The Thunderstorm—Awesome King of the Clouds
Probably the most impressive and awesome of clouds are the ones that can “talk.” Their name is cumulonimbus, or thunderhead. Dark and foreboding, these clouds might reach ten miles [16 km] or even higher into the sky, and they are the ones that bring thunderstorms. Storm clouds can sparkle with lightning and growl out warning peals of thunder as they build up in the sky. At night they may put on a dazzling sound-and-light show that surpasses any man-made fireworks display. They blow and shed their rain and hail and then move on, leaving behind the smell of fresh, clean rain, often on a previously parched earth. We can’t change the weather, be it rain, snow, or drought, but we can at least understand it.
How Thunderstorms Develop
It`s estimated that up to 15,000,000 thunderstorms occur on earth each year, and about 2,000 thunderstorms are active at all times.
A thunderstorm develops when dense cold air overlies moist air that is less dense. A trigger, such as solar heating, frontal weather, or rising terrain, causes the warm moist air to begin to rise through the cold air. Air currents develop, and heat energy stored in the air and the water vapor is then converted into wind and electrical energy.
The atmospheric conditions needed to form thunderstorms are most common in the lower latitudes. This explains why South America and Africa are the most thunderstorm-prone continents and why Central Africa and Indonesia have long been considered to have the world’s highest incidence of thunderstorm activity. However, thunderstorms also occur in many other parts of the earth.
Fireworks in the Sky
The two characteristics of a thunderstorm that are clearly obvious to all are thunder and lightning. But what causes these spectacular, often frightening, phenomena? Lightning is simply the discharge produced when the differences in the electrical charges in two locations are large enough to overcome the insulating effect of the air. This can occur within a cloud, between clouds, or between clouds and the ground. Lightning causes air to be heated momentarily to amazingly high temperatures—as high as 54,000 degrees Fahrenheit [30,000 C] at the moment the stroke discharges its electrical charge.
Lightning can be classified as either streak lightning, forked lightning, or sheet lightning. If the discharge is visible as a distinct channel, it is streak lightning. If the channel is visibly divided or branched, then it is called forked lightning. If the flash is within the cloud, or intracloud, it is known as sheet lightning. Authorities tell us that most of the lightning we see is cloud-to-ground lightning.
Lightning causes harm to living things—even death and injury to humans and animals. People on beaches and golf courses and outdoors in rural areas are at highest risk because they are not protected from the electrical charge.
Only about 30 percent of people struck by lightning actually die, and the incidence of long-term injury is low when first aid is applied promptly. However, contrary to popular myth, lightning can and often does strike more than once in the same place!
Lightning strikes start many fires. These can devastate large areas of land. Approximately 10 percent of forest fires in the United States are caused by lightning. These result in the burning of more than 35 percent of the total forest and bushland consumed by fire in that country.
But lightning strikes are also beneficial. For example, forests benefit in several ways. Fires that are started by lightning reduce ground cover in low-temperature fires. This can reduce the risk of more damaging high-temperature fires reaching the tree crowns.
Lightning also brings about a beneficial change to gaseous nitrogen, which as a gas cannot be used by plants. Lightning converts this gas into nitrogen compounds, essential to the formation of plant tissues and the development of seeds, which provide proteins vital to animal life. It has been estimated that from 30 to 50 percent of the nitrogen oxides present in rainfall are produced by lightning and that globally 30 million tons of fixed nitrogen are produced in this manner each year.
A Storm’s Greatest Benefit
A thunderstorm can release an enormous amount of water. The main reason for huge quantities of rainfall within a short time is that the intense updraft of the severe storm suspends a large amount of water and then releases it suddenly. Such rain has been measured as falling at the rate of up to eight inches an hour. Of course, there can be a negative side to such heavy rainfall.
When a storm moves slowly, only a relatively small area of land receives most of the rainfall, and this can result in a flash flood. During such a storm, the surface runoff causes streams and rivers to swell. It is estimated that about one third of all flood damage in the United States is caused by flash floods spawned by thunderstorms.
However, storm-related rainfall brings many benefits. Large quantities of water are supplied to the soil and to reservoirs and dams. Research has shown that from 50 to 70 percent of all precipitation in some areas comes from thunderstorms, so in these places storm rain is vital to life.
What About Hail?
A very damaging aspect of thunderstorms is that they are often accompanied by heavy hail. Hail is formed when raindrops freeze and then grow in size as they are cycled by updrafts and downdrafts. There have been some accounts of hailstones of incredible size and weight. A hailstone that measured ten inches by six inches by five inches [26 cm x 14 cm x 12 cm] reportedly fell in Germany in 1925. Its weight was estimated at over four pounds [2 kg]. One of the largest hailstones ever recorded in the United States fell in the state of Kansas in 1970. This hailstone measured 17 inches [44 cm] at its largest circumference and weighed 27 ounces [776 gm]. A hailstone of that size falling from high up in the clouds is large enough to kill a man.
Fortunately, hail is generally much smaller than that and is more likely to cause inconvenience than death. Also, because of the nature of the thunderstorms that produce hail, the areas affected by damaging hail are relatively small. However, losses to the world’s agricultural crops caused by hail are estimated at hundreds of millions of dollars annually.
Tornadoes and Thunderstorms
Perhaps the most dangerous result of thunderstorms is the tornado. Virtually all tornadoes are associated with thunderstorms, but not all thunderstorms are accompanied by tornadoes. When formed, a tornado is a violently rotating, narrow column of air, averaging several hundred yards in diameter, that extends to the ground from a thunderhead cloud. Wind speeds in the most intense tornadoes can be more than 200 or 300 miles [400 or 500 km] per hour. The combined action of powerful rotary winds and the updraft in the center can collapse buildings and hurl lethal debris through the air. Tornadoes occur in many countries of the world.
Less visually dramatic but still potentially damaging are the straight-line winds associated with downdrafts and microbursts. Downdrafts can cause damaging winds at or near the ground that can reach a speed of up to 100 miles [150 km] per hour. Microbursts are more intense and can reach 150 miles [over 200 km] per hour.
It is obvious that thunderstorms should be treated with respect and that we should always be aware of their dangers.
