J. Great Lakes Res. 22(4):845-863
Internat. Assoc. Great Lakes Res., 1996
Impacts of the Great Lakes on Regional Climate Conditions
Robert W. Scott and Floyd A. Huff
Illinois State Water Survey
2204 Griffith Drive
Champaign, Illinois 61820
ABSTRACT. Estimates of lake-induced spatial changes of six climate variables (precipitation, mean
minimum and mean maximum temperatures, cloud cover, vapor pressure, and wind speed) were derived
for the entire Great Lakes basin. These patterns were estimated by a comparison of maps of each weather
variable using: (1) all regional climate data, and (2) regional data when observations within an 80-km
zone around the lakes were removed. Results generally confirm expectations and prior findings, but point
to inadequacies in data collection that limit a highly precise analysis. Lake effects are most noticeable in
precipitation and temperature and vary considerably by season, time of day, and lake size. Greatest lake
influences are found near Lake Superior where up to 100% more precipitation falls downwind of the lake
in winter compared to that expected without its presence. During summer, all lakes cause a downwind
decrease in rainfall of 10% to 20%. Mean minimum temperatures in the basin are higher in all seasons
and over all lakes. Lake-induced reductions in mean maximum temperatures in the region are observed
during spring and summer. Effects on cloud cover are greatest during winter and show increases of
approximately 25% in areas downwind of Lakes Superior and Michigan. Conversely, the cool summertime waters of Lakes Michigan and Huron reduce cloudiness roughly 10%. Variations in vapor pressure
are consistent with observed changes in temperature. Amounts in winter are estimated to be 10% to 15%
higher across the center of the basin, but decrease by roughly 5% to 10% at many lake shore sites in
summer. Seasonal wind speed data were considered to lack an appropriate number of quality long-term
climate stations to determine spatial lake effects. Surface elevations, increasing east of the basin, complicated detection of effects due solely to the lakes.
INDEX WORDS:
Great Lakes, precipitation, temperature, clouds, vapor pressure, climate impacts.
INTRODUCTION
It is well known that the Great Lakes exert a
considerable influence on the climate of their region. Much of this influence is due to differences
in the heat capacities between the water and land
surfaces of the area, and to the large source of
moisture the lakes provide to the lower atmosphere. In addition, changes in terrain height adjacent to the lakes further alter climate variables.
Physical considerations suggests that major effects
of the lakes are: 1) to moderate maximum and minimum temperatures of the region in all seasons
(with perhaps little net effect on mean temperatures), 2) to increase cloud cover and precipitation
over and just downwind of the lakes during winter
due to the relatively large heat and moisture source
present, and 3) to decrease summertime convective
clouds and rainfall over the lakes because of the
greater atmospheric stability imparted at the surface by the relatively cooler water.
Past research has confirmed these expectations
for portions of several lakes or entire individual
lakes. However, seasonal c1im~te effects over the
Great Lakes basin as a whole are still somewhat illdefined. This is due largely to a continued lack of
quality long-term climatic data surrounding all
lakes obtained with a station spacing sufficient to
resolve and define adequately the boundaries and
extents of the effects the lakes superimpose on synoptic weather variables. Substantial climate research has been performed on portions of the Great
Lakes, specifically for Lakes Michigan and Ontario.
Indeed, recent work has been completed on shortrange mesoscale forecasting and monitoring of winter time variables over Lake Ontario (Reinking et
ai. 1993). However, the effects displayed by the remaining lakes and that of the entire basin on local
climate have received much less attention.
Climate studies over parts of the Great Lakes
basin are not new. Early work by Day (1926) and
845
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Scott and Huff
Horton and Grunsky (1927) documented seasonal
effects by the lakes on precipitation. These effects
were suspected to be due, in part, to increased snow
fall downwind of the lakes in winter. Blust and DeCooke (1960) quantified higher precipitation totals
over the land in summer and over the lake waters in
winter. These results were echoed by others, including Hunt (1959) and Wilson (1977) in studies conducted around Lake Ontario, and by Changnon
(1968) and Bolsenga (1977) in additional research
on Lake Michigan.
The increases in wintertime precipitation, for the
most part, result from lake-effect snow events, created by the transfer of heat and moisture to cold,
dry, continental polar air masses moving over the
lakes. Danard and McMillan (1974) estimated that
approximately 52% of the moisture that evaporates
from the lakes in winter immediately precipitates
downwind within the lakes' region, causing a substantial increase in the lee-side snowfall. Preferred
areas for this enhanced accumulation have been
well documented by Petterssen and Calabrese
(1959), Changnon (1968), Strommen (1968),
Eichenlaub (1970), Wilson (1977), Braham and
Dungey (1984), and others.
Day (1926) also suggested an enhancement of
lake-generated thunderstorm activity over land
areas adjacent to the lakes during warm months.
Changnon (1966), Lyons (1966), and Wilson (1977)
attributed these effects to shoreline low-level convergence generated locally by the lake breeze phenomenon. Conversely, Lyons (1966) provided
evidence that deep convection does not often form
over the lakes in summer, and existing storms substantially dissipate when moving over the lakes due
to the greater stability imparted to the lower atmosphere by the cooler lake surfaces.
