What Is the Difference Between Hail and Sleet?

These two forms of precipitation happen under very different circumstances.

That's some big hail you've got there. swa182/Shutterstock

Hail and sleet, though somewhat similar, actually occur during completely different weather and require a completely different set of circumstances to take form. So, what’s the difference? First, we'll talk about hail.

How Does Hail Form?

Hail usually forms in the summer months during a thunderstorm. Raindrops form in the bottoms of clouds during a thunderstorm. Updrafts during a severe storm cause those raindrops to be carried from the cloud bottoms to their tops, where the temperature is significantly cooler. This cooled water will freeze on contact with ice crystals, dust, or other matter and will form a tiny piece of hail.

It then falls to the bottom of the cloud only to be carried upwards again by an updraft. It comes into contact with more extremely cooled water, causing another layer to freeze around the hailstone. The hail finally falls to the ground when the updraft weakens or the hailstone becomes too heavy to stay in the cloud and gravity pulls it down to Earth.

Like a tree, a hailstone’s rings bear significance. If you pick up a piece of hail and slice it, you should be able to tell how many times it was carried to the top of a storm by how many layers it has. See the video below for an animated explanation.

Hail can vary in size, from about the size of a pea all the way up to the size of a softball. The largest piece of hail recorded to have fallen in the United States fell in Vivian, South Dakota, on June 23, 2010, with a diameter of 8 inches and a circumference of 18.62 inches. It weighed 1 pound, 15 ounces—yikes!

The world's heaviest hailstone fell in Bangladesh in 1986, weighing 2.25 pounds, where hailstorms frequently arrive in the lead-up to the summer monsoon season. Not surprisingly, hail can cause extreme damage.

Even though Florida ranks as the state with the most thunderstorms, hail is most likely to occur in Nebraska, Wyoming, and Colorado. In fact, the area where the three states meet is known as “Hail Alley,” averaging seven to nine days of hail a year.

How Does Sleet Form?

One major difference to understand between hail and sleet is the time of year. Hail occurs in warm weather, generally during spring, summer, and fall thunderstorms, while sleet occurs only during cold weather.

When the temperature falls below 32 degrees Fahrenheit (zero Celsius), precipitation falls out of a cloud as snow. When that snow falls through a warmer layer of the atmosphere, it will melt slightly and then turn into an ice pellet as it falls through a colder zone, causing it to hit the Earth in the form of sleet.

This has been described as a "warm air sandwich," where precipitation starts out cold high in the atmosphere, passes through warmth, and then refreezes again. It's the same phenomenon that creates freezing rain (more on that below) and it differs from snow, which travels through the atmosphere at a consistently cold temperature.

Unlike hail, sleet is tiny in size and falls only once from the sky. It's quite noisy when it hits your windshield or the ground, but it doesn’t cause the damage that hail can. Sleet that has accumulated on roads and sidewalks can make for hazardous conditions, but it's not the most hazardous form of winter weather. Generally the pellets just bounce off the surfaces they contact. In some cases, sleet can even provide a bit of traction to drivers—though that should not be reason to go faster in poor conditions.

Where Does Freezing Rain Fit In?

Freezing rain is worse than sleet when it comes to safety. This wintry precipitation comes down as snow and, like sleet, melts when it hits a warmer layer of air. However, this warm layer is deeper than in the sleet scenario and the pellet doesn't have time to refreeze before hitting the ground. When it does hit the Earth’s surface—and the ground is less than 32 degrees—it freezes, too. Since freezing rain only freezes upon contact with the roads and sidewalks, it is deceptively dangerous. It looks like rain but turns surfaces into a hazardous skating rink of sorts. You must walk and drive with extreme caution.

So, the next time you hear someone saying it’s hailing on a cold February day, make sure you correct them and explain the difference between sleet and hail. Everyone loves a know-it-all.