Large hail - photo: NOAA Photo Library, NOAA Central Library; OAR/ERL/NSSL Hail is formed when updrafts in thunderclouds carry raindrops upward into extremely cold areas of the atmosphere, where they freeze and merge into lumps of ice. When the lumps become too heavy to be supported by the updraft, they fall to the ground at speeds of up to 100 km/h or more. Hailstones as large as grapefruit have been reported in Canada.

Hail can be extremely dangerous and can cause extensive damage in only a few minutes. To avoid or minimize personal injury or property damage:

Move vulnerable items such as cars, equipment and machinery to shelter when a storm threatens, provided such action does not pose a personal risk.
If caught outside, reduce chances of injury to yourself by crouching down and protecting your head and neck as much as possible.
Protect pets & livestock. Many are injured each year by hail. Ensure that they have shelter available.

While hail may or may not precede a tornado, large hail often appears near the area within a thunderstorm where tornadoes are most likely to form. Once large hail starts to fall, it is safer to assume that a tornado could be nearby and to seek appropriate shelter. Once the hail has stopped, remain in a protected area until the storm has passed, usually 15 to 30 minutes after the hail stops.