It is 22 degrees F outside. Why is it sleeting and not snowing?
In: 20
The temperature of the air isn’t the temperature of the precipitation. The rain you see is falling down from higher altitudes where it apparently isn’t the same temperature as near the ground.
Rain often occurs as air cools, reducing the amount of water vapor it can hold. So we can expect there is a warmer mass of air overhead that is cooling, dropping out liquid water that only has time to partially freeze into sleet on the way down.
Because it depends on where the water formed into ice. If the water formed into ice in the upper atmosphere under calm conditions it’s snow. If it initially comes down as rain and freezes on it’s way down it’s sleet.
Snow is formed in the clouds when the temperature up there is cold enough. Sleet starts as rain in the clouds and freezes on the way down when it’s above freezing in the clouds and well below freezing below them.
Freezing rain is similar to sleet, except it stays liquid all the way down and only freezes once it hits the ground or other objects that are below freezing.
Because the air 2,000 feet above you is above freezing. Cold air is dense and heavy, it settles down to ground level, displacing warmer air above. It is very likely the precipitation is falling originally as snow, hitting this warm layer and melting, then the rain drop re freezes into a tiny ball of ice before reaching the ground.
It may be 22F on the ground, but it doesn’t follow that it’s the same temperature higher up — it can be warmer or cooler up there. You have warmer rain falling into cold air where it cools and some of it turns to ice on the way down. This is very dangerous because once it hits cold surfaces on the ground, it tends to freeze.
In the “standard” atmosphere, temperatures decrease height.
However, when frontal systems move through, you can wind up with layers in the atmosphere that are warmer higher up than the ones below. Depending on the thickness and height of these warmer layers, precipitation can freeze and melt (possibly several times) before finally reaching the ground.
This can result in clumpy “snowball” snow, freezing rain, sleet, or graupel (snow-filled balls of ice) depending on the vertical temperature profile of the atmosphere.
The air is warmer high in the sky, so it’s raining up there, and the rain gets partially frozen as it drops down through the colder air.
A [picture](http://wikipedia.org/wiki/Precipitation_types#/media/File%3APrecipitation_by_type.png) is probably the best way to show what’s going on..
This is why meteorological balloons are launched all over, among many other reasons.
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