Weather Whys: What determines precipitation type?

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When it’s cold, it snows, and when it’s warm, it rains! It seems obvious that temperature determines precipitation–or precip for short–type, but it isn’t always so black and white. We’ll take a closer look at how temperature and the different temperature layers in the atmosphere determine what you see on the ground when it storms. 

Within the Earth’s atmosphere there are many layers. We’re only going to focus on the one closest to the Earth’s surface. But even within that one layer we can see different temperature layers. Some are very cold, some are warm and some are just above freezing. The placement of these layers can manipulate the precip from a storm and create very different outcomes.

When precip falls through a cold layer from the cloud to the ground, it will remain frozen. This creates snow. The same concept is true when precip falls through a completely warm, or above freezing, layer. The precip will melt or stay liquid all the way to the surface and fall as rain.

Things get a little more complicated with these layers are not uniform.

If a storm forms above a thin warm layer with a thicker cold layer near the surface, the precip will initially melt and become rain. When it begins to fall through the cold layer, it will refreeze before hitting the surface. This will fall as sleet, little ice pellets often mixed with rain and snow. 

If a storm forms above a thick warm layer but the surface temperatures are freezing, we get one of our most problematic types of precip. The precip will melt and become rain, but refreeze upon contact with the freezing surface. This includes roads, trees, cars, houses… anything that is very cold. This is how freezing rain forms. All surfaces will have a dangerous coating of ice. 

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