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?
Let’s talk first about hail. Hail usually forms in the summer months during a thunderstorm, according to the National Severe Storms Laboratory, and here’s how it happens: Raindrops form in the bottom of clouds during a thunderstorm, and updrafts during a severe storm cause those raindrops to be carried from the bottom of the clouds to the top of the clouds, 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 then 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. 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.
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.
It's all about time of year
So what’s the difference between hail and sleet? About six months. Hail occurs in warm weather, while sleet occurs during cold weather. When the temperature falls below 32 degrees, 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. 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 can’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.
Another type of precipitation we have in the winter is freezing rain, which is similar to sleet. The precipitation comes down as snow, and like sleet, it melts when it hits a warmer layer of air. However, this layer is deeper than in the sleet scenario and doesn't have time to refreeze before hitting the ground. When it does hit the Earth’s surface (and the ground is below freezing), it freezes. Since freezing rain only freezes on contact with the roads and sidewalks, it’s deceptively dangerous, since it only looks like rain that’s coming down, and then freezes on contact.
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.