What is Sleet? Understanding its Hidden Dangers

When it comes to winter weather, blizzards and black ice are big perils. But what about sleet? It can be just as much of a troublemaker. Sleet doesn’t dent your car or tear up your roof like hail, but it is still dangerous, whether you’re on the road or just walking down the street.

What is Sleet?

Sleet is not the same as hail. Hail forms from thunderstorms towering high into the sky. Sleet forms from low, dark, and cold winter clouds. Sleet starts as liquid rain and freezes on its way to the ground.

Sleet is a Frozen Raindrop

Some pilots and meteorologists use the term “ice pellets,” but sleet is nothing more than a frozen raindrop, which forms in the air at temperatures of 32 °F (0 °C) or less. Sleet can mix with rain, snow, or freezing rain in some situations.

Slick Sleet vs Bouncy Sleet

Sleet may reach the ground partially unfrozen making it “slick sleet.” These pieces can stick together as they fall and make for unusually shaped pebbles of ice. A colder surface temperature or a longer fall through cold enough air before reaching the ground means the sleet may be totally frozen by the time it bounces off of your car’s hood.

How Does Sleet Form?

Snow forms when the air in the clouds is at 32 °F (0 °C) or less; ice crystals grow and fall out as snowflakes. Liquid precipitation forms when the temperature is 33 °F (0.55 °C) or greater. Here’s how that liquid precipitation winds up as sleet or freezing rain.

Sleet and freezing rain don’t come with every winter storm — it takes a special setup in the atmosphere for them to happen. If subfreezing air reaches from the clouds to ground level, you’ll get snow. When the freezing air is closer to the ground with warmer air above you’ll get freezing rain or sleet.

Weather.gov

How You Get Sleet

Sleet happens when liquid rain enters a freezing layer, usually a couple of thousand feet thick, between the cloud base and ground level. The raindrops turn to ice as they fall through the freezing layer. Sleet can fall when it's above freezing at the ground as long as there’s a subfreezing layer above. In this case, the sleet will turn into slush and make for sloppy driveways and sidewalks. Heavy sleet can accumulate several inches deep and paralyze traffic flow.

How You Get Freezing Rain

Freezing rain occurs when the cold layer in the air is very shallow, keeping the rain in liquid form on its way to ground level, where it then freezes on contact. Heavy freezing rain sometimes causes large accumulations of ice, damaging trees and causing widespread power outages. This is called an ice storm and they are always bad news!

Weather Service Advisories and Sleet Safety

The National Weather Service is committed to warning the public of dangerous winter weather. Advance notice for a freezing rain or sleet event may range from a day or just a few hours before, depending on the situation. You’ll want to pay attention to the specifics of any Winter Storm Warning or Advisory.

Winter Storm Warning or Advisory

A Freezing Rain Advisory gives you a heads up to the potential for accumulating ice. Freezing rain is most likely to accumulate on trees, power lines, and bridges. Sometimes freezing rain won’t stick to roads because of the warmer highway surface. An Ice Storm Warning is usually issued if heavy freezing rain is expected.

Winter Weather Advisories and Slush

A Winter Weather Advisory is issued when the primary type of winter precipitation falls as frozen raindrops. Sleet is less likely to cause an ice storm but more likely to create travel headaches as it piles up on roads. If the temperature is right around freezing and you're driving in heavy sleet, the accumulating slush on the roads can be treacherous. Beware!

Play it Safe in Slippery Conditions

If you find yourself in a sleet storm, slow down to a safe speed. Avoid sudden braking or steering changes. Make sure your headlights are on and in the low beam setting. Don’t hesitate to leave the roadway if it gets too icy! Avoid parking on the shoulder if at all possible, turn into a driveway or side road. Turn on your emergency flashers if you are forced to remain on the shoulder. Don’t be afraid to call for help.

The Bottom Line: Don’t Underestimate Sleet

Winter weather conditions can pose numerous challenges, especially when driving, and sleet is no exception. Plan ahead and check the weather multiple times if a potential ice or snow day is looming, as it will greatly reduce your chances of getting “stuck.”

While the NWS reports are crucial for staying safe, a personal weather station — like these from AcuRite — will give you a better feel for temperature conditions in your neighborhood than a far-away airport reading. Since we know it has to be 32 °F (0 °C) or less for sleet or freezing rain to stick, you can use this valuable resource to track the minute-by-minute temperature updates for your local area. Keep in mind that heavy sleet can make a slushy mess even if it’s a few degrees above freezing.

You can also use your home weather station’s rain gauge and liquid equivalent calculations to measure the amount of sleet dropped by a particular storm after it melts. If your gauge reads 2 inches of rain after a storm, you can estimate that you likely received 5 inches of sleet. If it’s below freezing, you’ll have to wait until temperatures rise and the ice in your gauge melts before your rain gauge measures the liquid equivalent.

Have a sleet story to share? Tell us in the comments below!

Steve LaNore is a Certified Broadcast Meteorologist with more than 30 years forecasting and technical experience. He has provided meteorological consulting for everything from insurance adjusters to court cases and is a nine-time award winning author and broadcaster. LaNore has authored two books available on Amazon. He resides in north Texas near beautiful Lake Texoma.

Sources:
https://www.weather.gov/lwx/WarningsDefined
https://www.weather.gov/images/rnk/LSR/RAIPZRSN.png
https://www.weather.gov/images/iwx/events/educational/misc/snowsleetfzrarain.png
https://www.weather.gov/rnk/Measure_SnowSleet
https://www.acurite.com/blog/karis-november-weather-report.html
https://www.acurite.com/shop-all/weather-instruments/shop-all.html