LITTLE ROCK, Ark. – Most often during the winter, cold air overlaps moisture resulting in a few different kinds of wintry precipitation. However, the kind of precipitation that occurs is all dependent on the temperature layers it falls through within a column of air. Let’s break it down further, shall we?
Snow is a form of frozen precipitation. More specifically, it’s a collection of ice crystals that cling to each other as they fall to the ground. In order to see snowfall, there must be a column of air that is at or below 32 degrees Fahrenheit that the precipitation falls through from the cloud all the way down to the surface.
Sleet is essentially snow that has melted and re-frozen. From the cloud, the precipitation falls in the form of snowflakes, passing through a layer of freezing air. Further down in the column of air, though, it passes through a shallow layer of warm air and partially melts. Before the precipitation reaches the surface, it re-freezes as it passes through another layer of freezing air. By the time sleet reaches the ground, it resembles small ice pellets and bounces on impact.
Freezing rain may sound like it’s frozen rain, but that’s not the case. Instead, its name more so represents what happens when it comes in contact with objects at the surface level of the atmosphere.
Freezing rain actually begins as snowflakes falling from a cloud. However, the snow will pass through a deep layer of warm air on its way to the ground, which melts the snow back to regular rain drops. Right above the ground, though, it will pass through a shallow layer of freezing air. As the rain passes through the cold layer before reaching the ground, it will become “supercooled.” What that means, is that the liquid rain has cooled below its freezing point, but it hasn’t solidified or crystalized yet. When the supercooled rain comes in contact with objects at the surface that have a temperature of at least 32 degrees, it will freeze immediately upon contact.
Freezing rain creates a glaze of ice on the ground, power lines, trees and other objects. If the accumulation is significant, it can result in power outages, dangerous driving conditions and other issues. If there are hours of freezing rain, the National Weather Service will define it as an ice storm.
Rain can either fall completely as rain through the atmosphere if the column of air only contains air at a temperatures above freezing, or it can start as snow and then make the transition to rain. If the latter is the case, the snow falling from the cloud into a shallow layer of freezing air will soon pass through a deep layer of warm air to the surface – resulting in the snow melting completely.
Graupel forms differently than the other types of winter precipitation and is a little less seen as well. Graupel is essentially a snowflake that passes through supercooled water vapor on its way to the ground. The supercooled vapor freezes to the snowflake, and as a result, a spongey bead forms. Graupel, like sleet, bounces upon contact but is softer than sleet.