How do hailstorms occur?

Hail is caused by a droplet of water being tossed up and down by winds inside the cloud. The droplet of water begins to gather ice around it within the cloud. It eventually becomes too heavy for the wind in the cloud to keep the ice pellets aloft. The larger the hail, the stronger the storm. In clouds with lower winds, therefore, water droplets don't get tossed around as much, so they don't collect ice as much--which explains why sometimes the storm only produces a small amount of hail.