winter period is characterized by surface water
temperatures generally below h°C. During this portion of the
year the lake surface may have a partial or a complete ice cover.
There have only been a few recorded instances of a complete ice
cover, once in the winter of 1935-36, and 1962-63.
The South Basin of Lake Michigan exhibited a different
pattern of temperature distribution than the North Basin. The
basin separation is a ridge between Milwaukee and Muskegon. In
the South Basin, inshore areas out to the 100-foot depth, cooled
to temperatures ranging from slightly above 0°C to 1°C, from
top to bottom, see Table 3> 12 to 19- The surface layers froze,
and in some cases were several feet thick. The inshore cooling
occurred rapidly, and the lake was isothermal by mid-January.
The central portion of the southern basin cooled at a much slower
rate, essentially because of the large volume of water, and was
constantly being mixed with the water from the deeper layers.
Studies off Milwaukee in the South Basin in January and
February showed a pseudo or false winter-type thermocline. Under
normal summer conditions the lake stratifies vertically due to
the great density changes. In the winter period the density
differences are extremely small. Because the density changes
are small near 4°C (temperature of maximum density) some striking
thermal variations can occur. The cruise on January 25, 1962
showed the inshore temperatures near 0.2°C whereas the mid-lake
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temperatures were almost 2.3°C. The "boundary zone between these
two water masses was extremely sharp, and the warmer offshore
water was found below the colder inshore water. This "boundary
zone suggests that lateral mixing did not occur very rapidly
during this period of the year. The isothermal conditions from
top to "bottom in the deep part of the South Basin, at temperatures
"below that of maximum density, would tend to show that vertical
mixing occurred throughout this basin.
Studies in the Northern Basin did not show complete mixing
from top to bottom as shown for the South Basin. A series of
temperature profiles in the deepest portion of the lake disclosed
that a winter-type thermocline existed for most of the winter.
The maximum depth of the winter thermocline was about the 600 foot
level; below this the water temperature was at the temperature
of maximum density. The existence of the thermocline approximately
at the 600 foot level shows that mixing did not occur below this
level. Mixing is known to occur to at least 600 feet. It is
likely that the position of the winter thermocline varies from
year to year, although the amount of variation is unknown.
The temperatures of the inshore waters and the upper layers
of the North Basin were similar to those in the South Basin.
Figure 5 shows some typical winter profiles for the winter
of 1961-62.
Spring, 1962
Definite dates marking the beginning or end of spring
conditions are difficult to establish, because Lake Michigan is
so large. It is possible that the complete spectrum of temperature
ranges, from mid-winter to summer conditions, can occur at one
time. Such wide variations would most likely occur in spring when
the lake is warming, but would not occur in the fall. Figure 6
shows sample temperatures which typify spring conditions in the
lake. The cruise of April 2k to May 7 (Table 3, 20 to 35) shows
both winter and summer conditions at the same time, in different
parts of the lake. The formation of a summer thermocline was
observed on April 26 in 55 feet of water with surface temperatures
at 6.4°C and bottom temperatures at 4.2°C. Yet, as late as May 1,
a winter-type thermocline was found in the Worth Basin. Spring
isothermal conditions still existed in the deeper waters of the
North Basin on June 20.
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8
Summer, 1962
Although summer conditions were found in the South Basin in
April, the entire lake was not stratified until late July. On July 18
the bottom of the thermocline varied from 25 feet to TO feet below
the surface. Thickness of the thermocline varied from 15 feet on
the l8th to 30 feet on the 20th. Secondary thermoclines were found
in some portions of the lake at 115 feet on the 19th and they
appeared unstable or weak. Typical profiles are shown in Figure 7
and Table 3, 36 to 75. The summer conditions continued through
October when the lake had already begun to cool. Variations in
thermocline conditions indicate that a great number of combinations
of temperature profiles exist simultaneously. A few soundings
can only describe the thermal range for a very limited locality.
