Does the weather forecast call for snow or ice? Here's what makes the difference.
Winter storms can bring all sorts of precipitation: snow, sleet, hail, freezing rain or even plain old rain. Why so much variety?
The answer involves temperature changes as the precipitation falls. Different temperatures at different parts of the journey have a major impact on what ends up hitting the ground.
Snow occurs when the atmosphere is "cold" all the way from the clouds to down here at the surface. And rain occurs when it's "warm" all the way from the clouds to the surface.
But it's freezing rain and sleet that are getting quite a bit of attention this week as an icy winter storm takes aim at portions of the central and eastern U.S. Here's what to know:
How do sleet and freezing rain form?
Sleet and freezing rain form because of a "warm-air sandwich" in the atmosphere above our heads (see graphic, below). 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. There is a larger wedge of cold air that refreezes the partially melted snowflakes into ice pellets (aka sleet).
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.
"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."
What about hail?
Though sleet and hail are both frozen precipitation, they form in completely different ways and usually at different times of year.
Sleet forms in winter storms. Hail, however, forms in thunderstorms that are more common in spring, summer, or fall. 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.)
The hailstones grow bigger in the clouds as ice crystals and cloud droplets freeze onto them. They are 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. 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 in Gopalganj district, Bangladesh, in 1986.