Lake effects on temperature have been quantified
as well. Kopec (1967) found substantially higher air
temperatures along the northern shores of Lake Superior in January and lower values in July compared to areas well away from the lake at the same
latitude. Danard and Rao (1972), Boudra (1981),
and Sousounis and Fritch (1994) found similar results for winter based on modeling efforts of cyclones over the Great Lakes. Similarly, Dare (1981)
showed a large negative departure of heating degree
day accumulations near the shores of Lake Michigan during the early part of winter due to the lag in
the cooling of lake waters compared to land surfaces.
Phillips (1972) statistically examined factors controlling temperature modification over Lake On-
tario and explained 86% of the variance by: the surface temperature of the upwind air mass, the surface water temperature, and the length of time air
was in contact with the lake. He concluded that
temperature modification happened quite rapidly as
air moved over the lake with more than half of the
change occurring within the first 10 minutes of
airnake contact and within about 3 km of the windward coastline. Wylie and Young (1979) found similar results over Lake Michigan, indicating that
surface-based temperature inversions in wintertime
rose to their maximum heights after only 50 km of
travel from the windward shoreline.
Examination of other climate variables has revealed measurable lake influences as well. Danard
and McMillan (1974) estimated that 48% of the
moisture evaporated from the Great Lakes remains
in the atmosphere as increased water vapor and
clouds well downwind of the lakes. Cox (1917),
Petterssen and Calabrese (1959), Eichenlaub
(1970), Danard and Rao (1972), Colucci (1976),
and Hjelmfelt (1988) are among many noting
changes in atmospheric pressure due to both lake
temperature and lake shape. All cited the Great
Lakes as a preferred region for wintertime cyclogenesis. Similarly, Strong (1972) documented anticyclonic development over the lakes in summer due to
cooler lake waters.
Changes in wind speed and direction and cloud
cover have been reported by Lyons (1966), Weber
(1978), Scott and Grosh (1979), and Comer and
McKendry (1993). All describe the effects of summertime lake-land contrasts and lake breezes on
ambient conditions. Similarly, Passarelli and Braham (1981) have pointed to a link between land
breezes and parallel shoreline snow bands in winter.
More recently, Kristovich and Steve (1995) documented wintertime lake-effect cloud frequencies
over the lakes using satellite data. Sousounis and
Fritsch (1994) show evidence that the lakes not
only alter the speed and direction of the surface
wind fields but also alter the structures and paths of
weather systems moving through the region on a
lake aggregate scale.
Interests in the impacts of the Great Lakes extend
far beyond their initial meteorological effects. Those
involved in lake hydrology and lake management
are interested in the long-term weather features and
feedback mechanisms in place within the basin.
What is not available for hydrologic research is detailed information on the average spatial patterns of
lake effects by all lakes on seasonal precipitation,
temperatures, winds, clouds, and vapor pressure.
Climatic Impacts of the Great Lakes
The primary objective of this research was to estimate, in as much detail as possible, the magnitude
of lake effects imposed by each of the Great Lakes
on the above climate variables. It was our intent to
remove substantial lake influences from the climate
data. In this created scenario, downwind climate
conditions likely are dominated to a much greater
extent by the large-scale synoptic weather patterns
moving across the area. All aggregate lake effects
may not be addressed adequately by this work, and
may remain in some analyses at a noticeable level.
Thus, reference in this paper to a "lake-effect" or
"no-lake-effect" area may not always encompass the
total of these larger scale lake influences.
DATA AND METHOD OF ANALYSIS
The climate of the Great Lakes basin was analyzed for lake effects on six climate variables.
These included: precipitation, mean minimum temperature, mean maximum temperature, cloud cover,
wind speed, and water vapor pressure. Although
each parameter is recorded within standard meteorological observations, all data were not obtained
from a single source. Temperature and precipitation
data were extracted from the archives of daily data
of the National Climate Data Center (NCDC) or
Canada's Atmospheric Environment Service. Cloud
cover, vapor pressure, and wind speed data in the
United States were averaged from hourly surface
airways observations at selected first-order stations,
obtained from NCDC, and described by Petersen
(1991). In Canada, these data were taken from averages listed in several data publications (Canada Department of Transportation 1968; Environment
Canada 1982, 1984).
Data for each element were averaged by season
(winter = December, January, and February, etc.)
for the period 1951-1980. This was the last 30-year
interval for which all data elements were available
(except for cloud cover data in Canada that were
from 1941-1960). Only sites with less than 10 percent missing data were included in the temperature
and precipitation analyses. Some of the hourly data
for the United States were of a lesser temporal quality as were some of the Canadian published data.
No attempt was made to adjust for missing values.
An analytical technique was devised to extract
lake effects from the full analysis of climate data
encompassing the basin. Past research at the Illinois
State Water Survey on the Lake Michigan basin
provided climatological techniques for defining the
extent of lake effects on monthly, seasonal, and an-
847
nual precipitation, temperature, and other weather
conditions (Changnon 1968). Inspection of these results and those of other studies found significant
lake effects within a variety of distances from the
lake shores. Gatz and Changnon (1976) considered
lake-induced visibility restrictions due to enhanced
condensation on hygroscopic nuclei to be confined
to within 10 km of the lake. Scott and Grosh (1979)
studied infrared satellite imagery to investigate
summertime cloud cover over southern Lake Michigan and found a tendency for pre-existing afternoon
cloud cover (cloudy or clear) to be advected over
the land-lake boundaries for distances up to 40 km.
