What is a Thunderstorm? A thunderstorm is a local storm produced by a cumulonimbus cloud, always with lightning and thunder, and usually accompanied by strong gusts of wind, heavy rain, and sometimes hail. In other words, a storm producing thunder and thus its companion, lightning. Consequentely, besides thunder and lightning, a typical thunderstorm produces showers, sometimes of torrential intensity, and occasionally hail or, more rarely, snow, along with gusty winds, sometimes in the form of dust storms or tornadoes. A vigorous thunderstorm generates a greater variety of violent weather than any other atmospheric circulation.

It is estimated that about 16 million thunderstorms occur every year, furthermore, at any given moment about 2000 thunderstorms are in progress over different parts of the Earth.  A single bolt of lightning discharges several hundred million volts and for a few seconds produces as much power as a nuclear power plant. It heats the surrounding air to 10,000 °C or more, hotter than the surface of the Sun.

In spite of the fact that thunderstorms are very violent events, they are local systems, often too small to be included on national weather maps. Typically, a thunderstorm forms and then dissipates in a few hours and covers from about ten to a few hundred square kilometers. It is not unusual to stand on a hilltop in the sunshine and watch rain squalls and lightning a few kilometers away. 

What Causes Thunderstorms? Thunderstorms develop when warm humid air rises, forming cumulus clouds that develop into towering cumulonimbus clouds. The following conditions cause these local regions of rising air:

1. Sea breeze convergence. As the subtropical Sun heats the Florida peninsula, rising air draws converging humid air from both the east and west coasts. Central Florida is the most active thunderstorm region in the United States.

2. Convection. Thunderstorms also form in continental interiors during the spring or summer, when afternoon sunshine heats the ground and generates cells of rising humid air.

3. Orographic rise. Humid air rising as it flows over hills and mountain ranges may generate thunderstorms.

Development of a Thunderstorm: Research conducted by a number of investigators in Ohio and Florida in the late 1940s led to a three-stage model of thunderstorm development. These are:

1) The cumulus or initial stage:  rising air condenses, forming a cumulus cloud; the cloud forms, the condensing vapor releases latent heat. 

Why is this air rising? The first stage in thunderstorm development is known as the cumulus stage; the airflow pattern in this stage is characterized by the converging and rising air flow, typical of any growing cumulus cloud. In comparing the temperature inside the cloud with that outside at the same altitude, it become evident that the air in the updraft is warmer than in the surrounding air; hence, it is buoyant. Buoyant, rising air cools adiabatically; if it rises high enough, condensation occurs. Condensation releases latent heat, which warms the air within the cloud, adding to its buoyancy. Thus, the thunderstorm begins as a typical cumulus cloud: it becomes larger and more buoyant than most cumuli, but its basic properties are the same.

And where does the growing cumulus come from? Some times it is convenient to visualize in the three-stage model an earlier, "zero stage," to span the period from clear skies to the cumulus stage. The mechanisms leading to the cumulus stage, like that stage itself, in part are just typical cumulus-forming processes, that is, warm, moist, conditionally unstable low-level air is heated to buoyancy. However, clouds large enough to become thunderstorms often grow due to other causes as well. For example, under the right conditions, neighboring cumuli sometimes merge, creating a single, much larger and more potent cell.

In a typical thunderstorm, 400,000 tons of water vapor condense within the cloud, and the energy released by the condensation is equivalent to the explosion of 12 atomic bombs the size of the one dropped on Hiroshima during World War II. Large droplets or ice crystals develop within the cloud at this stage, but the rising air keeps them in suspension, and no precipitation falls to the ground. .

2) The mature state (also some times refered to as "an overgrown cumulus cloud"): It has both updrafts and downdrafts; water droplets or hailstones become so heavy that updrafts can no longer support them, and they fall as rain or hail.  If conditions for growth remain favorable. the updraft in the cumulus cloud strengthens, drawing in more moist air at low levels, which in turn releases more latent heat, strengthening the updraft. Fifteen to 30 minutes after having reached the cumulus stage, precipitation begins falling from the storm's base, which marks its transition to the mature stage. During this stage, warm air continues to rise and may attain velocities of over 300 kilometers per hour in the central and upper portions of the cloud. The cloud may double its height in minutes. At the same time, ice falling through the cloud chills the lower regions and this cool air sinks, creating a downdraft. Finally, air currents rise and fall simultaneously within the same cloud. These conditions, known as wind shear, are dangerous for aircraft, and pilots avoid large thunderheads. 

