drizzle at 14*?

[hemi_newman]

John,

The NWS is calling for drizzle tonight,but what gets me is that its 14* outside and spose to get down to 7* here in NW WI. Shoudnt it be snow not drizzle?But it can snow at 33-35*?Im confused. Could you shed some light on this?

 

[mvedepo]

John correct me if I am wrong but it sounds like a temperature inversion. Warmer air above the colder air. Just like when you encounter freezing rain/drizzle/sleet in an airplane you actually climb into the inversion to get into above freezing air to limit ice accumulation on the aircraft.

 

[snowshoe]

I read the NWS forecast discussions everyday and what they describe is that the moisture is very shallow and doesn't reach the heights required to get ice crystal growth.

 

[mvedepo]

Drizzle is nothing more than small raindrops. It is usually associated with low level clouds. Fog is often associated with a drizzle event. Rain droplets get their size by updrafts in the clouds coase the water droplets to collide. So the more vertical development the more up drafts and generally results in larger rain drops. Since drizzle is such small droplets it indicates little lifting action also indicated by the low clouds and often poor visibility. So you are correct when you say its a low level event.

In winter a surface layer of very cold air can exist with a much warmer layer of air over it. Drizzle can form in this warm air as described above and eventually fall into the freezing layer. It is very possible for it to hit the surface prior to freezing. Freezing drizzle can happen when the surface temperatures are as low 10 degrees F.

 

[lenny]

I have no idea why but it was indeed drizzling at mid night last night

 

[jd]

hemi_newman-

All the answers provided were correct. The precip formed in a layer of the atmosphere where temps were not cold enough to get it to form as an ice crystal. I will not get into the micro physics of things, but just know that pure water (that which is found in the water vapor in the atmosphere) needs to be much cooler than 32 degrees F to go from the vapor to an ice crystal. That is unless it has what we call a freezing nuclei present and then it can go from vapor to a crystal at temps just below freezing.

So in this case, the vapor condensed into a water droplet and then fell. There was a layer of cold air at the surface, but the layer was not thick enough for the droplet to freeze before hitting the surface (sleet). So you end up with freezing drizzle. The same process works for freezing rain, only the droplets are bigger.

The reason it can snow with surface temps above freezing is the same, but opposite. Meaning the air aloft is cold enough for a snowflake to form, the flake falls to the ground. The air at the surface is above freezing, but that layer of above freezing air is not thick enough to melt the snowflake and thus it does not melt before reaching the surface.

-John

 

[hemi_newman]

thanks guys and john for clearing this up for me