Braham and Dungey (1984) used a 40-km wide
band to estimate the effects of Lake Michigan on
winter snowfall. Lyons (1966) reported on a case
study of the effects a lake breeze imposed on a transient squall line 80 km inland from Lake Michigan.
Strommen and Harman (1978) tracked lake-effect
snow over the Michigan shore from 65 km to as far
as 105 km inland (although the latter distance likely
was enhanced by topography).
Considering these and other studies and analyses
of air mass and storm characteristics, an 80-km
wide band around the lakes was selected that it was
thought would likely include most of the area with
detectable lake effects. Surrounding this band, a relatively large area was chosen within which a full
regional analysis was conducted of each climate parameter to define the spatial characteristics of the
elements well beyond the lake-effect region. This
latter region extended for approximately 300 km
beyond the lake-effect areas, except for only about
100 km in Pennsylvania and New York where large
topographic effects complicate the analysis. In this
manner, we attempted to exclude from our analysis
as much of the terrain influences on climate as possible in order to investigate those effects derived
solely from the lakes.
Averaged data were plotted and manually analyzed to establish a seasonal pattern of each element over the entire Great Lakes basin and the
surrounding areas. Then, a second map was constructed ignoring all data within the 80-km lake-effect band, and thereby, defining a "no-lake-effect"
pattern across the basin, derived only from regions
presumably unaffected by the lakes. These two
maps were compared, and the measured differences
were attributed to the "lake-effect" within the basin.
The variability of the above parameters across
the Great Lakes basin due solely to lake effects is
complex. That is, much of the basin is devoid of
observational data due to the lakes themselves; ap-
848
Scott and Huff
proximately 35% of the region is covered by water.
Unfortunately, much of the land area also has a lack
of quality, high resolution, long-term climate data.
Substantially fewer precipitation sites exist to the
north of Lake Huron and around nearly all of Lake
Superior compared to the remainder of the lake
shores and surrounding land areas of the United
States and southern Ontario (Fig. la). These are
critical locations to establish the magnitude and extent of lake influences. The concern is heightened
with temperature which is observed at fewer sites
(Fig. 3a), and even more so with the remaining parameters (Fig. 7b) that are observed largely from
first-order stations only. Thus, isopleth construction
over water and over land areas in data poor regions,
are subject to some analytical uncertainty.
Prior research has provided some useful guidance
in pattern estimation in data void areas. As stated
earlier, Phillips (1972) and Wylie and Young (1979)
reported rapid modification of surface air flowing
out over Lake Ontario. Thus, considering that the
prevailing wind in winter, for example, has a strong
westerly or northwesterly component, spatial patterns in the current research are drawn to reflect a
relatively rapid change in weather conditions on the
predominant windward side of the lakes.
Analytical difficulties are not limited to poor data
coverage. The substantial change in orography
around the basin can complicate detection of lake
influences. Air masses that approaches the Great
Lakes from the west are continental in nature.
These air masses are seasonally homogeneous because they move off the vast, relatively flat farmlands of the North American prairies. To the east of
the Great Lakes, however, the general terrain rises
abruptly into the rolling hills of the Appalachian
Mountains. Petterssen and Calabrese (1959), Wilson (1977), and Strommen and Harman (1978) have
documented the strong terrain effects in the climate
record around the Great Lakes, especially east of
Lake Ontario. Estoque and Gross (1981) and
Hjelmfelt (1988) have shown similar results from
numerical simulation.
Beyond this, another complication exists within
the data themselves. Along the common border between Canada and the United States, it became apparent that wintertime precipitation totals at
Canadian sites were 25% to 40% higher than totals
in adjacent areas of the United States. Other seasons displayed no such discrepancy. It is likely that
this difference results from contrasting methods of
quantifying melted precipitation from snow fall.
Most stations in the United States measure precipi-
tation during snow events by melting the amount of
snow collected in a standard 8-inch rain gage. In
Canada, however, the method employed is to estimate precipitation from a straight 1O-to-l ratio between snow depths and liquid precipitation.
The quality of snowfall measurements is poor
due to the fact that as wind speeds increase, gages
become less able to catch and retain fallen snow for
observation. This has been well reported in the literature, most recently by Groisman and Legates
(1994). Warnick (1956) and Larson (1971), among
many others, documented observation deficiencies
as high as 80% and 67%, respectively. Groisman
and Legates (1994) extend the problem to wind-driven rainfall as well. Bolsenga (1977) summarized
that these deficiencies substantially obscure lakeland differences in precipitation, especially differences that are small.
Snowfall measurements in the United States inevitably underestimate actual wintertime precipitation. However, the 10-to-l ratio of depth
measurement in Canada may frequently result in observations that are too moist during events of even
moderately dry snow. Therefore, the analyses of wintertime precipitation between the two countries were
kept separate, but with isohyets in the United States
being matched roughly with values 25% higher on
the Canadian side. Regardless, since this research
considered only the changes in precipitation induced
by the Great Lakes, the lake-effect charts should be
relatively unaffected by this procedure.