Oscillations of the Thermocline
Tilting of the thermocline in Lake Michigan has been
documented and reported by many observers (12). This phenomenon
has also been shown for smaller lakes (13)• Wind, blowing across
the lake surface can strip off the warm surface layers and pile
them on the windward shore of the lake. The stripping exposes
the cold deeper layers on the leeward side of the lake and upwelling
occurs. In addition to tilting, internal waves on the thermocline
will also produce temperature oscillations. The period of internal
waves may vary from a few minutes to over 17 hours. The internal
waves with a period of 17 hours, are called inertia! waves. The
term is derived from the fact that the wave travels with an inertial
period. Recent studies by C. H. Mortimer indicate that inertial
waves on the thermocline can also produce conditions which simulate
upwelling, but do not reach the surface (l4). These waves, originally
generated by wind energy, moved counterclockwise around the basin
(according to Mortimer) with a period of 17-5 hours (Lat. 43° north).
The wave period is a function of the latitude.
From August 8 through August 15 the Project had three
temperature recorders in the lake (Figure 2 of Special Report
Number LM7, position shown for May 15). The recorders were at
the 30, 50, and 75 foot levels. The records from the 30 ft. and
50 ft. levels showed no significant changes, whereas the recorder
at the 75 ft. level showed a pronounced wave on the thermocline.
The wave was found for the entire period of record of 150 hours.
The period of the wave averaged 17.5 hours and did not vary more
than an hour over the 150 hours of observation. Neither the
amplitude nor the velocity of the wave is known (see Figure 8).
It is known that the wave did not reach the 50 ft. level at this
time. The Project data, when compared to studies by Mortimer, show
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a much purer wave form with little or no distortion. Mortimer's
data, taken from water works intakes around the lake, is probably
complicated by inshore turbulence and other factors. The direction
of rotation during the August 8-15 period is not known. These data
shed light on possible temperature fluctuations, frequently found
at water works intakes, that do not appear to be related to wind.
The inertial oscillation of the thermocline is rhythmic and will
usually occur during calm periods. Upwelling (when the cold bottom
water comes to the surface) or downwelling will occur when strong
winds tilt the thermocline.
Observations in October 1961, from an anchored ship, showed
internal waves with periods of several minutes which changed the
position of the thermocline as much as seven feet in one hour.
Relationship of Study to Previous Work
Surface temperatures, in general, agreed with the observations
of Millar (6). In a few instances the temperature regime of 1961-62
was different. The winter temperatures in the lake for 1961-62 were
lower than those reported by Millar, whereas the mid-July temperatures
were identical. The winter and spring temperatures of 1962 were
generally cooler than the average conditions shown by Millar (6).
The winter temperatures reported by Church in 19^1-^2 were
very similar to the winter of 1961-62 (M(5)- '^ie summer season
of 19^2 appeared earlier and a thermocline appeared by mid-June.
The lagging of the temperature pattern of the Worth Basin behind
that of the South Basin, from spring to summer, was apparent
both in Millar's and Church's work.
The results of the four mid-summer synoptic cruises by
Ayers et al. are similar to the 1962 summer studies (7).
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10
SIGNIFICANCE OF RESULTS
Temperature soundings taken during the winter of 1961 through
the summer of 1962 indicated that the following conditions occur:
1. Temperature profiles show that a seasonal overturn
occurs in the southern basin of Lake Michigan, whose
maximum depth is about 565 feet.
2. Since the water below 600 feet in the northern basin
remains at maximum density throughout the year, no
evidence exists at this time to indicate that mixing
(due to overturn) occurs below this depth.
3. Inshore vertical cooling and mixing occurs rapidly
but the horizontal exchange with the main body of the
lake appears to occur at a slower rate. The rate of
exchange, vertically or horizontally, is unknown.
k. The northern basin lags thirty days or more behind the
southern basin during the late spring and early summer
warming period.
5. The southern basin cools at a more rapid rate than the
northern basin.
6. Typical temperatures for Lake Michigan for a season or
month of the year are difficult to define. The temperature
range during one month varies considerably between the
two basins at any one time. The temperature range for
any given month may be expected to vary widely from year
to year depending upon the severity of the winter or
the calmness of the summer.
T. Marked changes or configurations of the thermocline from
one end of the lake to the other are characteristic of
summer conditions in the lake.
