Had a lot of that lately. Two questions:
- How exactly does sleet so quickly turn into a solid sheet of slippery ice?
- What exact conditions make precipitation fall as sleet (vs rain/snow/hail)?
If the water vapor in the cloud condenses on some impurity in the air and falls without freezing, that’s rain.
If the vapor freezes in the cloud before forming into a drop, it’s snow. If it forms a drop, then freezes, that’s sleet. If that sleet gets suspended in the cloud by the wind so that layer after layer of water condenses on it then freezes, that’s hail.
If any of the frozen things melt before hitting the ground, that’s also rain.
Mainly when the ground is still warm, or the sleet is mixed with rain, or some is melting on the way down. When it’s cold enough sleet is like little needle-y bits of snow. Just a little warmer it quickly packs to form that solid ice sheet.
Also, according to some, sleet and frozen rain are not the same thing though they may appear to be the same. IIRC the difference is about freezing up in the clouds or on the way down. I don’t really know if there’s any significant difference.
“Sleet” means different things in different parts of the world, by the way. In (parts of?) Europe it means the same thing as what in the US has been dubbed “wintry mix.” Whereas in the US it is a type of frozen precipitation.
Sleet is not the same as freezing rain. Sleet is generally snow at a higher elevation that then starts melting before reaching the ground. Freezing rain happens when there are warmer temperatures at elevation, so it falls as rain and then freezes upon reaching the colder ground.
Here’s a simple graphic that shows the differences.
More details here: Winter Wx (weather.gov)
Sleet and snow are just occasionally annoying and inconvenient. Freezing rain is the devil’s own weather (conventional associations with heat being besides the point). Damn stuff lands wet, goes splat, and freezes. The drop that comes along behind it is going splat against a layer of frozen ice. So at any given split-second everything is covered in flat shiny ice with a thin layer of water on it, and it’s miserable to be out in (because you’re gettting RAINED on and it is freezing ONTO you) and you can’t walk on anything or drive on anything (because it’s slippery as buttered transmission fluid with goosegrease). And it’s bone-biting cold, subjectively feels colder than a Colorado mountain pass at -17°. Utterly miserable stuff.
Yes, we had a discussion about that just a year ago:
And then there’s graupel, which is snowflakes that accumulate extra ice crystals on the way down, beoming fluffy ice pellets.
Less fluffy than snow but much fluffier than sleet.
Yeah, my formative years around winter weather were in Europe. I call “wintry mix” sleet, and only learned about the difference in usage around a decade ago. I wish the US would use a different term for the falling ice bits.
Thanks for clarifying that, I was beginning to wonder what the heck was wrong with my understanding of what sleet is.
Well, if it’s any comfort, U.S. meteorology professionals don’t use the word “sleet” in any official machine-readable report format (which use numeric codes for reporting precipitation type in a observation).
NOAA uses World Meteorology Organization code tables which never uses the word “sleet” to decode any precipitation type. The two phenomena discussed here are called “rain and snow or ice pellets” (code 23) for the one type of sleet, and “ice pellets” (code 79) for the others. (Plus some other codes for variations in intensity and content of snow, ice, and liquid rain.)
“Sleet” does not appear in the code table, almost certainly because of the ambiguity in customary usage.
https://www.nodc.noaa.gov/archive/arc0021/0002199/1.1/data/0-data/HTML/WMO-CODE/WMO4677.HTM
Living in an area that is prone to all of them, the practical difference that sleet and rain don’t cause problems with traction. Sleet acts as pebbles, so the roughness doesn’t slip – it’s only slightly more slippery than wet pavement.
Snow is more slippery. The flakes slide more easily so you have to be more careful.
Freezing rain is a bitch. It turns everything into a skating rink and driving is extremely dangerous until it’s melted. It also often causes black ice – a clear coating of ice on the the road that you can’t differentiate from wet pavement. It may be in certain patches, surrounded by wet pavement so you find yourself skating suddenly.
We had a lot of sleet yesterday in the DC area. Sidewalks were covered, buy easy to walk on. I was walking on a dead end street, so I decided to go onto the road. It turned out to be black ice, so I slid. I made the quick decision just to keep sliding until I hit a small pile of snow. Dunno why I didn’t figure that out until my late fifties.
Sleet is just hail that never had the makings of a varsity athlete.
I was in my 30s when I figured out that if I started to lose my balance on ice or snow, I needed to squat down as far as I could, and lean forward a little. I might end up sliding pretty far, but I wouldn’t take a tumble.
I don’t think I heard the term “wintry mix” until fairly recently (within the last two decades at least). ‘Sleet’ was always one of those words I saw in print more often than I heard anyone actually use it, and I wondered what it was as a kid. Its lack of popular use (compared to ‘snow’, ‘hail’, and ‘rain’) and uncertainty of meaning may be why ‘wintry mix’ has come about here.
Rain, hail, snow, all easy to identify. When little frozen bits fall from the sky they could have started as raindrops and frozen on the way down, or snow that melted and refroze on the way down, or little bits of ice forming in the clouds to small to be hail and too dense to be snow, or any of the options thawing and freezing multiple times on the way down from the clouds.
If it’s not rain or snow or hail I’ll probably just call it sleet.
It’s also possible for falling water to be super-cooled - that is, it’s below the freezing temperature of water but it’s still liquid because… well, physics I probably can’t explain well but such conditions are possible.
As soon as super-cooled water hits something it instantly turns to ice.
This can also account for an ice coating/sheet.
To further complicate things, super-cooled water can co-exist with conditions that produce sleet, freezing rain, and snow. So definitely it can be part of a “wintry mix”.
Same. But it is fairly obvious what it means: a mix of frozen and unfrozen water.
Growing up, i distinguished among:
Snow – flakes, of varying types with other names, mostly depending on the temp.
Slush – snow mixed with cold rain, or snow that has partially melted as it fell
Sleet – bits of ice with cold water
Dry sleet, or ice – bits of ice without any melted parts, which was quite uncommon.
Hail – a mythical frozen thing I’d read about, that was a larger ball of ice with layers
Freezing rain – mostly water that froze when it landed
For what it’s worth, any rain heavier than a light drizzle actually forms as ice and melts on the way down. It just takes way too long to form large drops of liquid water (longer than it takes to precipitate).