SELECTED ANALYSIS AND RESULTS
The full compliment of charts concomitant with
this report is quite large. Only a select sample of results will be presented here. These include the "all
data" and "lake-effect" charts for: precipitation,
component temperature, cloud cover, and vapor
pressure during winter and summer. Charts delineating climate as it may appear with the lakes removed and all spring and autumn charts can be
found in Scott and Huff (1996).
Winter Precipitation
Figure la shows the total precipitation pattern for
winter (which includes lake-induced effects) for the
Great Lakes basin and the surrounding region. Precipitation site locations are indicated by dots. Isohyets on precipitation maps are drawn for every 25
mm. Dashed lines are used over the lakes, indicating that the analyses in these areas represent only a
Climatic Impacts of the Great Lakes
a. All data
(100)
o
,
..
(150)
'200
,',
b. Lake-effect
FIG. I. Average precipitation (mm) over the Great Lakes basin in winter: a) using all data
and b) showing lake-induced changes. Heavy line on b) represents the 80-km lake-effect
boundary. Dots indicate locations ofprecipitation sites. Values in parentheses are for Canada.
849
850
Scott and Huff
best approximation. As stated in the previous section, data across Canada and the United States are
analyzed separately for this season only; isohyet labels in Canada are shown in parentheses.
A general increase in precipitation is observed
from northwest to southeast across the entire area.
Superimposed on this overall pattern are large maxima in precipitation over the eastern portion of all
lakes and adjacent land areas. Highest magnitudes
are associated with Lakes Superior, Huron, and Ontario. Maxima found inland over central and western New York and northern lower Michigan likely
are enhanced by terrain effects (Strommen and Harman 1978). Unlike many previous studies, isohyets
in the current investigation are not analyzed in a
tight pattern along the eastern lake shore. Instead,
they are spread further to the west, reflecting the results of Phillips (1972), Wylie and Young (1979),
and Braham and Dungey (1995), all of whom suggested rapid modification of air masses flowing
over the lakes.
Results here compare reasonably well with those
from prior climatological studies. Gatz and
Changnon (1976) show similar locations of most
maxima and minima centers over Lake Michigan,
but with magnitudes that differed by ±1O%. A primary difference is the precipitation maximum in
northern lower Michigan. Eichenlaub (1970) placed
a similar feature further to the west while Gatz and
Changnon (1976) placed the feature even further
west. Strommen and Harman (1978) matched the
location given by the present work, but then presented evidence that the center moves from west to
east, and then back west throughout the late autumn
to early spring period. All of these differences
likely can be attributed to natural variability caused
by different temporal domains and the stations
used. In addition, broad pattern agreement is found
between this study and Wilson's (1977) I-year investigation over Lake Ontario. Both studies show
higher precipitation over the southwestern and eastern parts of the area, and the enhanced influence
from the rise in terrain well to the east.
A comparison of the analysis in Figure la to the
no-lake-effect chart for winter (not shown) reveals
the location and magnitude of lake effects on wintertime precipitation (Fig. 1b). (The location of the
80-km boundary is superimposed.) Precipitation
maxima attributed to lake influences are observed
to the east of all lakes. Greatest effects are found
over eastern Lake Superior and over the land just
east of the lake. Here, the increase reaches 125 mm,
corresponding to a percentage increase slightly
greater than 100% of the no-lake-effect estimate.
The maximum is exceptionally high due likely to
the lake's large surface area, great depth, and westeast orientation which aligns the lake approximately parallel to the prevailing wintertime wind
direction. The lake seldom freezes over a substantial portion of its surface. Thus, this allows for a
long fetch of airflow over open water, and conditions develop that promote a strong influence by its
warm moist surface to the air mass above it.
Lake-effect precipitation increases of 90% and
50% are observed downwind of Lakes Huron and
Ontario, respectively. East of Lake Huron, an area
of enhanced precipitation extends past the 80-km
boundary to about 100 km from the lake. These occurrences were uncommon throughout our analyses.
The high precipitation totals east of Lake Erie and
well east of Lake Ontario are influenced also by the
local topography, and thus are difficult to separate
by cause. Wilson (1977) found precipitation increases of approximately 25% near Lake Ontario,
and over 50% in regions of higher terrain just to the
east. The current study shows a 50-mm increase in
southwestern lower Michigan that represents a departure of about 35%. Changnon (1968) calculated
a 30% increase for this area, but a much higher increase in extreme northwestern Michigan of about
50% compared to just 20% for the current study. It
is likely that differences in precipitation patterns
and amounts between the current work and prior
studies are due mostly to natural temporal variability and variances in the number and location of stations used. Aggregate lake effects on wintertime
precipitation appear to be contained within the 80km boundary.
Summer Precipitation
Long-term summer precipitation patterns (Fig. 2a)
reveal a general trend of higher precipitation in all
directions away from the Great Lakes basin. Distinct
minima of precipitation are seen over each lake. The
cooler lake waters help stabilize the surface boundary layer above them and inhibit surface-based convection that commonly accompanies summertime
precipitation. The patterns over Lake Michigan
closely parallel those of Gatz and Changnon (1976),
although rainfall amounts in the current study are
about 10% higher. Similarly, Wilson (1977) also indicated lower precipitation over Lake Ontario, and
showed higher precipitation to the south and north
of the lake as is revealed here. Rainfall near Lake
Erie shows a less well defined rainfall minimum.
