What causes snow, sleet, and freezing rain?

If you’re from some God-forsaken outpost in Siberia, pretty much the only wintry precipitation you ever experience is snow. But, if you’re from the Southern US like me, your winters are often sprinkled with a few helpings of several different types of water falling from the sky. Why is that? What causes snow sleet and freezing rain? Let’s find out!

First things first

First of all, let’s start with some definitions, ’cause I’ve gotten into some pretty heated debates with people over what terms apply to what stuff falling from the sky.

  • Snow – We all know what snow is. The white fluffy stuff that comes in flakes.
  • Freezing Rain – This is the stuff that’s a liquid while it’s falling, but turns into a glaze of ice all over everything after it makes impact with the ground. It is not pellets of ice, as lots of people will try to tell you.
  • Sleet – This is little pellets of ice that turn into pellets of ice before they hit the ground. Sleet is not light snow or a snow/rain mix… unless you’re from England. It’s also not hail (more on that later).
Freezing Rain

The aftermath of some pretty serious freezing rain – note the thick glaze of ice over the branches.
(Image credit: Nicolas Perrault III on English Wikipedia.)

How does it form?

Whether water comes out of the sky in the form of rain, freezing rain, sleet, or snow all depends on the temperature of the air – and, more importantly, the temperature of various air pockets that often sit on top of each other… As the National Weather Service explains:

  • Snow – Snow forms when you have cold air from ground level all the way up to the clouds. Frozen water molecules in the clouds accumulate into snowflakes, then start falling to the ground. After a while, things start to pile up.
  • Sleet – Sometimes, snowfall doesn’t go quite that neatly. Suppose a big pocket of warm air comes between the cold air in the clouds and the cold air on the ground. Snow will form in the clouds, melt in the pocket of warm air on its way down, and then re-freeze in the cold air near the ground. By the time it hits the ground, it’s just a frozen water droplet.
  • Freezing Rain – Suppose you had a pocket of warm air (like in the sleet example) that started getting pretty close to ground level. Snow would form in the clouds, melt in the pocket of warm air, hit the cold air near the ground… but run out of time to re-freeze before impact. This causes freezing rain – the precipitation is still melted when it gets to the ground, but it quickly starts freezing up on whatever it lands on.
  • Plain Old Rain – You can get plain old rain when you’ve got warm air in the clouds and the water never freezes… or when that pocket of warm air underneath the clouds stretches all the way to the ground. In that scenario, snow will form, melt, and never re-freeze.

So what about hail?

Like sleet, hail is ice pellets that fall from the sky. But it’s slightly different, and it forms in a very different process. As we’ve already seen, sleet forms when water droplets freeze on their way down to earth – it’s one drop that freezes solid. Hail forms inside thunderstorms, freezes way up high, and has layers like an onion. In a hail storm, droplets of water get blown around by all the air inside a thundercloud. As they move around, they encounter cold air (which makes them freeze), then moist air (which adds another layer of water on the outside), then cold air (which freezes them again). This goes on for a while, until they fall down. Scientists are still debating exactly what the process is, but the results can be pretty terrifying…

Well, that’s all for now. Stay safe this winter!

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