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 Precipitation type and temperature

We’re getting close to Halloween; which means not only are we on the lookout for Vampires and Hobgoblins, but also questions, on How come its 36°F out and it’s snowing?

First, I will mention rain below 32°F:

Most of us know that it can rain even if the thermometer shows a temperature below 32°F. If there is a layer air aloft is above freezing, but the air at the surface is below freezing, As the raindrops approach the ground, they move through that thin layer of cold air and cool to temperatures below 32°F. This phenomenon is called supercooling (or forming "supercooled raindrops").  These supercooled rain drops are called freezing rain, where the rain freezes as soon as it hits something, like the ground, streets, cars, or trees.  

Sleet is a result of the same process, except the layer of freezing air is thicker. So, the raindrops end up freezing before reaching the ground.

Because of this supercooling, drizzle, which is composed of small liquid droplets, can form as liquid and remain unfrozen even when temperatures are continually below freezing.  In these cases, the clouds form as tiny liquid drops, even though the air temperature is below 32°F.  This happens in relatively shallow clouds in which no part of the cloud has temperatures too far below freezing, so the cloud doesn’t contain any snow.  Sometimes the drops grow large enough to become freezing drizzle at the ground, or liquid drizzle if there is warm air below.  Or it might even be crunchy round snow pellets if the air is very cold below the cloud. 

Snow above 32°F:

We see it here all the time when we get into late fall. The air temperature is 38°F, 40F and looking out the window we see it’s snowing! Weren’t we taught in school, that 32°F is the freezing point (of pure water) and that snow/ice melts at 33°F and above? So, how can it be snowing?

To get a snowflake to form, the temp must be 32F or lower. No exceptions. That flake then falls. Usually a few thousand feet or more, from the cloud to the ground. Once formed and falling, it has several thousand feet of air to deal with, before reaching the ground. Almost all precipitation begins as snow, as ice crystals in clouds absorb super-cooled water droplets (small liquid droplets in clouds), and grow big and heavy enough to fall from the cloud. As long as the air temperature is below freezing on the way to the ground, the precipitation will stay in snow form.

Suppose the air was below freezing, all the way down…until about say, 50 feet above the surface. That would mean that the flake only has to fall through 50 feet of warmer air before it reaches the ground.

Moisture falling through air, is not only affected by the air; it also effects the air it moves through So, in that last 50 feet, the air may be beginning to melt the flake, but, the flake…by the process of melting in and of itself, is evaporatively cooling that air! evaporative cooling is a basic thermodynamic principle 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, those first few minutes of flakes never reach the ground, because they are evaporating in to the air and are cooling that air, often several degrees or more.  As I said above the warm layer has to be thin; if it’s too thick the snowflakes will melt into raindrops; but if the layer is thin, the flakes evaporate at first, cooling down that 40°F air to close to 32°F. This allows the flakes behind the ones who melted to make it to the surface.

Yes, Virginia, there can be snow falling by your window, even if it's 41°F.

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