Sleet and hail have different properties, though they may look similar. They require different circumstances, weather conditions and seasons in order to be produced. Another category that should be compared to sleet and hail is freezing rain, which again has its own special conditions and characteristics. All three involve precipitation in the clouds, but each requires just the right set of circumstances to occur.
One difference between sleet and hail is the time of year in which each occurs. Typically, hail is most often a summer occurrence, while sleet occurs mostly in winter. You’re much more likely to see hail in warm and oppressive weather where a sudden thunderstorm occurs. In fact, you need cumulus clouds like thunderheads in order to produce hail.
Hail begins as raindrops at the bottom of clouds; "bottom" means those parts of the clouds that are closest to the earth’s surface. The action of updrafts sends raindrops toward the top of thunderheads, where the temperature is cool enough to cause the raindrops to freeze. The now frozen raindrops start to fall, but updrafts cause hail to swing up to the colder top of the clouds several times. Each time this occurs, hail accumulates more water from the lower clouds, which is then frozen to create layers. If it happens often enough, you might see huge pieces of hail fall to earth. More typically it goes through a few cycles and looks like pea-sized pieces when it hits the ground.
Sleet and hail may look similar, but the formation of sleet is quite different, and explains why sleet is more often a winter phenomenon. Clouds that might produce sleet are warmer than the air below, and don’t have significant updrafts. In fact when sleet falls from clouds it is still rain.
The temperature of the air, as rain falls from the clouds is cold enough to freeze it on the way down. Therefore sleet doesn’t freeze in upper cumulus, but instead freezes in the air. Sleet won’t be large since it doesn’t accumulate extra layers of frozen water. Instead, when it falls it looks quite small, like tiny glittering diamonds. It usually melts quickly, because the air on the ground is warmer than air a few feet up.
A phenomenon separate from sleet and hail is freezing rain. This is also typically a winter occurrence, but it requires freezing temperatures on the ground instead of in midair where hail freezes. Freezing rain tends to remain rain until it hits the ground, where it instantly hardens and creates a sheet of ice on the ground. Of the three types, freezing rain is often more dangerous to motorists than is sleet and hail. Due to low ground temperature, ice layers on the ground may remain for several hours to several days, creating slippery roads and very unsafe driving conditions.