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Name: Jessie and Patricia
Status: student
Grade: 4-5
Location: GA
Country: N/A
Date: 11/16/2005

What is supercooled rain?

Dear Jessie and Patricia-

Supercooled rain is liquid precipitation at temperatures below freezing. It is a very common occurrence throughout the year, although the rain warms to the air temperature as it nears the ground.

Here is the official definition from the glossary of meteorology from the American Meteorological Society...

supercooled rain—Liquid precipitation at temperatures below freezing. At midlatitudes, supercooled rain often forms first as an ice crystal or snow in the clouds, which then melts as it falls through an elevated layer of air warmer than freezing before reaching the thick bottom layer of cold air that cools the drop below freezing. A supercooled drop freezes instantly on contact with surfaces such as electrical power lines, trees, and roads during an ice storm. See glaze.

Here is a good link that explains how supercooled rain forms and what happens when it hits the ground or surface.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

Jessie and Patricia,

This may be a little hard to follow, but I will try my best to make it clear.

Liquids have a property called vapor tension that greatly control how easy or difficult it is for drops to grow or evaporate and for liquid to change state (from water to ice, for instance). On a curved surface like a water droplet, the vapor tension is greater than on a flat surface.

Clouds form when the air becomes saturated (100% relative humidity) with water vapor and the water vapor is drawn to minute particles to form water droplets. When the air is saturated, the vapor tension in the air (from water vapor) is the same as the vapor tension of a flat water surface, like a puddle. However, water droplets in the cloud can have a much higher vapor tension than a puddle (the smaller the water droplet, the greater the vapor tension), because they are round. For the droplets to grow larger or to freeze requires that there be even more water vapor in the air than when it is saturated; this is called super-saturation and therefore is normal in clouds.

One way that you can raise the relative humidity (and therefore the water vapor tension) of the air is to reduce the temperature (super-cool). The temperature must be reduced significantly (10 to 30 degrees Centigrade below freezing) to increase the relative humidity (or, in other words, to raise the water vapor tension of the air to an amount that is greater than the vapor tension of the drop) to the point that the drop will freeze.

Clouds with smaller droplets require lower temperatures for freezing of the drops to occur. In winter it is common to have smaller droplets in clouds since there is generally less water vapor in the air than in summer. This makes winter super-cooled clouds very dangerous for aircraft flying through them. Disturbed super-cooled water droplets freeze very rapidly when they hit a surface, such as an airplane wing, resulting in rapid ice build-up, a hazardous situation for an aircraft. So-called "freezing rain" (super-cooled rain) usually results from super-cooled droplets that fall from the clouds and freeze upon hitting the ground, your car, or your sidewalk.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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