A flake's journey down
Why Does It Sometimes Rain and Snow at Once?
Almost all rain starts as snow. What matters is what happens in the last kilometre.
-
Step 1 of 5
Most rain begins as snow.
Cloud tops in temperate places are well below freezing, so precipitation almost always starts as ice crystals, even in summer.
-
Step 2 of 5
The air below decides what lands.
As a flake falls it passes through layers at different temperatures. Whether it melts, half-melts or stays frozen depends entirely on those layers.
-
Step 3 of 5
A thin warm layer half-melts it.
If the warm layer is shallow, the flake only partly melts before dropping back into cold air. What lands is a wet, half-frozen lump — sleet.
-
Step 4 of 5
Refreezing near the ground makes freezing rain.
Sometimes the drop melts completely and then falls into freezing air just above the surface. It stays liquid until it touches something and freezes instantly.
-
Step 5 of 5
Which is why the type can change hour by hour.
The layers shift by a few hundred metres as a front moves through, and the same cloud switches from snow to sleet to rain and back.
Freezing rain is the most dangerous of the four — it coats every wire and branch in solid clear ice.
The short version
Precipitation starts as ice, and whether it lands as snow, sleet, freezing rain or rain depends on the warm layers it falls through.
Try it yourself
On a sleety day, check the forecast's temperature at 1,500 metres. Snow at ground level almost always needs it below freezing up there.
Topics