This is the season when the huge expanse of the Great Lakes is affected by the cold air of fall.
In the summer months, a large amount of thermal energy was stored in the deep water basins. The cold weather in Canada is advancing into these eight states to bring about some hydrological changes. Water level monitoring shows steady drops in water levels as vapor enters the atmosphere, a typical pattern in September. How does this steep thermal contrast generate such rapid atmospheric drainage?
How arctic air and warm water meet across the Great Lakes in fall
The U.S. Army Corps of Engineers has been carrying out hydrologic tracking of Great Lakes water levels. There is a clear annual cycle that starts with an accumulation of spring rainfall and end-of-winter snowmelt. These factors mean water levels are at their peak by the middle of summer.
Most observers would assume that it is the heat on summer afternoons that is behind the greatest evaporation loss in the region.
The pattern is actually the opposite, according to the hydrologic data. The lakes are so deep that they take months to heat up in the summer. This is because water possesses a high specific heat capacity. It is only by September that Lake Superior reaches its peak surface temperatures.
This coincides with the arrival of cold fall air coming down from Canada. A steep vapor pressure gradient results directly at the water surface, and water molecules escape into dry air at an accelerated rate.
Seasonal moisture loss and how NOAA measures it
The National Oceanic and Atmospheric Administration (NOAA) continuously tracks these environmental changes from several monitoring stations. There are 94,000 square miles of water to monitor, and the volume of evaporation at the beginning of fall is immense. The Great Lakes can lose tens of billions of gallons of water in one day when weather conditions reach their height.
The thermal storage in Lake Superior is relatively large based on the 31,000 square miles of surface area in the lake. Evaporation is enhanced due to the presence of strong winds that remove the moisture-filled air from the lake. New air is drawn in from the north.
The process is ongoing, and the lake levels decrease gradually from October to December according to the U.S. Army Corps of Engineers. It impacts commercial shipping. There is an alteration in the shoreline, and also the channels at the lake harbors are shallow.
The NOAA Great Lakes Environmental Research Laboratory explains that water vapor accumulation in the atmosphere does not disappear from the regional climate system.
Winter weather downwind depends on fall evaporation
Lake-effect precipitation patterns in states downwind of the Great Lakes are fueled by the open water evaporation. The cold air from the north warms as it drops and reaches high levels of water vapor accumulation over the lakes.
Snowstorms for downwind states
The consequence for residents of Michigan, New York, and Pennsylvania, in particular, is unusually heavy snowstorms.
If ice forms over the lake surfaces early in the winter season, the evaporation mechanism is halted. If freezing air arrives but ice does not form, the loss of moisture is high until late December.
If freezing air arrives but ice does not form, the loss of moisture is high until late December.
The climate of the eastern region of North America has much to do with the Great Lakes. The yearly decline in the level of the lakes in the autumn season is not a reduction in water volume permanently. Instead, it is a reallocation of moisture and energy that becomes part of the climatic cycle.
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