- Here’s an explanation of how freezing rain and sleet form.
- Sleet forms in winter storms, while hail is a warm-season type of precipitation.
- Snow occurs when the atmosphere is “cold” all the way from the clouds to down here at the surface.
In addition to snow, winter storms can sometimes unleash a surprising amount of sleet and freezing rain. But how do these different types of precipitation form? And what about hail?
While all forms of winter precipitation start as snow high up in the clouds, there are four primary types that eventually reach the ground: snow, sleet, freezing rain and plain old rain.
Snow occurs when the atmosphere is “cold” all the way from the clouds to down here at the surface.
A ‘warm-air sandwich’Â
Sleet and freezing rain form because of a “warm-air sandwich” in the atmosphere above our heads. Precipitation starts as snow in the cold layer at the top, then melts to rain as it falls through the warm layer, then refreezes into sleet or freezing rain as it falls through the cold layer near the surface.
For sleet to occur, the warm air layer is rather thin. A thicker wedge of cold air beneath the warm air refreezes the partially melted snow into ice pellets.
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For freezing rain to occur, the warm air layer is thicker. The snow melts into rain then refreezes just as it hits the cold ground.
Since the rain is not freezing until it reaches the surface, it still falls like regular rain and therefore looks and feels the same until it freezes on the ground.
Sleet is made up of ice pellets that bounce off objects. Even though this may sound more hazardous than freezing rain, that’s not the case.
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“Freezing rain is by far the most dangerous because it forms a solid sheet of ice, as opposed to sleet that just has small ice pellets that quickly bounce off of the surface,” AccuWeather meteorologist Brett Anderson said. “Actually, sleet can even provide a little bit of traction for drivers, as opposed to the obvious dangers of a solid sheet of ice that forms from freezing rain.”
Finally, when the air is warm enough all the way down to the surface, it’s just plain rain that reaches down here.
Sleet vs. hail
While sleet and hail are both forms of frozen precipitation, they form in completely different ways and usually at different times of year. Sleet forms in winter storms
Hail, however, forms in spring, summer or fall thunderstorms. First, soft, snow-like particles form in subfreezing air at the top of a thunderstorm. (Yes, even in the middle of summer, the tops of thunderstorms are below freezing).
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The hailstones grow bigger in the clouds as ice crystals and cloud droplets freeze onto them. They are held suspended in the clouds by strong winds that push up into the storm.
Finally, once the hailstones grow too heavy, gravity causes them to fall to the Earth. Hail is typically small, often the size of a penny, but can grow to monstrous sizes. The heaviest hailstone ever recorded was 2.25 pounds and fell in Gopalganj district, Bangladesh, in 1986.