8. Inertial waves on the thermocline, usually occurring
during calm periods following a strong wind, can produce
alternating periods of warm surface water and cold
deeper water at a water works intake.
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11
9. Under certain conditions, pollutants discharged into
the lake could lie on the thermocline (because of
similar densities as explained in Special Report
Number LMj) and be "brought to the surface during the
summer period, by tilting or oscillations of the
thermocline.
10. Internal waves with periods of several minutes were
observed to change the position of the thermocline
as much as seven feet per hour.
The studies clearly indicate the great variability with
respect to both location and time, of the water temperatures in
Lake Michigan.
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REFERENCES
1. Welch, P. S. Limnology. McGraw-Hill Book Co., Inc., New-
York (1952). p. 538.
2. Hough, Jack. Geology of the Great Lakes. Univ. 111. (1958).
p. 313.
3. Van Oosten, J. Temperatures of Lake Michigan, 1930-32.
United StatesFj.shjjirHi Wildlife.._Service. Special Scientific
Report, Fisheries No". 322 (1960)."
h. Church, P. E. The Annual Temperature Cycle_of Lake Michigan,
Wo. 1. Univ, of Chicago Press, Misc. Repts. No. 4 (19^2)7
p7T8.
5- Church, P. E. The Annual Temperature Cycle of Lake Michigan,
No.__2. Univ. of Chicago Press, Misc. Repts. No. l8 (I9h^).
p. 100.
6. Millar, P. C. Surface Temperatures of the Great Lakes.
Jour. Fish Res. Bd. Can., 9: 329-376 (1952).
7. Ayers, J. C., Chandler, D. C,, Lauff, G. II., Powers, C. F. and
Henson, E. B. Currents and Water Masses of Lake Michigan.
Great Lakes Res. Inst., Publication No. 3 (1958).
8. Spilhaus, A. F. A Bathythermograph. Jour. Mar. Res., 1: 95-100
(1937)-
9- Bralove, A. L. and Williams, E. I. A Study of the Errors of the
Bathythermograph. Notional Sci. Lab. Inc., No. NObsr 523U8
(1952). p. 47-
10. Welch, P. S. Limnqlorrlcal Methods. Blakiston Co., Philadelphia
(191*8). p. 38l7
11. Feyling, A. J?. Geodvne Tarogerature Recorder. Geodyne Corporation
(1962).
12. Moffett, J. W. An Inotance of Upwelling Along the East Shore of
Lake Michigan, 1955- Great Lakes_Res. Inst., Proc. Fifth Conf.
(1962). p. 126. ~
13. Verber, J. L. Currents in Lake Mendota, Wis. Ohio Jour. Sci.,
53: 72-76 (1953).
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-------
14. Mortimer, C. H. Internal Waves in Large Basins With Particular
Reference to Lake Michigan. XV Inter. Congress Limnology,
Madison, Wis.Abstracts (1962). p. 33.
15. Sverdrup, H. U., Johnson, M. W., and Fleming, R. H. The Oceans.
Prentice-Hall, New York (1946). p. 1049.
