The Thunderstorm—Awesome King of the Clouds
By Awake! correspondent in Australia
MANY people have been fascinated by clouds since they were children. One 80-year-old recalls how in early childhood he would often lie in the grass watching clouds “parade across the sky,” as he puts it. He remembers that he often wondered what clouds were made of. Was it cotton? Why did each one look so different? That one looked like a sailing ship, that one like a prancing horse. And then came a billowing castle. They continued to delight his childish imagination as they floated across the sky in an ever-changing array of shapes and sizes. To this day, he says, he still enjoys watching as clouds seem to “play charades” in the sky. Perhaps you too share this simple pleasure.
However, 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.
How Thunderstorms Develop
In recent times man has been able to view planet Earth from space. He sees a carpet of cloud hovering over much of its surface. Author Fred Hapgood informs us that “at any given moment half the globe’s surface, 100 million square miles [250 million sq km], is covered with [clouds]—sheetlike, rounded, quilted, filamentous, lacy, puffy, in all degrees of luminescence and opacity, blossoming, spreading, sailing and fading all around the world.” Thunderstorms form part of this cloud mass—in fact, 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. The accepted record is 242 thunderstorm days per year, recorded at Kampala, Uganda. 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.—See the box on page 15.
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 be aware of their dangers. They are just one of the many facets of creation about which we still have much to learn.
[Box/Picture on page 15]
Lightning Strike Precautions
Emergency Management Australia suggests the following precautions during a thunderstorm.
◼ Seek shelter in a hard-top vehicle or in a building; avoid small structures, fabric tents, and isolated or small groups of trees.
◼ If in the open away from shelter, crouch down (singly), preferably in a hollow, with feet together, and remove metal objects from head and body. Do not lie down, but avoid being the highest object in the vicinity.
◼ If your hair stands on end or you hear buzzing from nearby objects, such as rocks and fences, move away to a new position immediately.
◼ Don’t fly kites or model airplanes with control wires.
◼ Don’t handle long or metallic objects, such as fishing rods, umbrellas, and golf clubs, in the open.
◼ Don’t touch or move close to metal structures, wire fences, or metal clotheslines.
◼ Don’t ride horses or bicycles or drive in open vehicles.
◼ If driving, slow down or park away from such tall objects as trees and power lines. Stay inside hard-top vehicles and trailers, but do not touch or lean on metal body components.
◼ If swimming or surfing, leave the water immediately and seek shelter.
◼ If boating, go ashore as soon as possible. If unsafe to do so, seek protection beneath a high structure, such as a bridge or a jetty. Be sure that the mast and stays of a sailboat are adequately grounded to the water.
◼ Keep clear of windows, electrical appliances, pipes, and other metal fixtures.
◼ Avoid use of telephones. If an emergency call is required, make it as brief as possible.
◼ Before the storm arrives, disconnect external antenna and power leads to radio and television sets. Disconnect computer modems and power sources. Then stay away from electrical appliances.
From the publication Severe Storms: Facts, Warnings and Protection.