Before we get immersed in the causes and effects of thunderstorms we must learn one term which applies to most of them. “Derecho” is a Spanish word meaning “direction”. It applies to a wide-spread thunderstorm moving in a straight line, which Meteorologists use to forecast upcoming severe thunderstorms over a specific area. Canadian Media sources do not like that term because they might be called upon to explain why a Spanish word is being used to describe Canadian thunderstorms. Here we are simply given “severe thunderstorm forecasts”. In the southern US-Media however, the term is used extensively. I will leave the scientific explanation to the scientists who are experts in that field.
During July 12th to 15th, 1995 four derecho thunderstorms hit part of the USA and the Great Lakes during an intense and deadly heat wave over the Midwest.
Thunderstorms are certainly not infrequent across all of Canada. The Southern Georgian Bay Area is having its share almost daily this year during the month of July. Their formation is aided by the presence of the Blue Mountains to the west of Georgian Bay. A part of their formation is the water cycle. Warm, moist air from Lake Huron is lifted upward by the Blue Mountains into higher, colder air layers where condensation occurs, causing rain to fall on the lee side of the mountains. Cold air contains ice crystals. Warm air contains water droplets. During thunderstorms crystals and drops bump together, which builds up static charges in the clouds that must be discharged, causing lightning flashes and thunder.
Rain almost always accompanies thunder storms, as does wind, at times severe, with intense cloudbursts. Thunderstorms generally cause rapid and very welcome drops of air temperatures, but also increase the humidity in the air. Hail may also occur in different sizes. The heaviest recorded Canadian hailstone fell at Cedoux, Saskatchewan, in 1973, weighing in at 290 g, five times as much as a tennis ball.
In the Rocky Mountains typical mountain thunder storms are born similarly. Warm air rises above the summits, cooling as it ascends. Water droplets form and clouds appear. More condensation forms as the clouds rise. Some droplets freeze and then grow quickly by absorbing water from still liquid water droplets. The growing heavier ice particles begin to fall downward, gaining mass rapidly by collision with smaller droplets in their path. When the falling “graupel” droplets collide with tiny ice crystals, electrical charges occur in the cloud. When the cloud reaches sufficient depth, precipitation and lightning occurs in the foothills and beyond.
The map shows how far the effects of derecho storms can reach across the United States. It also shows how thunderstorm warnings are derived from these patterns and then broadcast by the media as “severe thunderstorm warnings”.
How far away is the lightning?
As in fireworks, the light comes first, then the bang. Light travels at 300,000km per second. Sound is much slower. It travels only 332m per second. When you see the lightning, count the time until you hear the thunder. It is rare to be able to hear thunder if you are located 25 km or more from the lightning strike.
Are you afraid of lightning?
If you, your children or your pets are afraid of thunderstorms, look for shelter in a basement or elsewhere, away from direct outside sound and light. Indoors is always the safest place during thunderstorms. Wait at least 30 minutes after the last thunder to leave the shelter.
How many people are killed in Canada by lightning strikes?
Each year on average in Canada, two or three people die and 180 are injured by lightning. Most lightning strikes happen during outdoor recreational activities, such as camping, hiking, walking or boating. Outdoor workers and soccer and baseball players are also at danger. Lightning generally hits the tallest object. Taking shelter under trees is another dangerous thing to do during thunderstorms. If you look at a tree hit by lightning you will quickly realize that standing near it puts you in extreme danger during a lightning storm.
(Note: the choice of photographs is the sole responsibility of the author under the Fair Dealing Provision of the Canadian Copyright Laws).