Sleet is a type of winter precipitation that forms when raindrops or partially melted snowflakes freeze into small ice pellets before reaching the ground. In the United States, sleet typically occurs when snow falls through a layer of warm air above a layer of freezing or subfreezing air near the surface. The result is precipitation that often bounces when it hits the ground rather than melting into a puddle or sticking like snow.
Although sleet, snow, freezing rain, and hail all involve ice, they form through different atmospheric processes. The temperature of the air at different heights, the path precipitation takes through the atmosphere, and the way ice develops determine which type reaches the ground. Understanding these differences helps explain why winter storms can produce very different conditions over relatively short distances.
What is sleet?
Sleet consists of small, hard ice pellets that fall from clouds and usually bounce or scatter when they strike a solid surface. In the United States, the term generally refers to frozen precipitation that begins as snow, partially melts while passing through warmer air, and refreezes before reaching the ground.
Sleet pellets are usually small, often measuring just a few millimeters across. They may accumulate on roads, sidewalks, roofs, and other surfaces, sometimes forming a layer of loose ice that makes walking and driving hazardous.
The defining feature of sleet is that the precipitation freezes into ice pellets before it reaches the ground. This distinguishes it from freezing rain, which remains liquid as it falls and freezes only when it contacts a sufficiently cold surface.
Sleet is also different from snow. Snow reaches the ground as ice crystals or collections of ice crystals that retain their snowflake structure, while sleet consists of pellets formed when falling snow melts partially or completely and then refreezes.
In everyday American usage, sleet is associated primarily with winter storms. The term can have different meanings in other countries, where it may describe a mixture of rain and snow or other forms of wintry precipitation. Here, sleet refers to the small ice pellets recognized by U.S. weather terminology.
How does sleet form?
Sleet forms when the atmosphere contains distinct layers of air at different temperatures. The most important feature is a relatively warm layer above a colder layer near the ground.
Precipitation often begins high in a cloud, where temperatures are below freezing. Water vapor can deposit directly onto ice crystals, allowing them to grow into snowflakes. These snowflakes then fall toward the surface, passing through the layers of air below the cloud.
Whether the snow remains frozen depends on the temperatures it encounters along the way.
When snow enters a layer of air above 32°F (0°C), it begins to melt. If the warm layer is deep enough and sufficiently warm, the snow may melt completely into raindrops. If the layer is shallower or less warm, the snow may melt only partially.
The precipitation then enters a colder layer of air closer to the ground. If this subfreezing layer is deep and cold enough, the partially melted snow or liquid droplets can refreeze into ice pellets before reaching the surface.
Those pellets are sleet.
The precise outcome depends on more than whether the air temperature is above or below freezing at a single location. The depth of each air layer, its temperature, and the time precipitation spends within it all affect whether snow melts, rain freezes in the air, or liquid water remains unfrozen until it reaches the ground.
This is why a winter storm can produce snow in one area, sleet in another, and freezing rain farther away. The precipitation may originate in the same storm system, but small differences in the atmospheric temperature profile can change what falls from the sky.
Why sleet forms instead of snow or freezing rain
The difference between snow, sleet, and freezing rain often comes down to the depth and temperature of the warm layer above the ground and the thickness of the cold layer beneath it.
If the air remains below freezing throughout most or all of the precipitation’s path, snow can reach the surface without melting. This is the typical outcome when the atmospheric column is sufficiently cold.
If snow passes through a warm layer and melts into rain, the precipitation’s next encounter with cold air becomes crucial. A sufficiently deep, cold layer near the ground gives the liquid droplets time to freeze before they arrive, producing sleet.
If the cold layer near the ground is too shallow or not cold enough to freeze the falling droplets, the precipitation can remain liquid despite having a temperature below freezing. The droplets are then described as supercooled: they remain liquid even though their temperature is below the normal freezing point of water.
When supercooled droplets strike a surface that is at or below freezing, they freeze on contact. This produces freezing rain.
The key distinction is where the water freezes. Sleet freezes into pellets in the air before reaching the ground, while freezing rain generally freezes after contacting cold surfaces.
These processes can occur in sequence during a single winter storm. As warm air moves into an area above a shallow layer of cold air near the surface, snow may change to sleet and then to freezing rain. If the atmospheric structure changes again, freezing rain may return to sleet or snow.
Such transitions are especially important because the different precipitation types create different hazards, even when the air temperature at ground level changes very little.
Sleet vs. snow: What is the difference?
Snow forms when ice crystals grow in clouds and combine into snowflakes. If the air remains cold enough along their descent, the flakes reach the ground without melting.
Sleet, by contrast, forms when falling snow encounters a warm layer that causes melting, followed by a sufficiently cold layer that causes refreezing.
The two forms of precipitation therefore differ in structure and appearance. Snowflakes often have delicate, branching shapes, although their exact forms depend on the temperature and moisture conditions in which they grow. Sleet consists of compact ice pellets that are usually rounded or irregular and lack the intricate structure of snowflakes.
