Everything you’ve ever wanted to know about Freezing Drizzle

Some of you might be wondering why we keep getting freezing drizzle. Let me explain.

Everyday at 12:00 UTC and 00:00 UTC weather balloons are launched almost simultaneously all over the United States. Attached to these balloons are a radiosonde. A radiosonde gathers data like atmospheric pressure, temperature, wind speed and direction, and humidity as it ascends through atmosphere. The data gathered is then transcribed to a skew-t log. You have seen me use these. They are very important.

A sounding or skew-t looks like this (graphic 1). It’s called a skew-t because all the data is skewed to the left. The red line is the temp as it rises through a column of air. The green line is the dew point.

Looking closer at the skew-t I have provided you can see some very important characteristics. The first I have highlighted in blue. That is the cold air. If you follow the red line you will see a sharp turn to the right. That indicates that the temp is rising with height, or a temperature inversion . If it was spring that would be called a cap. It would prevent any storms from forming. Back to the blue highlighted part. It indicates that the cold air is very shallow. In fact the temp doesn’t fall back to surface temps until the 600 millibar range (the area I have circled). That’s about 13,000 feet. So, after about 3,000 foot the temp would climb with height. The pink area is the warm air aloft.

Why are we getting freezing drizzle? On the skew-t you and see that the dew point and temp lines are pretty much on top of one another. That’s the cold air sucking all the moisture out of the atmosphere. Cold air cannot hold as much moisture as warm air. That left over moisture falls as drizzle. When they are equal you get condensation…or drizzle. Since the cold air is so shallow the drizzle does not have time to freeze as it falls. So it falls as rain, and freezes on surfaces in the colder layer.

The second graphic shows how different winter precip is affected by temps in the upper atmosphere.

The 3rd graphic is the skew-t without my scribbling