Climatic Impacts of the Great Lakes
a. All data
250
275
b. Lake-effect
FIG. 2.
Similar to Figure 1, except for average precipitation (mm) in summer.
851
852
Scott and Huff
Since it is the shallowest of the Great Lakes, it
warms faster seasonally. Therefore, its water surface
temperature more quickly approximates the air temperature of adjacent land areas, and thus, reduces the
negative effect of a cooler lake surface on convective precipitation.
The summer no-lake-effect chart (not shown)
suggests that some influence due to the lakes' presence may remain in the analysis. The rainfall pattern for the region outside the basin was quite
variable and created a minimum within the basin
with higher rainfall towards the southwest, southeast, and northeast. The cooling effect of the lakes
likely generates a concomitant large scale subsidence in the summer season (Strong 1972) that
may extend beyond the 80-km boundary set by this
study. A much larger region of extended analysis
than is used here would be needed to test for this
effect.
The resulting lake-effect analysis (Fig. 2b) indicates less rainfall across most lakes during the summer due to the inhibiting influence of the lakes.
Maximum departures exceed the no-lake-effect estimates by 50 mm over north central Lake Huron,
eastern Lake Ontario, and south central Lake Superior, corresponding to a change of about 20%. Elsewhere, decreases were about 25 mm (10%).
Increases found well to the southeast of Lake Erie
are likely terrain enhanced.
The general summertime lake-effect pattern has
similar attributes to those of Changnon (1968) and
Lyons (1966), both showing decreases in rainfall
over Lake Michigan. In addition, results here are
comparable to work by Augustine et al. (1994) who
used a satellite-based rainfall estimation technique
and found mean summer decreases on precipitation
of 18%, 14%, and 32% over Lakes Michigan, Superior, and Huron, respectively. Changnon (1968)
shows a 20% increase in rainfall over northern
lower Michigan (perhaps terrain enhanced) and a
10% increase in its Upper Peninsula (likely convergence of lake breezes). Only the latter of these is
weakly detected here. Part of the region in northern
lower Michigan is outside the 80-km zone and,
therefore by definition, is not affected by the lakes
in the present study.
Over-lake precipitation, expressed as a percentage of change attributed to lake effects, are summarized in Table 1. In general, lake-induced
precipitation is highest during winter and autumn
when lake waters are typically much warmer than
the air flowing over them. This scenario provides
moisture for enhancing clouds and precipitation.
TABLE 1. Approximate seasonal change in precipitation (in percent) from the no-lake-effect
analysis for each of the Great Lakes by season.
Lake
Superior
Michigan
Huron
Ontario
Erie
Winter
Spring
Summer
Autumn
100
40
60
15
15
40
-25
0
-20
10
-20
-10
-20
-20
-10
50
20
20
20
20
Conversely, beginning in spring over Lakes Michigan and Ontario and spreading to all lakes in summer, the lakes act as a stabilizing force on the lower
atmosphere, decreasing convective rainfall.
In all seasons, effects are greatest over Lake Superior, a result of its large size, and east-west orientation (being approximately parallel to the
prevailing westerlies). These factors maximize the
time available for air to move over the lake and be
modified. Conversely, being the smallest water
body, Lake Erie, as would be expected, creates the
least influence. In addition, it must be pointed out
that air masses reaching Lakes Erie and Ontario
typically have been modified by first passing over
the larger lakes to the north. This would tend to decrease the intensity of lake influences here compared with that measured near the larger lakes.
Departures east of Lakes Erie and Ontario are also
substantially influenced by the rapid increase in elevation of the adjacent terrain, masking the effects
due solely to the lakes.
Temperature
Lake effects on mean seasonal temperatures are
very evident due to differences in the heat capacities of land and water. The Great Lakes provide a
significant source of warmth to the atmosphere during colder seasons, but conversely, they generally
cool the region in summer. Seasonal analyses indicate that the greatest lake influence occurs on mean
minimum temperatures in winter and on mean maximum temperatures in summer.
Wintertime mean minimum temperatures increase substantially from north to south across the
Great Lakes region (Fig. 3a). Superimposed on this
pattern are relatively high values over and south of
each lake. Temperature contours are drawn tightly
on the northern shores of most lakes, based on the
Climatic Impacts of the Great Lakes
a. All data
-24
b. Lake-effect
FIG. 3. Similar to Figure I, except for mean minimum temperature CC) in winter. Dots
indicate locations of temperature sites.
853
854
Scott and Huff
findings of Phillips (1972) and others, with the
strongest gradient found over Lake Superior. A 6°C
change in air temperature occurs in a distance of
about 50 km into the lake from the northwestern
shore, with a total increase of goC across the lake.