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Manistique
LAKE MICHIGAN
Sheboygaw
MILES
L 1 i— * ' f
25
Saugatuck
South Haven
GREAT LAKES 8 ILLINOIS
RIVER BASINS PROJECT
STATION LOCATIONS
Chicago
DEPT. OF HEALTH, EDUCATION, 8 WELFARE
PUBLIC HEALTH SERVICE
REGION V CHICAGO, ILLINOIS
FIGURE I
-------
TEM PERATURE
CO
LJ
O
1.00000-
0.99977-
0.9 9 9 5 4 -
09993 I-
0.99908-
0.99885-
0.99862-
0.9 9 8 3 9 -
0.998 I 6-
09979 3-1
01 4 6 8 tO 12 14 16 18 20 22
I I i i_J L_| i I i 1 I I 1 I i I i I i I I I
NOTE
Welch, reference 10, page 350
GREAT LAKES 8 ILLINOIS
RIVER BASINS PROJECT
DENSITY OF FRESH WATER
DEPT. OF HEALTH, EDUCATION, 8 WELFARE
PUBLIC HEALTH SERVICE
REGION V CHICAGO, ILLINOIS
FIGURE 2
-------
GREAT LAKES 8 ILLINOIS
RIVER BASINS PROJECT
THE BATHYTHERMOGRAPH
OEPT. OF HEALTH, EDUCATION, & WELFARE
PUBLIC HEALTH SERVICE
REGION V CHICAGO, ILLINOIS
FIGURE 3
-------
-------
t-
LJ
UJ
U-
CL
LJ
Q
25
50
75
100
125
150
175
200
0
"Z
5 10
TEMPERATURE
SLIDE N0.5I
NOV. 15,1961
TIME 1319 CST
BT NO 5834
15
20
0
25
50
75
100
125
150
175
200
0
5 10 15
TEMPERATURE °C
20
SLIDE NO. 73
NOV 29,1961
TIME 1229 CST
BT NO 5834
900
10
0 5
TEMPERATURE
SLIDE NO. 79
DEC. 21,1961
TIME 1345 CST
BT NO. 48599
°C
SOUTH BASIN
LAKE
MICHIGAN
GREAT LAKES 8 ILLINOIS
RIVER BASINS PROJECT
TEMPERATURES
FALL, 1961
DEPT. OF HEALTH, EDUCATION, 8 WELFARE
PUBLIC HEALTH SERVICE
REGION V CHICAGO, ILLINOIS
FIGURE 4
-------
LJ
0
o
25
OU
IQU
125
1 7S
«iuuo
5 10 15 20
TEMPERATURE °C
SLIDE 1*40.1-1
MARCH 20, 1962
TIME
BT NO.
1933
16122
CST
TEMPERATURE
SLIDE NO. 7-1
MARCH 22, 1962
TIME 1820 CST
BT NO. I24I-B
TEMPERATURE
SLIDE NO. 92
JANUARY 25, 1962
TIME 1403
BT NO. 48599
CST
NORTH a SOUTH
BASIN
LAKE MICHIGAN
GREAT LAKES 8 ILLINOIS
RIVER BASINS PROJECT
TEMPERATURES
WINTER, 1962
DEPT. OF HEALTH, EDUCATION, 8 WELFARE
PUBLIC HEALTH SERVICE
REGION V CHICAGO, ILLINOIS
FIGURE 5
-------
ui
a.
UJ
Q
0
25
50
75
100
125
150
175
200
'
0
5 10
TEMPERATURE
SLIDE NO. 2-1
APRIL 24,1962
TIME 1347 CST
BT NO. 5834
15
°C
20
0
25
50
75
100
125
150
175
2OO
0
5 10 15
TEMPERATURE °C
SLIDE NO. 6-1
APRIL 26,1962
TIME 1008 CST
BT NO. 5834
20
900
0 5
TEMPERATURE
SLIDE NO. 7-3
JUNE 20,1962
TIME 0850 CST
BT NO. 1241 B
NORTH a SOUTH
BASIN
L AKE MICHIGAN
GREAT LAKES 8 ILLINOIS
RIVER BASINS PROJECT
TEMPERATURES
SPRING, 1962
DEPT. OF HEALTH, EDUCATION, 8 WELFARE
PUBLIC HEALTH SERVICE
REGION "V CHICAGO, ILLINOIS
FIGURE 6
-------
-------
Ul
175
200
1
0
5 10
TEMPERATURE
SLIDE NO. 6B-5
JULY 19, 1962
TIME 1103 CST
BT NO. 5834
15
°C
20
200
0
5 10
TEMPERATURE
SLIDE NO. IOA-S
JULY 20, 1962
TIME 1427 CST
BT NO. 5834
15
20
900
TEMPERATURE
SLIDE NO. 22A-5
JULY 27, 1962
TIME 1035 CST
BT NO. 48599
°C
NORTH a SOUTH
BASIN
LAKE MICHIGAN
GREAT LAKES 8 ILLINOIS
RIVER BASINS PROJECT
TEMPERATURES
SUMMER, 1962
DEPT. OF HEALTH, EDUCATION, ft WELFARE
PUBLIC HEALTH SERVICE
REGION V CHICAGO, ILLINOIS
FIGURE 7
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FIGURE 8
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