Lightning strikes an Edmonton subdivision in August 2008 (Photographed by Raj Boora)
Lightning and hail can occur in any thunderstorm. Death resulting from hail is rare, but injuries are more common. Hail destroys crops, shreds roof coverings, and damages vehicles. Lightning kills and injures dozens people in North America each year, and is the leading cause of damaging wildfires.
Lightning Lightning is an electrical discharge between positive and negative regions of thunderclouds, or between a thundercloud and the ground. Discharges between clouds and the ground are the most dangerous and damaging form of lightning.
All thunderstorms have the potential to generate lightning. Particles in clouds are in constant motion and they become charged when they interact. Gravity and updrafts work to separate these charged particles. The upper portion of tall clouds becomes positively charged, while the lower levels become negatively charged. This produces enormous electrical potential.
Lightning starts in the part of the cloud with strong electric fields. A streamer moves downward toward the ground. It moves in discrete steps of about 50 metres each. As this nears the ground, a leader is launched from the earth and intercepts the descending streamer before it reaches the ground. Once a connecting path is established, a return stroke flies up the established path at close to the speed of light. This return stroke releases tremendous energy, bright light and thunder. The air near a lightning strike is hotter than the surface of the sun. The rapid heating and cooling of the air results in thunder.
Hail Hail is ice that forms, grows and ultimately falls from thunderclouds. Severe storms can drop enough hail to blanket the ground, flatten crops or clog storm sewers. Up to two percent of crop value is destroyed by hail each year. There have been many storms that led to hundreds of millions of dollars of hail damage to buildings and vehicles.
Raindrops freeze when they are lifted up into the cool air mass. A new layer of ice is added each time gravity pulls the stone down through the cloud then an updraft causes it to rise again. Three elements are critical for hail formation: 1) Large thunderstorms with hot air rising from heated ground. 2) Air temperatures at upper levels of the thundercloud that are much colder. 3) Very strong updrafts within the cloud.
Destructive hailstorms are most common during the late spring and summer months. The largest storms are often in southern Canada and the central regions of the United States. In April, hail is common in Texas and Oklahoma. By June, most hailstorms are found in southern Canada and the north-central region of the United States. The largest hailstone on record fell on Coffeyville, Kansas in 1970. It had a diameter of 14 cm and weighed 766 grams.
