Winter Precipitation Types
Updated: Wednesday, August 14 2013, 10:13 PM EDT
When a winter storm takes aim on the Ohio Valley, chances for sleet, freezing rain, rain, and snow are usually all in play. This is because warm air and cold air are battling it out at the ground and aloft. When air above freezing gets sandwiched between layers of air that are below freezing, different types and amounts of precipitation can fall.
|An example of Live Precision Doppler 12 HD during a winter precipitation event. Green on the radar is where our radar and weather center computers believe rain is falling. Pink colors show where sleet, freezing rain, or a mix of precipitation types is falling, and white colors show where snow is likely falling.|
Because temperatures well above the ground are below freezing, precipitation at these high altitudes tends to be in the form of snow. Because air near the ground is warmer than air well away from the Earth’s surface, snowflakes melt as they fall from the upper-levels of the atmosphere. Depending on the temperature and depth of the layers of air below it, this snowflake miles above the ground can become an ice pellet, a rain drop, a rain drop that freezes, or stay a snowflake when it comes in contact with the ground.
Freezing rain is rain that freezes after contact with a surface having a temperature below 32°F. When rain freezes on road surfaces, ice can lead to potentially dangerous driving conditions. Freezing rain occurs when a snowflake falls into a thick layer of air with a temperature above freezing then a shallow layer of air with a temperature below freezing before reaching the ground. Because the layer of air near the ground with a temperature below freezing is shallow, the snowflake turned rain drop doesn’t have time to freeze in the air.
|An animation showing how snowflakes in the upper-level levels of the atmosphere can either remain snowflakes nearing the ground or be transformed into ice pellets, raindrops, or raindrops that freeze on contact (via UCAR/NOAA)|
Ice pellets, or sleet, are spherical ice balls that typically bound off of the ground on impact. Sleet occurs when a snowflake falls into a relatively shallow layer of air with a temperature above freezing then a thicker layer with a temperature below freezing before reaching the ground. Because the layer of air near the ground with a temperature below freezing is relatively thick, the snowflake turned rain drop has time to freeze in the air.
When a snowflake in the upper-levels of the atmosphere either falls through a column air with a temperature below freezing or falls through a layer or layers of air with a temperature above freezing (usually very shallow), we see snow at the ground. When a snowflake in the upper-levels of the atmosphere falls through a column air with a temperature above freezing, we see rain.
The temperature of the air at various levels of the atmosphere can change dramatically during a winter storm, so it is important that a meteorologist follow these changes while forecasting. Not following these changes can result in a poor snow or ice forecast.