Sleet, freezing rain, and snow are all forms of winter precipitation, but they form under different atmospheric conditions. Understanding the differences can help you better prepare for winter weather. This article will focus on sleet, explaining what it is and how it differs from other types of winter precipitation, drawing information from NOAA’s National Severe Storms Laboratory.
Sleet Formation Explained
Sleet occurs when snowflakes only partially melt as they fall through a shallow layer of warm air aloft. As these partially melted snowflakes, which resemble slush, continue their descent, they pass through a deep layer of freezing air closer to the ground. This refreezes the slushy drops, and they reach the surface as small, translucent ice pellets. These ice pellets are what we know as sleet, and they often bounce when they hit the ground. The key element is a shallow warm layer aloft and a significant freezing layer near the surface.
Depending on the intensity and duration of the sleet, it can accumulate on the ground, much like snow. This accumulation can create hazardous conditions, similar to those caused by snow or ice.
Sleet vs. Freezing Rain
While both sleet and freezing rain involve frozen precipitation, they differ in how they freeze. Freezing rain forms when snowflakes melt completely while passing through a layer of warm air. The resulting liquid water then falls through a thin layer of freezing air just above the surface. However, this layer isn’t cold enough or the water droplets do not spend enough time in this layer to refreeze the water droplets before reaching the ground. Instead, the supercooled water droplets freeze instantly upon contact with surfaces that are at or below freezing (32 degrees F). This creates a glaze of ice, which can be extremely dangerous. The main difference is that sleet freezes before hitting the ground, while freezing rain freezes upon impact.
Even light accumulations of freezing rain can make travel hazardous, and heavier accumulations can cause significant damage to trees and power lines, sometimes resulting in ice storms.
Sleet vs. Snow
Snow forms when the atmospheric temperature remains at or below 32 degrees F from the cloud base to the ground. In this case, snowflakes, which are collections of ice crystals, do not melt as they fall. Most precipitation in wintertime clouds starts as snow because the upper levels of storms are usually cold enough to create ice crystals. However, if the snowflakes encounter a layer of warmer air, they may melt and become rain, freezing rain, or sleet, depending on the temperature profile of the atmosphere.
Understanding the Weather Dynamics
The type of winter precipitation that occurs depends on the temperature structure of the atmosphere. Sleet requires a specific temperature profile: a layer of warm air aloft to partially melt the snow, followed by a deep layer of cold air near the surface to refreeze the precipitation into ice pellets. Freezing rain needs a warm layer to melt the snow and then only a shallow layer of freezing air near the surface. Snow, on the other hand, requires temperatures to stay at or below freezing all the way to the ground.
Being able to differentiate between sleet, freezing rain, and snow can help you better understand and prepare for the potential hazards associated with winter weather. Sleet can create slippery conditions, while freezing rain can lead to widespread power outages and dangerous travel. Snow, depending on its accumulation, can also cause significant disruptions. By understanding the nuances of each type of precipitation, you can stay informed and safe during winter storms.
