Freezing rain vs Sleet??
Sunday
November 16, 2014
During the winter we experience many different kinds of precipitation… that is, the water that falls from the clouds can do so in many ways. It can be liquid (rain) or it can be frozen (snow or sleet) or it can be something in between (freezing rain). The purpose of this post is to clarify these different types of winter weather, and how they form.
First off… all types start as ice crystals (aka snow). Very high up in the clouds it is almost always cold enough for everything to start as snow.
Snow… when the air is below freezing all the way between the clouds and the ground, the ice crystals stay as snow for their entire fall to the ground. This results in us observing snow on the ground.
Sleet… sleet is similar to snow, but forms differently. In order for sleet to form, there must be a small layer of warm air between the clouds and the ground. This layer of warm air melts the falling snow, changing it briefly to rain. When the rain drop finishes falling through the layer of warm air, it starts to refreeze back into a snowflake. However, there is not enough time left for it to complete this process and we end up with partially formed snowflakes that we call sleet.
Freezing rain… freezing rain happens when the layer of warm air between the clouds and the ground is very thick. The ice crystals in the clouds melt to rain as they fall. When they come out of the bottom of the warm layer, there is not enough time for any refreezing, they hit the ground as liquid rain. This liquid rain then freezes AFTER it is on the ground, causing a glaze of ice. This can result in widespread black ice.
Because the layer of cold air at the surface must be thin enough for the rain to remain liquid until it gets to the surface, and because minor amounts can have large impacts, freezing rain can be difficult to forecast.
Pure rain occurs when there is no cold air at the surface, and the liquid water never refreezes even after it has hit the ground.
What about hail??
Although people often confuse hail and freezing rain, the two types of weather form completely differently. In order to have hail you must first have a very strong thunderstorm. Thunderstorms create winds that blow upward (updrafts) that allow them to form very tall cumulonimbus clouds. Hail forms when ice crystals in the cloud get sucked into one of these updrafts. The ice is blown and tumbled throughout the cloud many times, and as it does it gains new layers of ice and grows larger. Eventually the ball of ice gets too heavy for the updrafts to keep in the cloud, and it falls to the ground as hail. Larger hailstones are created by stronger updrafts in strong thunderstorms. Small hail stones are created by weaker updrafts in weaker thunderstorms.
While it isn't impossible, it is very rare for New York State to experience hail in the winter. Usually it forms in the spring and summer months during strong thunderstorms.
The National Weather Service provides a glossary of weather terms, and is a great resource if you ever hear a weather term for the first time.
http://w1.weather.gov/glossary/
-Ethan
