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Precipitation Climatology Handout

This page is part of the project: Basic Climate Information for Great Lakes

Precipitation Changes in the Great Lakes

Precipitation throughout the Great Lakes region has undergone important changes in its geographical distribution and intensity.  Although much of the Great Lakes region has observed increases in average precipitation, notable localized decreases occurred in the north over Michigan’s Upper Peninsula, northern Wisconsin, and Minnesota. Some locations, such as the central portion of the Great Lakes, experience increases during part of the year and decreases during other times.  Differences in the timing and geography of changes in precipitation highlight the importance of studying local changes within the regional context.

Precipitation Regime

Precipitation throughout the Great Lakes region is unique due to the strong influences of the lakes on the regional and local climate.  The five Great Lakes are very important to the geographical distribution of precipitation, and they influence the timing and amount of precipitation that falls. 

During the cold-season (November-April) the lakes are relatively warm compared to the air temperature over them, and they act as a source of moisture for precipitation.  Locations directly downwind from the lakes, known as lake-effect zones (Figure 1), receive the greatest amounts of lake-induced precipitation.  Evidence of lake-effect precipitation is seen in the seasonal observations for SON and DJF (Figure 2) where the downwind coastlines experience more precipitation than regions outside of the lake-effect zone.  The driest areas during the cold season are in the western Great Lakes, and precipitation amounts increase farther to the east.

Figure 1: Lake-effect snow regions across the Great Lakes

During the warm-season (May-October) the lakes create conditions for a stable atmosphere and precipitation over the lakes is decreased.  Warm-season stabilization is seen in JJA (Figure 2) where the least amount of precipitation is observed over the lakes.  Lands adjacent to the lakes experience precipitation generated from lake breezes, but this affect is localized.  Non-lake-effect precipitation during the warm season is mainly a product of convective storms that move across the region and vary in size and intensity.

Figure 2: Seasonal mean precipitation (mm/day) amounts for the period 1981-2010 for December, January, February (DJF) (a), March, April, May (MAM) (b), June, July, August (JJA) (c), and September, October, November (SON) (d)

Observed Precipitation Changes

The World Meteorological Organization’s definition of climate (a period of at least 30 years) is used to describe climatic changes in precipitation for the Great Lakes region between the periods of 1951-1980 and 1981-2010.  Precipitation data are from the CPC US Unified Precipitation data set that is provided by the NOAA/OAR/ESRL PSD from their Web site at http://www.esrl.noaa.gov/psd/.  The data are regarded as one of the current best sources for precipitation observations.

On average Great Lakes annual precipitation has increased 55 millimeters in the latest climate period.  The greatest increase occurred in the southwest portion of the domain (Figure 3), but overall decreases were observed in the north and parts of the southeast.  Locally, increases and decreases are not observed equally across all seasons.  Locations in the central Great Lakes (i.e., IL) experienced an increase in precipitation during the months of September through February but a decrease for the remainder of the year. Parts of the north, such as Michigan’s Upper Peninsula, experienced declines in precipitation during all seasons.  

Figure 3: Average change in precipitation (mm/day) between the periods of 1951-1980 and 1981-2010

Figure 4: Average change in seasonal precipitation (mm/day) between the periods of 1951-1980 and 1981-2010 for December, January, February (DJF) (a), March, April, May (MAM) (b), June, July, August (JJA) (c), and September, October, November (SON) (d)

 

Table 1: Mean, min, and max changes for the period averages and seasonal precipitation (mm/day) in the Great Lakes region between the periods 1951-1980

 

Period Change

DJF

MAM

JJA

SON

Mean

0.17

0.13

0.12

0.18

0.28

Min

-0.81

-0.84

-0.87

-1.07

-0.73

Max

0.81

0.86

1.02

1.00

1.04