Thunderstorms are transient, sometimes violent storms of thunder and lightning, often accompanied by rain, and sometimes hail or tornados. Thunderstorms can occur singly, in clusters, or in lines. Lightning, which causes thunder, is a major threat in a thunderstorm. Each year, lightning kills 75 to 100 people in the US. Thunderstorms are classified as severe if the storm produces a tornado, if it has winds that are 58 mph or higher, or if it produces hail of at least 3/4 inch in diameter.
Hail is precipitation of ice pellets when there are strong rising air currents. Hail forms in cumulonimbus clouds. Raindrops go through a process called accretion, when the updraft forces the raindrop up into the cloud and then it freezes it and then it starts to fall in the cloud again and then the updraft carries it back up into the cloud where it is made bigger (this gives the stone another ring of ice). Then continues the cycle until the hailstone gets too heavy for the updraft to carry it up into the cloud. It will then fall to the ground and may cause damage.
A hailstone can be as small as a pea or as large as a grapefruit. There are layers/rings in a hailstone. The number of layers tells you how many times it was sent up in an updraft. Updrafts winds can be from 100 to 120 mph.