Sleet is smaller and occurs during the winter, whereas hail is larger and happens during warmer months, according to AccuWeather. Both sleet and hail are frozen precipitation, but the two major differences revolve around size and timing. Hail is larger because strong updrafts in thunderstorms keep hail aloft longer. Sleet occurs when snow melts in the cloud and then freezes again in a lower layer to create small pellets.
Hail occurs when an object, such as small dust particles, come in contact with very cold water droplets. A small ice pellet forms in the cloud. The pellet stays aloft until the warm updrafts in a thunderstorm no longer support the weight of the ice pellet. Hail can stay in a cloud for five to 10 minutes before falling, depending on how strong the winds are in a thunderstorm.
Sleet happens in winter time among cold-air clouds. Most precipitation in cold weather starts out as ice in the atmosphere. When ice or snow drops, the precipitation may find a warmer layer in the cloud that melts the ice. Before sleet hits the ground, water droplets refreeze to form round pellets. Sleet falls once through a cloud, unlike hail stones that may go up and down several times before falling to Earth.