What is...Evaporative Cooling?

[Nick L]

As we head further into winter, the term "evaporative cooling" will no doubt make an appearance or two when snow chances arise. It is a process that can see rain "turn into" snow over the course of a few hours. But what exactly is this often mentioned phenomenon? And in what circumstance does it occur?

 

First of all, it's crucial to know the basic thermodynamic principles surrounding evaporation. When liquid water evaporates, it requires energy to change from a liquid to gaseous state, which it draws in the form of sensible heat from its surroundings, thus cooling the air. This is how sweat keeps us cool - when it evaporates, it takes in heat from your skin. Exactly the same happens when snow melts into rain as it falls.

 

So how does this apply to our weather? First of all, you need constant, fairly heavy rainfall, and secondly you need very little wind. Let's say at the start of our scenario that the temperature at the surface is a few degrees above freezing with constant heavy rain and light winds - thoroughly miserable conditions. As the rain falling evaporates, as well as the snow melting at a higher altitude, it cools the air due to the process explained above. However, the lack of wind now comes into play. If there was sufficient wind, the cooler air would mix with warmer air in the vicinity, offsetting the effects of the cooling. If the cooler air can't mix, it just keeps on cooling. If the rain keeps falling at a reasonable rate and winds do not increase, the air can cool enough for the snow to reach the surface.

 

So evaporative cooling is a fairly simple process, but one that can give massive headaches to forecasters, and one that will almost certainly be mentioned over the coming months across the forum

 

I hope this explains it sufficiently, but do point out any glaring errors and ask any questions! 

Edited December 19, 2014 by Nick L

[Backtrack]

Fantastic write up Nick, I certainly didn't know the part about the energy process, from the liquid to the gas. Will certainly help newcomers!

 

Worth adding too, the heavier the precipitation, the quicker the process, lowering the temperature even further. Hail is a supreme example of evaporative cooling at its best!

[Nick L]

Fantastic write up Nick, I certainly didn't know the part about the energy process, from the liquid to the gas. Will certainly help newcomers!

 

Worth adding too, the heavier the precipitation, the quicker the process, lowering the temperature even further. Hail is a supreme example of evaporative cooling at its best!

 

I think with hail it's more to do with the fact it falls at a much faster velocity than rain/snow, so it doesn't have chance to melt fully before it reaches the ground.

 

And thanks

Edited December 19, 2014 by Nick L

[Mark Neal.]

Very handy to know and good insight Nick!

[conor123]

http://www.wetterzentrale.de/pics/archive/2007/avn/Rtavn00220071118.png

 

I remember 18th November 2007 sticking out in particular here, although the snow never really stuck it was notable as upper's never fell below -2 throughout.

 

http://www.wetterzentrale.de/pics/archive/2013/avn/Rtavn00220130211.png

Last Feb was the most recent one here.