Ask the Met: Forecasting Rain & Snow

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How do you know if it is going to rain or snow?

Emily Robinson, Ashland Middle School

When forecasting different precipitation types, meteorologists look at the vertical temperature profile of our atmosphere. Basically what that means is, how warm and how cold is the temperature through our atmosphere? This determines what type of precipitation we see on the ground.

All precipitation falls from clouds initially as snow. For rain, most of the atmosphere is warm …above 32 degrees Fahrenheit. Therefor, it will fall from the cloud as snow and melt into a raindrop as it approaches the ground. For snow, it is opposite — the atmosphere needs to be cold, near or below freezing. This means a large layer of the atmosphere is 32 degrees Fahrenheit roughly, or below this. This supports the precipitation to fall from the cloud as snow, and remain snow when approaching the earth’s surface.

It’s quite challenging when having to forecast for freezing rain or even sleet. This requires three different temperature layers. Freezing rain will fall from a cloud as snow, melt to rain through a warm layer of air, and then freeze in a shallow layer of cold air at the surface ..either just before hitting the ground, or freezing on contact with the ground. Sleet it similar, however the cold layer of air at the earth’s surface is not as shallow. It falls as snow, melts through a warm layer, and partially refreezes through a cooler layer of air at the surface.

Meteorologist Alyssa Caroprese