Sleet and hail do look similar – both are a form of frozen rain, yet are not snow – but are technically separate categories of precipitation. (Another similar form of precipitation is the more dangerous freezing rain.) The difference is that hail forms in thunderstorm updrafts and falls to the ground after freezing, whereas sleet initially forms as snow, but then melts and re-freezes on its way to the ground.
Sleet – and here I use the American term for falling pellets of ice, not the British and Canadian term for mixed rain and snow – forms when snow in a cold upper layer falls as precipitation, but passes through a layer of warm air perhaps a mile above the ground. As it passes through this layer, the snow melts into rain. However, if the warm layer is narrow rather than extending to the ground, it will re-freeze before it reaches the surface, becoming hard pellets. (If the warm layer is too close to the ground, the rain does not have time to re-freeze and therefore becomes freezing rain instead.)
When it reaches the surface, this sleet can look somewhat like hail – it appears as ice pellets which bounce off the ground and, if the temperature is cool enough, can accumulate somewhat like snow, requiring shovelling and road clearing.
Hail itself, however, forms through a quite different process. According to the National Oceanographic and Atmospheric Administration (NOAA), hail “occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice.” Essentially, supercooled raindrops accumulate together and are blown upwards in the updrafts which characterize violent thunderclouds. As they rise, they cool further and eventually freeze. The larger the storm, the higher these updrafts can blow the growing hailstones, and therefore the larger they will grow.
However, this accumulation and freezing process means that the ice particles, now called hailstones, become increasingly heavy. Eventually, they will grow too heavy to be sustained by the updraft, and will fall to the Earth due to gravity. This is not necessarily the end of the journey: more, stronger updrafts may yet blow the hailstone up again, so that more layers of water freeze onto its surface. By the time these hailstones reach the Earth’s surface, however, they have ultimately reached their final frozen form and grown heavy enough to fall out of the updrafts in that particular thunderstorm.
Hail is likely to melt quickly as it is more likely than sleet to fall in an area where the temperature at the surface is not actually cold enough to sustain frozen water.