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Why precipitation falls in the form it does

Posted by David Epstein  March 17, 2014 10:41 AM

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Precipitation type is an interesting thing. There are times you think itís cold enough for snow and it rains and times you think itís warm enough for rain and it snows. To understand they type of precipitation we see here at the ground think about the atmosphere like a layer cake.

In the clouds, where precipitation forms is analogous to the top of the cake and the ground where the precipitation reaches is the bottom or crust (I like pie). What happens in the middle is quite critical for determining what for the precipitation will take.

Notice in the diagram below the red color represents air above freezing. In the snow situation, there isnít any air above freezing. As the snow falls from the clouds, itís able to make it to the ground without melting. In the other three situations, the warm air melts the snow before it reaches earth.

In the sleet example, the warmth is very limited and the air below it is cold. The rain, which resulted from the melting snow, was able to refreeze into a little ball of ice or sleet. Upon reaching the ground the sleet bounces and even can accumulate. I once saw 4 inches of sleet accumulate in Portland, Maine during a winter event. Sleet is about 3 times as heavy as a typical snowflake.
In the freezing rain situation the snow still melts into rain, but the water only has time to chill. The water becomes so cold itís temperature falls below 32F, but it remains liquid. So unlike what you have learned about water freezing at 32 degrees, supercooled water can be liquid well below freezing.

When supercooled water hits the ground, your car, the street, it freezes into a sheet of ice and creates glaze. This is one of the most hazardous types of precipitation.
Rain is simply snow that melted and never refroze or cooled below 32F.

Hail is an entirely different form of precipitation caused by big thunderstorms carrying rain drops high into the storm where they freeze into a ball of ice. Itís similar to sleet in that both are ice, but they form for entirely different reasons and hail can grow very large as it gains layer upon layer of ice inside the strong storm.

The end of March can still bring some snow or sleet to the area, freezing rain is much less likely in spring because the ground is often too warm.
Iíll be updating the forecast on Twitter @growingwisdom.

This blog is not written or edited by or the Boston Globe.
The author is solely responsible for the content.

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About the author

David Epstein has been a professional meteorologist and horticulturalist for three decades. David spent 16 years at WCVB in Boston and currently freelances for WGME in Portland, ME. In 2006, More »
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