Where does the water come from in Midwestern snow storms?

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Where does the water come from in Midwestern snow storms?
ENLARGE

Satellite image of US from the east coast to the Rockies shows the land covered by white clouds

Water can travel a long way to dump onto the Midwest as snow. This picture shows the storm system that cause the massive February 2011 storm.

Last week we were visited for the second time this winter by a sizable snowfall. A reasonable question to ask is where does the water come from to make the snow? A general answer is not available as the variety of snowstorms that we get in Wisconsin result from different sources of water. In fact, even for this last storm in the southern part of the state, the answer depends on where you live.

In Madison, the bulk of that water that fell as snow in that last storm most likely originated in or near the Gulf of Mexico, transported northward on southerly winds in advance of the storm center itself. As the air containing the water vapor rose, it condensed or was deposited onto ice particles in the clouds and eventually grew to a sufficient size to fall from the cloud as a snowflake.

In Milwaukee, however, an additional source of water vapor to form the snow was Lake Michigan. Air carried over the lake was able to gather more water vapor from the lake surface due to evaporation, thus enhancing the amount of water vapor in the air near Milwaukee. Upon rising, this increased water vapor content led to increased snow amounts there.

Finally, the process of transforming water from invisible vapor to ice releases a huge amount of energy to the surrounding atmosphere. In fact, the amount of energy released by this phase change during a 5″ of snowstorm over Dane County alone is enough to power the Madison metro area for 400 days!

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.