If a snowflake falls through a warmer part of the cloud it can get coated with water, then frozen again as it’s tossed back into a colder part. It can go round and round, adding more and more layers of new ice. When it’s too heavy to stay up, what finally comes down is hail. If the updrafts in a thunder cloud are strong enough, the hail stones can get pretty big before they become too heavy to stay up. http://scijinks.gov/rain