Freezing Drizzle Wednesday Morning

It was a slick commute back to work for many Wednesday morning as freezing drizzle developed, prompting Winter Weather Advisories for much of northern Illinois.  Both freezing drizzle and freezing rain can be very dangerous for driving in the winter, often times more dangerous than snow, but they form in different ways.

There are millions of water molecules in the atmosphere and all around us.  You can't see them, but they're there.  As those water molecules move further up in the atmosphere they encounter temperatures that are well below freezing.  While some of the water molecules freeze once they've reached a temperature below 32 degrees, some of the molecules remain in the liquid form because there is nothing for the water droplet to freeze to, something known as a condensation nuclei.  These are what meteorologists refer to as 'supercooled' water droplets.  These supercooled water droplets can remain in the liquid form up to temperatures to -40°C (-40°F).  In fact, between roughly -4°C (~25°F) to -40°C, there is a mix of supercooled water droplets, ice and liquid water droplets.

Typically whenever there is drizzle, low clouds are usually present in the atmosphere.  These clouds tend to form in the atmosphere where the temperature is roughly between 0°C (32°F) and -10°C (14°F).  Since most of the water droplets in this type of cloud are not too far below freezing, the water droplets will most likely be supercooled.  If the cloud has enough moisture, it will produce drizzle with the supercooled water droplets.  Once the droplets reach the ground they are 'disrupted' and form a thin layer of ice, coating windshields, roads, parking lots and sidewalks.  Freezing drizzle can be very deceptive and often times catch drivers off guard.  While there were few major accidents Wednesday morning, there were quite a few cars that ended up in ditches because of the slick roads.  And I imagine many folks were doing the 'bambi splits' when walking in the parking lot or sidewalk.

Thankfully the threat for freezing drizzle is gone for Thursday morning.  But residual moisture on the ground could refreeze with temperatures Wednesday night staying below freezing.