Snow, rain, hail...we can see just about everything Mother Nature has to offer here in Western Wisconsin. This past weekend brought something a little more uncommon, freezing drizzle. It started late Sunday afternoon and continued well into the night and left most places with more than 1/10" of ice. Not fun for those who park their cars outside, a lot of scrapping to get ride of that ice.
The circumstances that led to the freezing drizzle are something we don’t see very often. Most of the time the warmest temperatures are close to the ground and then get colder the higher up you go in the atmosphere. But in the case of freezing drizzle, it get’s a little more complicated. Below is a typical temperature profile that can give us freezing drizzle.
To us meteorologists, that picture tells a story. Without getting too scientific on everyone, what you’re looking at is a plot of temperature and dew point through the atmosphere. Those are the black lines going from bottom to top, with the temperature line on the right, and dew point on the left.
For the purpose of freezing drizzle, we’ll focus mainly on the temperature line. At the surface (lowest part of the graph) we have an air temperature that is below the freezing mark, 0 degrees Celsius (purple/pink line going diagonally from the center to the upper right). But higher up in the atmosphere, about one mile up, the temperature is above freezing so the precipitation that is formed in the clouds will fall as a liquid. In Sunday evening’s case, there wasn’t enough moisture to get rain, but enough to form that mist/drizzle.
Once the drizzle fell to ground, it encountered temps in the middle 20s degrees Fahrenheit. With those air temps below the freezing mark, things like the ground, cars, trees and pretty much everything else were also below freezing, so once the mist/drizzle comes in contact with those objects, it freezes, and forms the layer of ice on everything.