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A thunderstorm, also called an electrical storm or lightning storm, is a form of weather characterized by the presence of lightning and its attendant thunder. Thunderstorms have had a lasting and powerful influence on early civilizations. Romans thought them to be battles waged by Jupiter, who hurled lightning bolts forged by Vulcan. Thunderstorms were associated with the Thunderbirds, held by Native Americans to be a servant of the Great Spirit.

Thunderstorms occur throughout the world, even in the polar regions, with the greatest frequency in tropicalrainforest areas, where they may occur nearly daily. Kampala and Tororo in Uganda have each been mentioned as the most thunderous places on Earth, an accolade which has also been bestowed upon Bogor on Java, Indonesia or Singapore. Thunderstorms are associated with the various monsoon seasons around the globe, and they populate the rainbands of all tropical cyclones. In temperate regions, they are most frequent in spring and summer, although they can occur along or ahead of cold fronts at any time of year. They may also occur within a cooler air mass following the passage of a cold front over a relatively warmer body of water. Thunderstorms are rare in polar regions because of cold surface temperatures.

Some of the most powerful and dangerous thunderstorms occur over the United States, particularly in the Midwest and the southern states. These storms can produce large hail and powerful tornadoes. Thunderstorms are relatively uncommon along much of the West Coast of the United States, but they occur with greater frequency in the inland areas, particularly the Sacramento and San Joaquin Valleys of California. Furthermore, in spring and summer, they occur nearly daily in certain areas of the Rocky Mountains. In the Northeast, storms take on similar characteristics and patterns as the Midwest, only less frequently and severely. Probably the most thunderous region outside of the Tropics is Florida. During the summer, violent thunderstorms are an almost daily occurrence over central and southern parts of the state. In more contemporary times, thunderstorms have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies to explore the visual and scientific aspects of storms and tornadoes.

This image represents the estimated frequency of Lightning Flashes to ground derived from an analysis of lightning detected over Canada and the United States during the hour ending at the time noted above the image. Times are expressed in Universal Time Coordinates (UTC). This image is updated each hour at approximately 25 minutes past the hour.

StrikeStar is a network of Boltek lightning detectors around the United States and Canada.These detectors all send their data to a central server where the StrikeStar software developed by Astrogenic Systems triangulates their data and presents the results in near real-time.

Below are links providing the latest thunderstorm conditions in the Souther Ontario area.

• Southern Ontario Live Lightning Data - provided by Beamsville site
  
• Eastern StrikeStar Lightning Data - provided by Beamsville site and StrikeStar
• WASP2 Regional Lightning Data w/ Nexrad Radar - provided by Beamsville site

Beamsville Weather Station is a non-commercial, privately owned and operated weather station and weather information site. Its purpose is to provide our region with local weather observations along with other publicly available weather information. Beamsville Weather Station also provided lightning information to the SrikeStar network.

StrikeStar is a network of Boltek lightning detectors around the United States and Canada. These detectors all send their data to a central server where the StrikeStar software developed by Astrogenic Systems triangulates their data and presents the results in near real-time.

Life Cycle

Warm air is less concentrated than cool air, so warm air rises within cooler air, similar to hot air balloons. Clouds form as warm air carrying moisture rises within cooler air. As the warm air rises, it cools. The moist water vapour begins to condense. When the moisture condenses, this releases energy that keeps the air warmer than its surroundings, so that it continues to rise. If enough instability is present in the atmosphere, this process will continue long enough for cumulonimbus clouds to form, which support lightning and thunder.

All thunderstorms, regardless of type, go through three stages: the cumulus stage, the mature stage, and the dissipation stage. Depending on the conditions present in the atmosphere, these three stages can take anywhere from 20 minutes to several hours to occur.

Cumulus stage

The first stage of a thunderstorm is the cumulus stage, or developing stage. In this stage, masses of moisture are lifted upwards into the atmosphere. The trigger for this lift can be insolation heating the ground producing thermals, areas where two winds converge forcing air upwards, or where winds blow over terrain of increasing elevation. The moisture rapidly cools into liquid drops of water, which appears as cumulus clouds. As the water vapor condenses into liquid, latent heat is released which warms the air, causing it to become less dense than the surrounding dry air. The air tends to rise in an updraft through the process of convection (hence the term convective precipitation). This creates a low-pressure zone beneath the forming thunderstorm. In a typical thunderstorm, some 5×108 kg of water vapour are lifted, and the amount of energy released when this condenses is about equal to the energy used by a city (US-2002) of 100,000 during a month.

