How Is Sleet Made?

Sleet, when combined with freezing rain, can form a slippery coating.

Sleet is a form of precipitation composed of small balls of ice, smaller than hailstones. These frozen raindrops usually bounce when they strike the ground, and rarely freeze to the ground unless they fall along with freezing rain. Sleet does not form often, and requires particular weather conditions to occur. Some areas of the United States and southeastern Canada suffer from serious sleet storms.

Conditions

Sleet forms when a warm front meets an area of cold, dry air in the wintertime, sandwiching rain in between. These conditions are normally associated with extratropical cyclones, which are low-pressure areas between warm and cool air masses and are the source of most winter storms. These conditions often pass quickly, leading to freezing rain or mixed rain and snow.

Formation

Sleet forms when rain produced in the warm layer of air falls into the cold air closer to the ground, or when snowflakes fall through the warm layer, melt, and refreeze near the ground. Sleet forms more readily when the cold air is very dry. If the cold layer is not deep enough to freeze the water before it hits the ground, the result is freezing rain.

Misconceptions

Many people refer to the phenomenon of mixed rain and snow as sleet. However, this is not strictly accurate. The U.S. National Weather Service considers sleet to refer only to falling ice pellets. Mixed rain and snow may be referred to as a wintry mix or as wintry showers. Sleet may be part of these weather conditions, since the atmosphere during a winter storm rarely remains the same for long.

Hail

Sleet is very similar to hail, but is much smaller and can form without the presence of a thunderstorm. Unlike sleet, hail requires an updraft that keeps the ice balls from falling until they become so large they can no longer be supported by the air. Sleet balls are smaller because they form without the presence of an updraft.

Forecasting

The special environmental conditions required to produce sleet make it particularly difficult to forecast. It can be difficult for meteorologists to determine whether the meeting of a warm and cold front will create freezing rain, ordinary rain or sleet. It may also be difficult to tell what impact the sleet will have on the ground, as it may form a layer of loose pellets, a sheet of ice or a layer of cold slush.

References

• University of Illinois: Sleet
• Weather Questions: What causes ice pellets (sleet)?
• National Severe Storms Laboratory: Hail basics
• National Oceanic and Atmospheric Administration: All About Mixed Winter Precipitation