Similar increases of 4°C to 5°C are found along
northern shores of Lakes Huron and Ontario. Conditions over Lake Michigan display a latitudinal
component due to the north-south orientation of the
lake with a 3°C to 4°C increase along its eastern
shore. The effect over Lake Erie is minimal, again
due to the lake's smaller volume and likely more
rapid temperature moderation, concomitant with adjacent land surfaces.
The rather zonal appearance in the isotherms west
of the lakes was extended across the region to create
the no-lake-effect chart with little assistance from
isotherms east of Lakes Ontario and Erie. Values at
stations in the east were highly variable due to the
topographical influences that complicated the analysis. Regardless, the resulting lake-effect chart shows
positive departures of mean minimum temperatures
near each lake (Fig. 3b). A warmer over-lake air
mass in excess of goC is seen over south central
Lake Superior. Northern Lake Michigan and central
Lake Huron generated a 4°C increase. The smaller
and more southerly lakes (Ontario and Erie) created
a +1°C to +2°C departure.
Wintertime mean maximum temperatures likewise increase from north to south across the domain
(Fig. 4a) but with less range than was observed
nocturnally. The pattern across the region is again
strongly zonal. Warmer latitudinal conditions are
observed again over the basin and are quantified in
the temperature anomalies of the lake-effect chart
(Fig. 4b). Differences are much smaller than were
observed in mean minimum temperatures. A 2°C
temperature increase attributed to lake-effect exists
over parts of extreme western and southeastern
Lake Superior and northern Lake Michigan. A 1°C
increase is observed over most larger lakes and
south central Lake Ontario. Once again, effects attributable to Lake Erie are negligible.
This research included analyses on the components of daily temperature. A comparison of Figures
3b and 4b describe lake effects on mean temperature. Maximum departures in winter approach 5°C
over southern Lake Superior and 2°C to 3°C over
the northern portions of Lakes Huron and Michigan.
Lake Ontario shows departures of 1°C to 2°e. Lake
Erie indicates a positive 1°C change in mean temperature that maximizes in the eastern part of the
lake. These seasonal data (December - February) are
in reasonable agreement with findings of Kopec
(1967) who showed higher mean temperatures for
January alone of 7°C over Lake Superior, 2°C to
3°C over Lakes Michigan, Huron, and Ontario, and
1°C to 2°C over eastern Lake Erie. Gatz and
Changnon (1976) revealed a 3°C lake-effect warming over northern Lake Michigan for January.
Mean minimum temperatures in summer (Fig.
5a) continue the pattern found for nocturnal conditions during winter. That is, the substantial effect of
the Great Lakes on nighttime temperatures in summer is to warm the overlying air. The effect is most
evident around the more southerly lakes. Maximum
temperature gradients of about 2°C to 3°C are
found around Lakes Erie and Ontario. These
smaller lakes likely warm faster seasonally, and
thus, at night will be warmer than adjacent land surfaces. Lake Superior stands out as an exception
with essentially no shoreline temperature differences, except for one small region adjacent to the
Keweenaw Peninsula. The warmer conditions in extreme southwestern Lake Michigan may be enhanced by an urban effect of Chicago.
In quantifying these effects, the extent of warmer
conditions is greatest (in excess of 3°C) over northern Lake Huron and on the western sides of Lakes
Ontario and Erie (Fig. 5b). Temperatures at least
2°C higher are found over the balance of these
lakes, and also in smaller areas of eastern Lake
Michigan and extreme south-central Lake Superior.
Thus, diurnal warming of the smaller and more
southerly lakes in summer raises over-lake temperatures to values higher than the temperatures that
occur over the land due to nocturnal cooling. A near
neutral influence on temperature is shown over
Lake Superior. Due to its size (hence, slower warming) and more northerly location (i.e., less insolation), temperatures over Lake Superior more
closely parallel nocturnal land temperatures. Only
near the Keweenaw Peninsula, which may trap
water to its east and allow it to warm more, do effects on mean temperature occur at levels similar
than that observed near other lakes.
Typical diurnal heating increases summertime
mean maximum temperatures over land to values
well above those observed near the lake shore (Fig.
6a). Substantially cooler conditions are indicated
over all of the Great Lakes, but not outside the 80km buffer region. Temperature anomalies of 3°C to
4°C are found over the larger lakes, and maximize
at 5°C over Lake Superior. The coolest air mass
within the domain is actually found in this area
rather than far north of the basin.
Climatic Impacts of the Great Lakes
a. All data
855
-10
------
-10
-9
-ll
___________ -7
--~-~:
~~~
~-3
b. Lake-effect
FIG. 4.
Similar to Figure 1, except for mean maximum temperature
eC) in winter.
Scott and Huff
856
a. All data
8
~7
8
8
b. Lake-effect
FIG. 5.
Similar to Figure 1, except for mean minimum temperature (0C) in summer.
Climatic Impacts of the Great Lakes
857
21
a. All data
-----
21
~22
22_______
23
24
(
27
----¥---3O
29
28
b. Lake-effect
FIG. 6.
d
~\~
Similar to Figure 1, exceptfor mean maximum temperature (OC) in summer.
858
Scott and Huff
Isotherms away from the basin again suggest that
a rather zonal pattern would exist across the region
without the presence of the lakes, and thus, the
lake-effect chart (Fig. 6b) displays substantially
cooler conditions due to the lakes. Greatest departures in mean maximum temperatures (-6°C) are
observed over northern Lake Superior. Temperature
changes of at least -3°C are present over much of
the three largest lakes. Lakes Erie and Ontario show
changes of -2°C.
