Tuesday's snow didn't form where snow usually forms

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Some of you saw some light snow Tuesday morning. We said snow wasn't likely, so what's up with that?

Your snow fell after the main area of precipitation had moved away. The image in this post is a plot of the data we got back from the weather balloon at 6:00 Tuesday morning. The red line is the vertical temperature profile of the atmosphere over Mississippi.

Look at the red line. It shows the temperature was well above freezing above about 4,000 feet. It's above freezing all the way up to 10,000 feet. The problem is that's a vertical temperature profile common for sleet and does not support snow.

Again I ask... what's up with that? Sleet forms when snow falls from a cloud and melts to become a rain drop. That rain drop refreezes into an ice pellet - sleet. Snow cannot reform from that melted raindrop. Snow forms by a different process called deposition. Most of us have heard of condensation, when water vapor becomes water. Deposition is when water vapor becomes ice crystals.

What happened here is as the cold air was building southward Tuesday morning, deposition was occurring in that cold layer near the ground. Ice crystals were forming in the humid air that was growing colder and colder. In other words, snow was effectively being squeezed out of cold air below the clouds. What formed were what are called snow grains, not snow flakes. They tend to kind of flutter down to the ground.

Usually with a vertical temperature structure like we had, sleet or freezing rain are the result. This was a less common occurrence this morning that led to snow, though it has been known to cause light snow in mountains regions.