Monthly Archives: February 2017

How unusual is freezing precipitation in Colorado?

9:35 AM, Thursday, February 2, 2017

The local media has asked me how unusual the recent freezing precipitation is. I thought I’d share with the blog readers.

First let meĀ  answer the question: freezing rain is quite rare in Colorado, but freezing drizzle is a rather regular visitor. What we had recently was freezing drizzle. Now you’re really confused, right?

 

What is freezing drizzle?

Drizzle is not light rain, it’s small rain. It’s the small, misty droplets that don’t make a sound when they hit the ground. It forms in low stratus clouds that don’t reach high into the atmosphere. Cloud droplets slowly grow large enough to fall to the ground as mist and drizzle. When it’s below freezing those drizzle droplets are supercooled and freeze on contact with the surface. If the cloud grows tall the top of the cloud would be cold enough to form ice crystals and those crystals then help convert the cloud droplets to snowflakes.

 

How is freezing rain different?

Rain is a larger droplet than drizzle and forms in deeper cloud layers. Most of our rain (even in summer) starts as hail or snow in the cloud and melts on its way to the ground. Freezing rain occurs when hail or snow melt into rain on the way to the ground, but then the rain enters a below-freezing layer of air near the ground and becomes supercooled. Freezing rain typically requires that a warm & moist oceanic air mass ride up and over the cold low-level air, something that rarely happens in Colorado. Freezing rain is responsible for truly devastating ice storms that we sometimes get in the central and eastern U.S. I’m not aware of any really severe ice storm along the Front Range.

 

What do we get in Colorado?

Freezing rain is rare in Colorado. But freezing drizzle, although we don’t get it every winter, is a fairly dependable occurrence.

Maybe some patchy fog and freezing drizzle tonight

3:00 PM, Wednesday, February 1, 2017

A shallow cold air mass is banked up against the lower foothills and it will probably be Saturday before it’s pushed east. During this afternoon locations such as Nederland and Idaho Springs basked in the 40s while Denver and Boulder were 20 degrees colder with low stratus. The cold air didn’t fully get over the Palmer Divide, so Colorado Springs is not quite as cold as places to the north.

 

During the night and into Thursday there may be some areas of fog, light freezing drizzle, and possible some light snow grains in and along the lower foothills mainly north of the Palmer Divide. Amounts should be quite light, but it doesn’t take much freezing drizzle to make for some icy surfaces. The overcast may thin a bit by Friday, but it will likely be Saturday before westerly winds bring a fresher air mass.