Sublimation?

Most people know what evaporation is, when a liquid transfers into a gas, such as wet roads after a rain storm when the sun shines and you can see steam rising from the surface.  When temperatures are cold, generally below freezing, snow will also behave in the same matter as liquid water, but this process is referred to as sublimation and not evaporation.  Sublimation is the transfer of a solid into a gas, in this case ice turning into steam directly before melting.  Unfortunately sublimation does not provide the ground with any beneficial moisture since all the moisture is released back into the atmosphere.  A good time to catch this phenomenon visually is in the early morning as the sun rises providing just enough heat, while surface temperatures are well below freezing.  Here is an image I took Sunday morning when temperatures were hovering around 20 degrees at the surface and the sunlight was just hitting some frost on the fence allowing for sublimation.  In order to confirm it was sublimation and not evaporation was that the fence never became wet or even looked like there was any type of moisture on it once all the frost sublimated into the air.

Another process that can create sublimation is wind.  Cold, windy days with below freezing temperatures where snow does not melt by warmth, but visually dissipates is usually caused by sublimation as well.  The tell tale sign this has occurred will be that the top layer of snow is smooth and crunchy with dry, crumbling snow below and the ground is still hard and dry beneath.  This indicates that the moisture from the snow has sublimated into the atmosphere and did not melt into the ground below.  Sublimation is a process that is not beneficial for providing drought relief, although, fortunately it does not occur that often since Colorado needs all the moisture it can get!    

 

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