Hail forms during warm weather while sleet forms in cold weather

Hail forms via freezing within a cloud, while sleet forms via freezing as precipitation falls

Hail forms within cumulonimbus clouds while sleet forms within cirrus clouds

Hail is the term for frozen raindrops larger than 0.5 millimeters, while sleet is the term for frozen raindrops less than 0.5 millimeters