Brochures

Thunder, lightning and hail storms

• Lightning
• Thunder
• Hail
• Thunderstorm detection at Environment Canada
• Weather watches and warnings
• What to do when:

The thunder cloud looms on the horizon like a warship, long, broad and dense, with an anvil-shaped prow. It skims closer, blackens the entire sky and hurls down a salvo of heavy rain, lightning, thunder and sometimes hail and even tornadoes.

Violent winds often accompany the thunder cloud and wreak havoc. Gusts of 50 to 80 km/h are common and can often exceed 100 km/h.

Sudden cloudbursts can cause flash floods in creeks, ravines and streets, and can wash out or submerge culverts and bridges. As much as 5 cm of rain may fall in less than half an hour.

Although thunder and lightning are occasionally spotted during a snowstorm, April to October are the prime thunderstorm months; late afternoon and just before sunrise are the prime thunderstorm times.

A thunderstorm develops in an unstable atmosphere. Warm, moist air near the earth's surface rises rapidly and cools. The moisture then condenses to form ice crystals, rain droplets and voluminous thunder clouds, called "cumulonimbus clouds". (Many mechanisms can trigger strong updrafts: for instance, heat from the sun and ground, a colder air mass, or a hill, mountain or other obstacle.)

About 44 000 thunderstorms happen every day worldwide on average and produce 100 lightning flashes per second.

Storms can last for hours but usually disperse in less than one hour, perhaps to form again elsewhere. Storms generally move from the west at 35 to 50 km/h. Frequently, the more severe ones are found in broken lines or bands up to 80 km in width.

Lightning

During a thunderstorm, the air is charged with electricity. Sometimes it's even enough to make your hair stand on end!

The most striking sign of the electricity is lightning. Lightning is electricity--a megavolt electric discharge, like the static shock you get when you rub your feet on the carpet and then touch something or someone. In fact, if you see a static shock in the dark, it looks like a tiny lightning bolt.

Thunder clouds build up electric charges, which are polarized and attracted to their opposites, just like a magnet.

The bottoms of thunder clouds carry negative charges and the tops carry positive charges. Influenced by the charged clouds, the ground also becomes positively charged.

When the electric build-up gets too great, the lightning jumps from the negatively charged cloud base and strikes a positively charged object--usually another cloud. However, about one-third of lightning bolts strike the ground.

Streaking at 40 000 km per second, lightning usually appears as a single main bolt with forks. In fact, the main bolt is usually a whole series of lightning strikes taking the same path, too quickly for the eye to distinguish between them.

The air can smell scorched by lightning. As a searing bolt breaks up molecules of oxygen, nitrogen and water vapour, these combine to form new, highly pungent substances, particularly ozone, ammonia and nitrogen acids.

It's not true that lightning never strikes the same place twice. The Empire State Building in New York has been struck as frequently as 15 times in 15 minutes.

Each year in Canada, lightning kills an average of 16 people and causes more than 20 per cent of all forest fires (40 per cent in British Columbia.) Lightning also starts about 2000 fires annually on private property (about 2.5 per cent of all recorded fires.) You have a one in 350 000 chance of being killed by lightning; your chances of being killed in a car accident are 50 times greater.

Thunder

Thunder is the sound of the lightning moving rapidly and explosively through the atmosphere. The air expands and then compresses violently, producing sound waves.

The waves are heard as a single thunder crash when the lightning bolt is straight and short. When the bolt is long and forked, there is a succession of crashes and rumbles.

To judge how close lightning is, count the seconds between the flash and the thunder clap. Each second represents about 300 m.

Hail

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 high speeds.

Hail can be extremely dangerous, particularly when accompanied by strong winds. On July 28, 1981, a single 15-minute hail storm in Calgary caused over $100 million in damage.

Thunderstorm detection at Environment Canada

Weather Radar

Weather radar is a prime tool of Environment Canada in detecting heavy precipitation, rapidly developing cloud systems, and other signatures of severe thunderstorms. Weather radars cover the areas inhabited by 95 per cent of Canada's population. Each radar scans a radius of about 325 km, although the best range for detecting thunder clouds is 120 to 150 km.

Just as a searchlight beam picks out and reflects off an object, weather radar sends out microwave pulses that bounce off rain, hail and snow. The radar measures the pulses reflected back by precipitation and relays the patterns to a video screen for a meteorologist to interpret.