Their behavior after reaching the ground also differs. Snowflakes can accumulate into a soft, porous blanket containing considerable air between the crystals. Sleet tends to form a denser layer of small pellets, which may roll, bounce, or collect in low spots.
Snow can make roads slippery, especially when compacted by traffic or when it melts and refreezes. Sleet can also reduce traction, and its pellets may accumulate on roadways. When sleet mixes with snow or freezing rain, the resulting surface conditions can be particularly difficult to assess.
Neither precipitation type is automatically more dangerous in every situation. The effects depend on accumulation, temperature, road treatment, traffic, and other local conditions.
Snow also plays an important role in the water cycle. A snowpack can store water until it melts, releasing it gradually into streams, reservoirs, and soils. Sleet can contribute to winter snow and ice accumulation as well, but its denser pellets settle differently and may become embedded in snow or form compact layers within it.
Sleet vs. freezing rain: Why one bounces and the other coats surfaces
Sleet and freezing rain can form under similar atmospheric conditions, but the distinction between them is particularly important for winter weather safety.
Both often begin as snow falling through a layer of air warmer than 32°F (0°C). The difference is how the precipitation behaves in the colder air below that warm layer.
In sleet-producing conditions, the cold layer is deep or cold enough for the falling precipitation to refreeze into ice pellets before reaching the surface. When these pellets hit a road, window, or sidewalk, they generally bounce or scatter.
In freezing-rain conditions, the cold layer near the ground is not sufficient to freeze the falling droplets in the air. The precipitation reaches the surface as supercooled liquid water and freezes when it comes into contact with a sufficiently cold object.
Freezing rain can create a smooth, transparent layer of ice known as glaze ice. Because the ice often conforms to the shape of the surface beneath it, it may be difficult to see, especially on dark pavement.
This makes freezing rain particularly dangerous. Even a thin coating can substantially reduce traction on roads and sidewalks. Ice can also accumulate on tree branches and power lines, adding weight that may cause branches to break or lines to fail.
Sleet can also create dangerous travel conditions, but its loose pellets typically produce a rougher surface than glaze ice. They can roll underfoot, collect in wheel tracks, and become compressed into a slippery layer by passing vehicles.
The distinction matters for forecasting, too. A forecast calling for sleet suggests that precipitation is expected to freeze before reaching the ground. A freezing-rain forecast indicates that liquid precipitation is likely to freeze on contact, potentially producing a more continuous coating of ice.
Some storms produce both types of precipitation. If sleet changes to freezing rain as the layer of cold air near the ground becomes shallower, roads that were previously covered with loose pellets may develop a smooth, hazardous ice coating.
Sleet vs. hail: Two kinds of ice with different origins
Sleet and hail both fall as solid ice, but their formation processes are fundamentally different.
Sleet is associated with winter precipitation and forms when falling snow melts and then refreezes in a cold layer of air. Its formation depends on the vertical temperature structure of the atmosphere.
Hail forms inside strong thunderstorms, where powerful rising currents of air, called updrafts, carry water droplets into very cold parts of a cloud. The droplets freeze onto ice particles, allowing hailstones to grow as additional layers of ice accumulate.
Some hailstones develop through repeated cycles of movement within a storm, encountering supercooled water droplets that freeze onto their surfaces. Hailstones can become large when strong updrafts support them long enough for substantial growth.
Unlike sleet, hail does not require snow to melt in a warm layer and refreeze in cold air near the ground. It develops within the storm cloud through the interaction of ice, liquid water, and strong air currents.
The two types of precipitation also differ in typical size and potential impact. Sleet pellets are generally small. Hailstones can range from small pellets to large chunks of ice capable of damaging vehicles, roofs, crops, and other property.
Hail is most closely associated with thunderstorms, particularly strong thunderstorms that contain vigorous updrafts. It can occur even when the ground-level weather is warm. Sleet, on the other hand, is generally associated with cold-season storms and subfreezing air near the surface.
Although small hail and sleet can sometimes look similar, their shapes alone do not always provide a reliable way to identify them. The surrounding weather conditions and the processes that produced the ice are more informative.
What atmospheric conditions favor sleet?
Sleet is most likely when a storm brings moisture into a region where the atmosphere has a layer of above-freezing air over colder air near the surface.
This arrangement often develops when warmer air moves over a shallow mass of cold air. Because cold air is denser than warm air, it can remain near the ground while warmer air flows above it. This arrangement is common in some winter storms, especially where cold air becomes trapped near the surface.
The boundary between the warm and cold layers is not necessarily horizontal or uniform. Winds, terrain, and the movement of the storm can alter the temperature profile over short distances.
For example, one town may remain cold enough throughout the lower atmosphere for snow to reach the ground. A nearby town may have a warm layer aloft and a deep enough subfreezing layer near the surface to produce sleet. Farther along the storm’s path, the surface cold layer may become shallow enough for freezing rain to develop.
