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How does sleet form?

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Quick answer

Sleet forms when snowflakes fall through a thin layer of warm air, partially melting into rain, and then pass through a thick layer of freezing air near the ground. The raindrops refreeze into small ice pellets before reaching the surface, so sleet hits the ground as bouncing ice, not snow or liquid rain.

Sleet forms when snowflakes fall through a shallow layer of above-freezing air, partially melt into rain, then refreeze into ice pellets as they pass through a deeper sub-zero layer near the ground. The result is small, translucent ice pellets that bounce when they hit a surface.

Step by step

  1. Snowflakes form in a cloud where the temperature is below freezing, growing from ice crystals.
  2. The snowflakes fall into a layer of air that is above 0°C, causing them to partially melt into water.
  3. The partially melted flakes continue falling and enter a deeper layer of air that is below 0°C.
  4. In this cold layer, the liquid water refreezes around the remaining ice core, forming a solid ice pellet.
  5. The ice pellet, now called sleet, reaches the ground without melting completely.

Key things to understand

  1. Snow forms high in a cloud where the air is well below freezing, creating ice crystals that grow into snowflakes.
  2. As the snowflakes fall, they enter a layer of air that is above 0°C, causing the edges to melt into liquid water.
  3. The partially melted snowflake continues falling and enters a second, deeper layer of sub-zero air near the ground.
  4. In this cold layer, the liquid water refreezes around the remaining ice core, forming a solid ice pellet.
  5. If the warm layer is deep enough to melt the snowflake completely, it becomes freezing rain instead of sleet.

Labelled diagram

  1. Snow cloud

    High, cold cloud where snowflakes form and start falling.

  2. Warm layer

    Above-freezing air that partially melts the snowflakes into water.

  3. Cold layer

    Deep sub-zero air near the ground where the water refreezes.

  4. Sleet

    Ice pellets reach the surface and bounce on impact.

How sleet forms in the atmosphere

Real-world examples

  • A winter storm in the US Midwest often brings sleet when a warm front lifts over a cold air mass at the surface.
  • In the UK, the word sleet usually means a wet mix of rain and melting snow; the bouncing pellets described here are called ice pellets there.
  • During a freezing rain event in Canada, if the warm layer is shallower, the same setup produces sleet instead.
  • Sleet can accumulate on roads like tiny ball bearings, making driving hazardous even without heavy snow.

Common mistake

✗ Myth: Sleet and freezing rain are the same thing.

✓ Fact: Sleet (ice pellets) freezes in the air and bounces on landing, while freezing rain stays liquid as it falls and freezes only on contact with cold surfaces, forming a glaze.

People also ask

What causes sleet?
Sleet is caused by a temperature profile in the atmosphere where snow falls through a shallow warm layer, partially melts, and then refreezes in a deep cold layer near the ground. The key is that the warm layer is not deep enough to melt the snow completely, so an ice core remains for the water to refreeze around.
How is sleet different from hail?
Sleet forms from snow that partially melts and refreezes during a winter storm, producing small ice pellets. Hail forms in thunderstorms with strong updrafts that carry raindrops high into freezing air, where they freeze and grow into larger, layered ice balls. Hail is usually bigger and occurs in warmer months.
Does sleet mean snow is coming?
Not necessarily. Sleet often occurs during a transition between snow and rain, but it can also fall steadily for hours. It indicates a layer of warm air aloft and cold air at the surface. Whether it turns to snow depends on how the temperature layers change over time.
What is the difference between sleet and freezing rain?
Sleet is ice pellets that form when snow partially melts and then refreezes before hitting the ground. Freezing rain is liquid rain that stays liquid until it hits a surface below 0°C, where it freezes on contact. Sleet bounces; freezing rain coats surfaces in ice.
Why does sleet bounce when it hits the ground?
Sleet is a solid ice pellet, so it behaves like a tiny hailstone or bead of ice. When it strikes a hard surface, the elastic collision causes it to rebound slightly. Rain, being liquid, splashes instead of bouncing.
Can sleet form without snow at the start?
Usually it starts as snow, but not always. If falling raindrops pass through a deep, very cold layer near the ground, some can freeze into ice pellets before landing. A shallow cold layer instead gives freezing rain, which freezes only on contact.
How big can sleet pellets get?
Sleet pellets are typically small, around 1 to 5 millimetres in diameter, roughly the size of a peppercorn. They are smaller than most hailstones because they form from partially melted snowflakes, not from strong updrafts in thunderstorms.

Part of the guide: Water & Weather Phenomena →