Conventional weather radar can show the type, amount and rate of precipitation. A relatively new radar, known as Doppler radar, can also measure the speed at which precipitation is moving in the area the radar covers, as well as wind shifts, gust fronts and cyclonic patterns.

Satellites

Day and night, satellites send us pictures of cloud formations over large areas of the globe. These pictures can show what is happening out of range of surface weather observation stations and radar sites. Thunderstorms are usually less than 80 km across, and some might not show up on a radar screen, but can appear in satellite pictures. Combined with radar data and conventional observations, satellite imagery can give a complete picture of the life-cycle of a thunderstorm.

Volunteer weather watchers

All across the country, thousands of specially trained volunteers watch the skies. When they spot thunderstorms, tornadoes or other severe weather, they report it to Environment Canada, which in turn broadcasts a warning.

Severe thunderstorms, despite the havoc they wreak, are small-scale and short-lived as weather phenomena go. Often it is difficult for a forecaster to get enough information to know how large a particular storm will be. Therefore, volunteers' timely and accurate observations of severe weather are an invaluable supplement to Environment Canada's own full observation network.

Weather watches and warnings

Environment Canada is closely monitoring weather systems so as to ensure your safety.

If a severe storm is brewing, the weather service will issue a "SEVERE WEATHER WATCH." Be on the lookout and keep listening for updates on your radio or television.

If a storm is imminent or confirmed in your area, Environment Canada will issue a "SEVERE WEATHER WARNING." Take safety precautions until the danger passes.

A "SEVERE THUNDERSTORM WARNING" means a thunderstorm has been detected or is imminent. Be prepared to take cover at a moment's notice and listen for updated forecasts and warnings.

Remember: a WATCH means there is a potential for severe weather; be on the lookout. A WARNING means severe weather is occurring; take precautions.

What to do when:

• You hear a severe thunderstorm watch for your area (this means that conditions are favourable for the development of severe thunderstorms.) You should secure or put away loose objects such as outdoor furniture, put your car in the garage, and bring livestock and pets to shelter.

  Prepare and maintain emergency packs containing food, clothing, blankets, a first aid kit and medication, a battery-powered radio with fresh batteries, a flashlight, lantern and tools for emergency repair jobs. Keep one pack in your home and another in your car.

• You hear a severe thunderstorm warning (this means that severe thunderstorms are highly probable or are occurring.) Keep calm and take shelter. If boating or swimming, head for land and shelter immediately.

  Note that lightning may strike several kilometres away from the parent cloud. Take precautions even if the thunderstorm is not directly overhead.

• You are caught outdoors in a thunderstorm. Go inside if you can. If not, move, if possible, to a canyon, or under a cliff.

  Stay clear of high ground and open spaces, including golf courses and sports fields. Also keep away from isolated trees, and avoid the edge of a forest or woods. Low trees located well within a forest, but not close to tall trees, are less dangerous than trees in open country.

  Stay away from objects that conduct electricity, such as tractors, golf carts, golf clubs, motorcycles, lawn-mowers and bicycles. Swinging a golf club or holding an umbrella or fishing rod are particularly dangerous activities. Take off shoes with metal cleats. Never get closer than 30 m to any wire fence, because you might receive a jolt from a lightning discharge that has hit a section of the fence as far as a kilometre away.

  During a severe storm, don't travel. If caught in your car, open the windows slightly and park off the road away from power lines. Set the brakes. Be wary of downed power lines and don't try to step outside the car if they are near or touching the car.

• You are indoors during a thunderstorm. It serves no purpose to close the doors and windows, except to keep the rain out. Lightning can come through a brick wall as easily as through an open window.

  Radio aerials, electrical lines, telephone lines and water pipes are excellent conductors of electricity and lightning. If properly installed and grounded, they are not hazardous, and there is no need to disconnect plugs and aerials. But don't tempt fate. Just postpone that hot bath, telephone call or last bit of ironing.

• Someone is struck by lightning. Victims of lightning receive an electrical shock but do not carry an electric charge and can be safely handled. They may be suffering from burns or shock and should receive medical attention immediately. If breathing has stopped, administer mouth-to-mouth resuscitation. If breathing and pulse are absent, perform cardio-pulmonary resuscitation.

This document is published by Authority of the Minister of the Environment. Copyright Minister of Supply and Services Canada 1995. Catalogue Number En57-24/22-1995E. ISSN 0715-0040. ISBN 0-662-22989-4.

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