Mature stage

In the mature stage of a thunderstorm, the warmed air continues to rise until it reaches existing air which is warmer, and the air can rise no further. Often this 'cap' is the tropopause. The air is instead forced to spread out, giving the storm a characteristic anvil shape. The resulting cloud is called cumulonimbus incus. The water droplets coalesce into heavy droplets and freeze to become ice particles. As these fall they melt to become rain. If the updraft is strong enough, the droplets are held aloft long enough to be so large that they do not melt completely and fall as hail. While updrafts are still present, the falling rain creates downdrafts as well. The simultaneous presence of both an updraft and downdrafts marks the mature stage of the storm, and during this stage considerable internal turbulence can occur in the storm system, which sometimes manifests as strong winds, severe lightning, and even tornadoes.

Typically, if there is little wind shear, the storm will rapidly enter the dissipating stage and 'rain itself out', but if there is sufficient change in wind speed and/or direction the downdraft will be separated from the updraft, and the storm may become a supercell, and the mature stage can sustain itself for several hours.

In certain cases however, even with little wind shear, if there is enough atmospheric support and instability in place for the thunderstorm to feed on, it may even maintain its mature stage a bit longer than most storms.

Dissipating stage

In the dissipation stage, the thunderstorm is dominated by the downdraft. If atmospheric conditions do not support super cellular development, this stage occurs rather quickly, some 20-30 minutes into the life of the thunderstorm. The downdraft will push down out of the thunderstorm, hit the ground and spread out. The cool air carried to the ground by the downdraft cuts off the inflow of the thunderstorm, the updraft disappears and the thunderstorm will dissipate.

Classification

There are four main types of thunderstorms: single cell, multicell, squall line (also called multicell line) and supercell. Which type forms depends on the instability and relative wind conditions at different layers of the atmosphere ("wind shear").

Single cell

This term technically applies to a single thunderstorm with one main updraft. Within a cluster of thunderstorms, the term "cell" refers to each separate principal updraft.

Thunderstorm cells can and do form in isolation to other cells. Such storms are rarely severe and are a result of local atmospheric instability; hence the term "air mass thunderstorm". These are the typical summer thunderstorm in many temperate locales. They also occur in the cool unstable air which often follows the passage of a cold front from the sea during winter.

While most single cell thunderstorms move, there are some unusual circumstances where they remain stationary. When this happens, catastrophic flooding is possible. In Rapid City, South Dakota, in 1972, an unusual alignment of winds at various levels of the atmosphere combined to produce a continuous, stationary cell which dropped an enormous quantity of rain, resulting in devastating flash flooding. A similar event occurred in Boscastle, England on 16 August 2004.

Multicell cluster

Multi cell storms form as clusters of storms but may then evolve into an organized line or lines of storms. They often arise from convective updrafts in or near mountain ranges and linear weather boundaries, usually strong cold fronts or troughs of low pressure.

Multicell lines

Multicell line storms, commonly referred to as "squall lines", occur when multi cellular storms form in a line rather than clusters. They can be hundreds of miles long, move swiftly, and be preceded by a gust front. Heavy rain, hail, lightning, very strong winds and even isolated tornadoes can occur over a large area in a squall line. Bow echoes can form within squall lines, bringing with them even higher winds.

An unusually powerful type of squall line called a derecho occurs when an intense squall line travels for several hundred miles, often leaving widespread damage over thousands of square miles.

Occasionally, squall lines also form near the outer rain band of tropical cyclones. The squall line is propelled by its own outflow, which reinforces continuous development of updrafts along the leading edge.

This kind of storm is also known as "Wind of the Stony Lake".

Supercell

Supercell storms are large, severe quasi-steady-state storms which feature wind speed and direction that vary with height ("wind shear"), separate downdrafts and updrafts (i.e., precipitation is not falling through the updraft) and a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the cloud (or anvil) can break into through the troposphere and reach into the lower levels of the stratosphere and can be 15 miles (24 km) wide. These storms produce destructive tornadoes, sometimes F3 or higher, extremely large hailstones (4 inch or 10 cm diameter), straight-line winds in excess of 80 mph (130 km/h), and flash floods. In fact, most tornadoes occur from this kind of thunderstorm.

Severe thunderstorm

A severe thunderstorm is a term designated to a thunderstorm that has reached a predetermined level of severity. Often this level is determined by the storm being strong enough to inflict wind or hail damage.

In the United States, a storm is considered severe if winds reach over 93 km/h, hail is 2 cm diameter or larger, or if a funnel cloud or tornadoes are spotted. Though a funnel cloud or tornado indicates the presence of a severe thunderstorm, a tornado warning would then be issued in place of a severe thunderstorm warning.

In Canada, a severe thunderstorm is defined as either having tornadoes, wind gusts of 90 km/h or greater, hail of 2 centimetres in diameter or greater, a rainfall rate greater than 50 millimetres in 1 hour or 75 millimetres in 3 hours.