Lake effects on summertime mean temperatures
(combining Figs. 3b and 4b), suggest a net cooling
effect imposed by the lakes over Lake Superior
(about -3°C) and Lake Michigan (-1°C). Conversely, the western ends of Lakes Erie and Ontario
show slightly warmer conditions, while the effects
of Lake Huron on temperature at night are largely
canceled by opposite effects generated during the
day. Results over Lake Superior are comparable
with July mean temperatures analyzed by Kopec
(1967) where departures of -4°C to -5°C were indicated. He likewise found only small lake effects
elsewhere, about _1°C. Gatz and Changnon (1976)
show a _1°C departure in July mean temperatures
over most of Lake Michigan and _2°C in the extreme southern portion of the lake. Once again, departures there may have an urban effect
superimposed. Besides natural variability, differences between these past studies and the present investigation were influenced from the inclusion of
June and August data in the current work only.
Magnitudes of lake effects on temperature during
other seasons are not as great (Table 2). Mean minimum temperatures increase as a result of the lakes'
presence during all seasons and over all lakes. Conversely, the influence of the lakes on mean maximum temperatures results in a cooler nearby air
mass during spring and summer, but with slightly
warmer conditions over some lakes in autumn and
winter. Overall, effects on component temperatures
TABLE 2.
season.
are smallest during spring with mean minimum
temperatures and in autumn with mean maximum
temperatures, both transitional periods. Not unexpectedly, largest and smallest absolute changes in
temperature are found over Lakes Superior and
Erie, respectively, largely a function of their respective sizes and, thus, the amount of water mass avail~
able for air temperature modification. Once again,
air masses passing over the southeasterly most
lakes typically have been influenced by first passing over the larger lakes to the north. That is, the
temperature differences between these air masses
and the lake water surface is not the same as that
observed over the larger lakes. This suggests the
tendency for a smaller lake effect on temperature
over the smaller lakes.
Summary of Other Climate Conditions
The seasonal values of cloud cover, vapor pressure, and wind speed were computed from surface
airways hourly data provided by NCDC (1986) and
prepared by Petersen (1991). These data possess a
much reduced station density compared with precipitation and temperature (shown in Fig. 7). Nevertheless, lake influences on some of these
variables are detectable and are displayed in Figures 7 and 8 and summarized in Table 3.
Cloud cover parallels the results observed with
precipitation. That is, cloud frequency is most affected by the lakes during winter due to the vast
heat and moisture source the lakes provide to the
lower atmosphere into the cold, dry, continental
polar air masses moving over them (Fig. 7a). Maximum effects occur over the larger lakes and tend to
reach greatest magnitudes on their eastern sides.
Conversely, summertime cloudiness is reduced due
to the increased stability imparted to the lower atmosphere from the relatively cooler lake waters
(Fig. 7b). As might be expected, lake-influences in
Maximum temperature departures (0C) attributed to lake-effect for each of the Great Lakes by
Mean minimum temperature
Lake
Superior
Michigan
Huron
Ontario
Erie
Winter
8
4
4
2
1
Spring
3
1
1
1
1
Summer
0
2
3
3
3
Mean maximum temperature
Autumn
3
3
4
3
2
Lake
Superior
Michigan
Huron
Ontario
Erie
Winter
Spring
Summer
Autumn
2
2
-3
-6
~
~
1
1
0
-3
-3
-1
-2
-2
-2
0
0
1
1
0
Climatic Impacts ofthe Great.Lakes
a. Winter lake-effect
b. Summer lake-effect
FIG. 7. Lake-induced changes in cloud cover (tenths of sky covered) over the Great Lakes
basin in a) winter and b) summer. Dots indicate locations of hourly data sites.
859
Scott and Huff
860
a. Winter lake-effect
b. Summer lake-effect
FIG. 8.
Similar to Figure 7, except for average water vapor pressure (mb).
Climatic Impacts ofthe Great Lakes
861
TABLE 3. Maximum departures (0C) in cloud cover and vapor pressure attributed to lake-effects for the
Great Lakes by season.
Vapor pressure
Cloud cover
Lake
Superior
Michigan
Huron
Ontario
Erie
Winter
Spring
25
25
15
7
7
0
0
0
0
0
Summer
0
Autumn
-9
-12
0
11
0
0
0
5
5
summer are smaller in magnitude than those during
winter. Summertime cloud reductions are related
strongly to daytime heating over the adjacent land
while the relatively warmer lake water conditions
influencing wintertime cloud cover occur nocturnally, as well.
Even so, it is likely that the extent of summertime
reductions in cloud cover are greater than what is
shown here. In general, cloud reductions in summer
are limited predominantly to the skies directly over
the lakes. However, due to the lake breeze phenomenon, clouds frequently form just inland from the
lake shore and offset cloud reductions as viewed
from lake shore sites. Cloud data from over-lake regions would be required to document the full extent
of lake-induced summertime cloud reductions.
