Winter Storms: Freezing Rain vs Sleet Quiz

Weather balloons provide a vertical "slice" of the atmosphere, showing exactly where the freezing line is at different altitudes. By tracking how the warm layer is growing or shrinking, meteorologists can determine if a city will see harmless snow, messy sleet, or a destructive and icy glaze of freezing rain.

Normally, the atmosphere cools as altitude increases. However, freezing rain and sleet require a "warm nose" or a layer of air above freezing sandwiched between two freezing layers. This inversion allows frozen precipitation to melt and then encounter sub-freezing air again near the surface, creating dangerous winter weather conditions.

Sleet begins as snow but passes through a warm layer thick enough to melt it into a liquid drop. Crucially, it must then pass through a deep enough layer of sub-freezing air near the surface to refreeze into a solid ice pellet before impact. This resulting ice bounces when it hits the ground.

In freezing rain, the snowflake melts completely in a deep warm layer. As it falls into a shallow cold layer at the surface, it becomes "supercooled." It remains liquid even though the temperature is below freezing, only turning to ice the moment it makes contact with cold objects like trees or power lines.

The transition between snow, sleet, and freezing rain is very delicate. If the cold layer at the bottom is deep, the water has time to refreeze into sleet. If the cold layer is shallow, it remains a liquid drop until impact, resulting in freezing rain. Surface temperature also dictates if the ice will stick.

These conditions are typical of "overrunning," where warm, moist air is forced up and over a wedge of cold, dense air at the surface. This setup is frequently found ahead of an approaching warm front. The vertical stacking of warm air over cold air provides the exact temperature profile needed for these precipitation types.

Sleet usually bounces off surfaces and accumulates like sand. Freezing rain, however, coats everything in a heavy, clear glaze of ice. This added weight can snap tree branches and bring down power lines, and it creates extremely hazardous, frictionless driving conditions that are much more difficult to manage than sleet.

Supercooled water is the essential component of freezing rain. Because the water droplets are pure and lack "freezing nuclei" while falling through a shallow cold layer, they stay liquid. They are in a state of unstable equilibrium, ready to turn into solid ice instantly upon physical agitation or contact with a surface.

Sleet is easily identified because it consists of small, hard grains of ice. Unlike snow, which is an aggregate of ice crystals with air trapped inside, sleet is a solid pellet that results from a frozen raindrop. Its solid nature causes it to make a distinct "tapping" sound when hitting windows or pavement.

This is often called "cold air damming." Cold air is more dense than warm air, so it acts like a heavy fluid that settles into valleys or hugs the ground. When a warm front arrives, the lighter warm air slides over the top of the cold air rather than pushing it away immediately.

For snow to reach the surface, there can be no significant warm layer aloft. If any part of the atmosphere is significantly above freezing, the snowflake will begin to melt. Snow requires a consistent sub-freezing column of air to maintain the crystal structure that defines a snowflake until it reaches our level.

Freezing is an exothermic process, meaning it releases heat into the surrounding environment. When freezing rain turns to ice on a tree branch, a tiny amount of latent heat is released. While not enough to stop the storm, this energy transfer is a fundamental part of the thermodynamics of the Earth's water cycle.

Ice storms usually need a "source" of cold air, often a high-pressure system in Canada, and a moisture source like the Gulf. The front acts as the boundary where the warm air is lifted over the cold air. This combination provides the moisture and the temperature inversion necessary for long-duration freezing rain.

In freezing rain, the warm layer (the warm nose) is deep enough to ensure the snowflake melts completely into a raindrop. For sleet, the warm layer is often thinner or less intense, which may only partially melt the flake or result in a drop that has a better chance of refreezing before impact.

Sleet is a winter phenomenon caused by the melting and refreezing of precipitation in a layered atmosphere. Hail is a summertime phenomenon caused by strong thunderstorm updrafts that toss water droplets into the freezing upper atmosphere. While both are ice, their formation processes and synoptic conditions are completely different.