Behind the Forecast: How temperature influences winter precipitation

LOUISVILLE, Ky. (WAVE) - Temperature and moisture content are critical to the types of precipitation we see during the winter. Everything from rain to graupel may fall from the sky.

For ice crystals to form in the atmosphere, relative humidity must be ay 70 percent or higher in what’s known as the snow growth or dendritic zone. In this part of the atmosphere, temperatures sit between 10.4° and 0.4°F. If there’s not enough moisture here, most wintry weather (snow and sleet) won’t occur.

For snow to fall, the atmospheric temperature must be at or below 32° from inside a cloud to the ground.

There are situations where snow can fall when parts of the atmosphere are above freezing. If there is a shallow layer of the atmosphere above the ground where the temperature remains below 33.8°F, then snow can still fall. Snow can still fall when the wet-bulb height is below 1,500 feet. The wet-bulb height is where the evaporatively cooled temperature goes below 32°F.

These situations usually lead to wet snow since the snowflake partially melts.

Sleet happens when the snow melts and refreezes. The melting layer is thin (less than 2,000 feet thick), and the temperature within that layer is usually between 33.8° and 37.4°F.

Freezing rain occurs when a snowflake melts completely, but the ground temperature is below freezing. Once the temperature in the melting layer is above 37.4°F, the snowflake will melt, becoming a raindrop. The layer of below-freezing temperatures near the ground is so shallow that the raindrop does not have time to freeze into sleet. Instead, it freezes on contact with surfaces.

Freezing rain can also occur when there’s not enough moisture in the dendritic zone (snow growth zone).

Graupel are “soft, small pellets formed when supercooled water droplets (at a temperature below 32°F) freeze onto a snow crystal, a process called riming.” Graupel is also called soft hail or snow pellets (it looks like small styrofoam balls). For graupel to occur, snowflakes fall through a layer of supercooled air and water and freeze. Surface air temperatures are typically below 45 degrees. Cloud temperatures are below freezing, however, some parts of the cloud can be below 15°F.

With temperatures and moisture so vital to winter forecasting, it’s important to be weather aware. A few degrees can make a huge difference.