Most winter precipitation starts as snow high in the cloud (see how raindrops grow). What reaches the ground depends on the temperatures the falling snowflake meets on the way down. Forecasters read these layers from weather-balloon soundings.
Key idea
If the air is below freezing all the way down: snow. If a warm layer (above 0 °C) melts the flakes and the air stays warm to the ground: rain. If the melted drops then fall through a deep cold layer, they refreeze into ice pellets: sleet. If the cold layer near the ground is too shallow to refreeze them, they land as liquid and freeze on contact with cold surfaces: freezing rain.
Why it works
Melting and freezing take time and energy. A flake has to fall through enough warm air, for long enough, to melt completely; a thin or barely warm layer leaves it partly melted. A melted drop has to fall through enough cold air to give up its latent heat and freeze; a thin cold layer lets it reach the ground still liquid but colder than 0 °C ("supercooled"). Supercooled drops freeze the instant they touch a cold road, branch or power line, coating it in a glaze of ice.
| Layers from the cloud down | What lands | What it looks like |
|---|---|---|
| all below 0 °C | snow | flakes |
| warm layer that melts the snow, warm to the ground | rain | liquid drops |
| warm layer aloft, then a deep cold layer (about 1 km or more) | sleet | ice pellets that bounce |
| warm layer aloft, then a shallow cold layer at the ground | freezing rain | liquid that freezes into a glaze of ice |
Worked example: Read the sounding
Snow forms at 3 km. Below it, the air is above freezing (up to +4 °C) from 2.4 km down to 1.2 km, then below freezing from 1.2 km to the ground, reaching −8 °C at the ground. What falls?
- The thick warm layer melts the snowflakes into raindrops.
- The cold layer below is deep (1.2 km) and cold, so the drops refreeze before landing.
- Ice pellets: sleet.
Answer: Sleet
Worked example: A thin cold layer
Same warm layer aloft, but now the air is above freezing down to 200 m, with only −2 °C at the ground. What falls?
- The warm layer melts the snow.
- The cold layer is only 200 m deep and barely below freezing: the drops stay liquid.
- They freeze on contact with the cold ground: freezing rain.
Answer: Freezing rain
Common mistake
Thinking sleet and freezing rain are the same. Sleet freezes in the air and lands as ice pellets that bounce; freezing rain lands as liquid and freezes on contact, building a smooth glaze on everything it touches. Check where the drops freeze: in the air (sleet) or on the ground (freezing rain). The difference comes from the depth of the cold layer near the ground.