Values of average water vapor pressure parallel
those of temperature and show strongest lake influences (increasing values) across the central part of
the basin during winter and autumn (Table 3).
These are seasons when the relatively warmer lake
waters provide a high moisture source to the surface
boundary layer above the lakes and a direct effect
on vapor pressure. Wintertime increases appear
most associated with Lakes Huron, Michigan and
Ontario (Fig. 8a). However, the lack of sufficient
lake shore sites, especially east of Lake Superior,
was a large limiting factor in the analysis. Although
actual vapor pressure is higher in summer than in
winter, substantial sources of moisture exist over
the land in the warmer seasons (evaporation and
transpiration) that are not present in the cooler seasons. Therefore, variations across the lake-land
boundaries are reduced. Due to the relatively cooler
water temperatures, lake effects in summer generally decrease water vapor pressure, but in reality,
changes are small. Maximum effects that straddle
southern Lake Superior and Michigan's Upper
Peninsula do not exceed 8%.
Lake effects on wind speed, generated by differ-
Lake
Superior
Michigan
Huron
Ontario
Erie
Winter
Spring
Summer
Autumn
0
-8
-6
0
3
6
14
4
0
0
0
0
0
-2
15
17
-4
12
3
3
ences in land-lake surface roughness and land and
lake breeze thermal circulations, are well-documented in the literature. Nevertheless, data from
the sparse number of long-term climate stations
presented here do not reveal detectable lake influences. Averaging of hourly reports greatly smooth
short term diurnal wind speed anomalies generated
by the lakes. In addition, wind speed measurements are influenced strongly by local topography
and instrument exposure. All of these factors may
have masked lake influences beyond the point of
detection.
SUMMARY AND CONCLUSIONS
This paper summarizes the results of an effort to
incorporate the findings of previous investigators,
along with those generated by our recent research,
into a ready reference on much of the quantitative
impacts of the Great Lakes on regional climate conditions within the entire basin of the five lakes.
Such information is not presently available, but represents a pertinent need to hydrologists, meteorologists, climatologists, and others concerned with the
problems generated by this huge inland water body.
Average spatial distributions of seasonal climate
conditions over the entire Great Lakes basin were
investigated to derive estimates of lake-induced influences on six weather parameters: precipitation,
mean maximum and mean minimum temperatures,
cloud cover, vapor pressure, and wind speed. Effects were estimated by first analyzing each variable using all data within the basin, and then
performing a second analysis which eliminated data
within an 80-km buffer zone around the lakes. A
quantitative measure of lake-effects was obtained
by a comparison of these two analyses. The analytical intent was to remove as much lake effects as
possible from the climatic record, leaving primarily
the synopticly-derived influences on each parame-
862
Scott and Huff
ter. Aggregate lake effects likely were not resolved
adequately by this research as was indicated in
some analyses. However, the substantial local effects observed just downwind of each lake appears
to have been documented well in most cases for the
parameters investigated.
Lake effects are observed most clearly in precipitation and temperature with considerable variation
between seasons. However, lake influences are
found also in other weather variables examined. Results generally confirm theory and analyses from
previous research. Analyses indicate that the greatest modification occurs over and just downwind (to
the east and southeast) of all lake shores while upwind regions are only minimally affected. Observed
differences between this study and those in the past
are likely due to natural temporal and spatial variability across the basin. Due to the strong subjective
nature of this and many prior works, it would be
difficult to suggest anyone study as presenting a
more accurate analysis. Nevertheless, this is the
first work to specify the local seasonal influences
by all lakes on these variables.
The use of an 80-km band around the Great
Lakes in order to encompass all lake effects appeared to be adequate for this study. Discrimination
between lake effects and topographical influences
on climate conditions, especially large just to the
east of the basin, was not undertaken. Although,
only partial results are presented here, a full report
is available (Scott and Huff 1996).
In conclusion, our work has provided new quantitative evidence of the magnitude of lake effects on
several climate variables. Nevertheless, the lack of
a good quality, high density, long-term climate data
network across the entire basin is a limiting factor
in any future effort to define lake effects more precisely. This is especially true in the documentation
of cloud cover, vapor pressure, and wind speed, but
also to a lesser degree in quantification of precipitation and temperature variations within sub-basins
around the lakes. One suggested improvement
would be a cloud climate study using infrared satellite imagery over the region as the period-of-record
of these data increase towards an acceptable climate
norm. Improved sensors within current and future
satellite imagery and NEXRAD radar facilities,
combined with surface measurements, should assist
in quantifying conditions in data void areas. Finally,
development of an alternative technique to incorporate greater resolution of aggregate lake effects
would be beneficial.
ACKNOWLEDGMENTS
The authors wish to thank Stanley A. Changnon
and Kenneth E. Kunkel of the Illinois State Water
Survey for their review and comments on the manuscript. We extend thanks to Linda Mortsch of Environment Canada for arranging access to the
Canadian data. In addition, we sincerely appreciate
the work of Water Survey staff members, Dave
Cox, for his assistance in preparation of the illustrations, and Jean Dennison for her professional word
processing support. This work is supported under
NOAA Grants COM NA16WN0351-01 and COM
NA27RA0173-0l.
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Submitted: 4 December 1995
Accepted: 1 August 1996