These differences can occur even when locations are experiencing the same broad storm system.
The temperature at the ground alone cannot determine which type of precipitation will fall. A surface temperature of 30°F (−1°C), for example, does not by itself tell a forecaster whether precipitation will arrive as snow, sleet, freezing rain, or a mixture.
Forecasters examine the temperature and moisture structure of the atmosphere at multiple heights. Weather balloons, radar observations, surface measurements, and computer weather models help them assess how precipitation is likely to evolve as it falls.
The depth of the warm layer is especially important. If snow passes through a warm layer for only a short time, it may melt partially and then refreeze in the cold air below. If it remains in sufficiently warm air for longer, it may melt completely, increasing the likelihood of rain or freezing rain depending on conditions near the surface.
The cold layer’s temperature and depth then determine whether that liquid water freezes before it reaches the ground or remains supercooled until contact.
How sleet affects roads, sidewalks, and buildings
Sleet can create hazardous conditions even when it does not produce the dramatic accumulation associated with a major snowstorm or an ice storm.
The pellets can accumulate on roads and sidewalks, reducing traction for vehicles, bicycles, and pedestrians. Their small, loose structure means they may roll or shift beneath tires and footwear, making surfaces unstable.
When vehicles travel over accumulated sleet, they can compress the pellets into a denser layer. If temperatures remain below freezing, that layer may persist. If temperatures rise above freezing, the pellets can melt into water, which may later refreeze when temperatures fall.
The combination of sleet, melting, and refreezing can produce uneven or deceptively slippery surfaces. Drivers may encounter changing traction over short distances, particularly where precipitation mixes with snow or freezing rain.
Sleet can also accumulate on roofs, steps, and outdoor equipment. Its effects depend on how much falls, how long temperatures remain below freezing, and whether other forms of precipitation accompany it.
Unlike freezing rain, sleet generally does not immediately form a smooth coating on every surface it touches. Its pellets tend to remain distinct at first, although they can become compacted or freeze together under suitable conditions.
For transportation, the distinction between sleet and freezing rain is useful but should not create a false sense of security. Both can make roads dangerous, and a surface that appears merely wet or lightly covered may offer very little traction.
People should be particularly cautious when temperatures are near freezing, because small changes in temperature can cause precipitation to melt, refreeze, or change type during a storm.
How meteorologists identify sleet
Meteorologists distinguish sleet from other forms of precipitation by examining observations at the ground and conditions throughout the atmosphere.
At the surface, sleet is identified by the presence of small ice pellets that bounce or scatter on impact. Freezing rain appears as liquid droplets that freeze on contact with cold surfaces, while snow arrives as flakes or clusters of ice crystals.
However, identifying the precipitation type is not always straightforward. Sleet can mix with snow or rain, and pellets may melt or become coated with liquid water as conditions change. An observer may encounter a mixture of precipitation types within a short period.
Radar can help meteorologists track precipitation and identify features associated with melting snow, but radar alone does not always reveal precisely what is reaching the ground. The same radar signal can be consistent with different precipitation types, depending on the atmospheric conditions and the characteristics of the particles.
Weather balloons provide measurements of temperature, humidity, and pressure at different heights. These observations help reveal whether the atmosphere contains a warm layer above subfreezing air and whether that lower cold layer is deep enough to support sleet formation.
Computer weather models use atmospheric observations and physical equations to estimate how temperature profiles and precipitation will evolve. Forecasters combine model guidance with radar, surface reports, and other evidence to predict where rain, freezing rain, sleet, and snow are likely to occur.
Because precipitation type can change over small distances, forecasts may describe a transition zone rather than a sharp boundary. The exact location of that boundary can shift as a storm develops or its path changes.
This uncertainty is especially important when a storm is expected to produce freezing rain or a mixture of ice and snow. A modest error in the predicted temperature profile can change the forecast precipitation type and the hazards people should expect.
Why the differences matter
Sleet, snow, freezing rain, and hail all involve ice, but they represent different pathways through the atmosphere.
Snow generally reaches the ground as ice crystals that have not melted. Sleet forms when falling snow melts partially or completely and then refreezes into pellets before reaching the surface. Freezing rain remains liquid until it strikes a cold surface, where it freezes. Hail grows inside thunderstorms as ice particles collect additional frozen or supercooled water.
These differences explain why the same storm can produce distinct hazards in different locations. Snow may reduce visibility and cover roads, sleet may create loose or compacted layers of slippery pellets, freezing rain may coat surfaces with glaze ice, and hail may cause impact damage.
The most useful distinction is not simply how the precipitation looks, but how it formed and where the water froze. For sleet, the decisive step occurs in the air: liquid water or partially melted snow refreezes before reaching the ground.
That process connects a familiar winter weather phenomenon to the larger workings of the atmosphere, where temperature, moisture, and the movement of air determine whether precipitation arrives as snowflakes, ice pellets, liquid droplets, or hailstones.