Severe thunderstorms can occur from any type of thunderstorm, however multicell and squall lines represent the most common forms. Supercells are often the most powerful type of severe thunderstorm.

Back-building thunderstorm

A back-building thunderstorm is a thunderstorm in which new development takes place on the upwind side (usually the west or southwest side in North America), such that the storm seems to remain stationary or propagate in a backward direction. Although the storm often appears to be stationary or even moving upwind on radar, this is actually an illusion. The storm in reality is a multi-cell storm with new, more vigorous, cells being formed on the upwind side replacing older cells which continue to drift downstream.

Ontario "Hot" Spots

Lightning flashes occur in Canada about 2.7 million times a year, including about once every three seconds during the summer months. This is based on observations collected during the past five years from the Canadian lightning detection network. Lightning can be deadly. A lightning bolt can carry up to 100 million volts of electricity - a million times more powerful than household current. Each year in Canada, lightning strikes kill about half a dozen people, seriously injure about 70 others, and ignite some 4,000 forest fires. In a recent study, Environment Canada scientists identified several lightning hot spots across the country.

Between Windsor and Sarnia, east of Lake St. Clair, where lake breezes converge, there is a large amount of lightning activity. A second hot spot is located along a line from the southern tip of Georgian Bay to southeast of Barrie. The two highland areas in southern Ontario, Algonquin Park and the Dundalk Highlands experience lightning much less frequently than the low land areas surrounding them.(The map shows the "flash density" -- the number of flashes per square kilometre per year -- for cloud-to-ground lightning and cloud-to-cloud lightning combined, average data from 1998 to 2002.)

 

Lightning Safety

Electrical conductors including power lines, tall objects, high places and bodies of water attract electricity and lightning. Metal items like golf clubs, fishing pole, and bicycles will conduct electricity so avoid using them when a storm approaches. Never get closer than 30 metres to metal fences and take off shoes that have metal cleats.

Have a lightning safety plan

Find a safe place where you can wait out the storm, preferably in a building or a vehicle. Keep a cell phone for emergencies. When you see lightning or hear thunder take shelter immediately. Maintain an emergency kit for your home or automobile with first aid supplies, battery powered flashlight, radio, food items and blankets, in the event of a major storm.

If you are involved in group activities outdoors ( with children at a summer camp, a golf game with friends etc.) an action plan should be established in advance for all persons involved. Management, event coordinators, organizations, and groups should designate a responsible, person(s) to monitor the weather to initiate the evacuation process when appropriate. Monitoring should begin days and even hours ahead of an event. Safer sites must be identified beforehand, along with a means to route the people to these locations.

Listen to the weather forecast

Be alert for Environment Canada weather warnings announced on radio or television; especially warnings of severe thunderstorms, which can produce lightning, rain, hail, strong winds, and even tornadoes.

Watch for severe weather

Keep an eye on the sky. Dark, heavy clouds in the afternoon of a warm spring or summer day can signal an approaching thunderstorm. Take precautions even if the thunderstorm is not directly overhead. Lightning may strike several kilometres from of the storm. If you see lightning or hear thunder you should take shelter immediately.

Seek shelter

• In a building

  Use a substantial and permanent structure, not a flimsy shelter.

• In a vehicle

  Lightning moves over the outer surface of metal objects, so you are safe inside an all-metal vehicle. But avoid touching anything metal inside, keep your hands in your lap! Don't park near trees or power lines that may collapse during the storm. If a power line falls on or near your vehicle, don't step outside! Smaller equipment (such as riding mowers, golf carts, utility wagons) are not safe and neither are convertibles because it is the outer body that offers protection and not the rubber tires!

• In the open:

  If you can't reach shelter, keep away from tall objects such as trees and towers. Stay clear of high ground and open fields. Seek shelter in low-lying areas such as valleys, ditches and depressions. Keep alert for flash floods in low-lying areas. If you're caught in a level field, crouch down, don't lie flat and put your hands over your ears to protect your hearing. If you are in a group, spread out, keeping several metres apart.

• When Indoors:

  Stay away from windows and doors and keep them closed. Avoid using tap water or the telephone and disconnect electrical appliances. Do not go back outside unless absolutely necessary.

• After the storm:

  Wait about 30 minutes to resume outdoor activities.

Help victims of lightning strikes

Call for immediate medical help! Victims may be suffering from burns or shock. Victims will not carry an electric charge and can be safely handled. If breathing has stopped, administer mouth-to-mouth resuscitation or cardio-pulmonary resuscitation.

Come join us at StormChat IRC room. During the big storms, there is a lot of action, news and updates in the real-time chat. Chat with the experts and amateurs alike.

IRC - StormChat!

Thanks goes to Wikipedia, Environment Canada and The Weather Network for content, information and help.

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