It becomes evident that the storm is now in full thunderstorm development, with thunder, lightning, heavy precipitation of various kinds, and turbulent winds. A strong, deep updraft may lift air as high as the tropopause, whose stability caps the upward motion and causes an anvil to begin forming; however,  the air in a vigorous updraft may be rising so fast it "overshoots" the tropopause, creating a cloud dome that bulges upward from the anvil into the lower stratosphere. 

Typical updraft speeds in the storm's mature stage are on the order of 10 to 20 meters per second (20 to 40 mph). Thus, in one minute, the ascending air may rise as much as a kilometer or more. (Traveling for 60 seconds at 20 meters per second, the distance covered is 60 s X 20 mls = 1200 m = 1.2 km). At these speeds, it takes only 10 minutes or so for updraft air to ascend from ground level to the tropopause.

This rapid lifting has important implications for precipitation formation since rising air cools adiabatically at 10°C per kilometer if unsaturated and somewhat less rapidly (6°C per kilometer, as a typical value) if saturated. Thus, the air in a strong updraft may cool by 10°C or more per minute for several minutes of ascent.  The sudden cooling causes most of the air's water vapor to change phase to liquid water or ice, which has two important consequences: (1)  vast amounts of latent heat are released as the water vapor changes phase; this heating enhances the buoyancy of the rising air. (2) the sudden copious supply of water in liquid and frozen form leads to the rapid growth of precipitation-sized particles. Just before the rain arrives, the wind strengthens and switches direction, coming straight from the darkest part of sky; also, the temperature may fall sharply. The downdraf has reached you; the heaviest rain and lightning are close hand. Time to take cover, fast.

During this stage flash flood can occur since rainfall from a cumulonimbus cloud can be unusually heavy in mountainous regions; for example  in 1976 Big Thompson Canyon flash flood, during this event a stationary complex of thunderstorms dropped 25 centimeters of rain over Big Thompson Canyon on the eastern edge of the Colorado Rockies in about 4 hours. The river flooded the narrow canyon, killing 139 people.

A cumulonimbus stages is often included between the mature and dissipating stages; which includes huge clouds that extend to elevation of 12,200 to 18,300 m with updrafts of 12 to 27 m/sec. Massive downdraft, mostly in the leading portion of the cloud, reach about the same velocities as the updrafts. This type of cloud is the most violent stage of the storm with torrential rain or hail, incessant lightning, and likely tornadoes.

3) The dissipating stage: Fifteen to 30 minutes after a typical thunderstorm has entered mature stage, the cold downdraft air has spread out beneath cloud base and has cu t off the storm's supply of warm, humid. Deprived of its source of heat energy moisture, the storm soon weakens. Now it is in its final, or dissipating, stage. Weak downdrafts, light precipitation, and temperatures colder than those at the same level outside the c characterize the lower regions of the cloud in this stage. Thunder and lightning diminish, then cease altogether. Rain or hail and lightning, usually lasts for about 15 to 30 minutes and no longer than an hour. The cool downdraft reduces the temperature in the lower regions of the cloud. As the temperature drops, convection currents weaken and warm, humid air is no longer drawn into the cloud. Once the water supply is cut off, condensation ceases, and the storm loses energy. Within minutes the rapid vertical air motion dies and the storm dissipates. Although a single thundercloud dissipates rapidly, new thunderheads can build in the same region, causing disasters such as that at Big Thompson Canyon. Much of the middle high-level cloudiness observed in warm climates consists of remnants of dissipating thunderstorms.

Lightning is an electrical spark that jumps from a cloud to ground or to another cloud. Lightning is a gigantic spark discharge: a sudden, intense flow of electricity along a narrow atmospheric path. They are the most distinguishing feature of the electrical activity of thunderstorms. The thunder is the explosive heating of the air along the lightning bolt's path which produces the rolling rumble (if distant) or earsplitting crash (if nearby). It occurs when the buildup of static electricity is great enough to overcome the insulating properties of air. Charges separate within cumulonimbus clouds and build until a bolt of lightning jumps from the cloud. 

If you walk across a carpet on a dry day, the friction between your feet and the rug shears electrons off the atoms on the rug. The electrons migrate into your body and concentrate there. If you then touch a metal doorknob, a spark consisting of many electrons jumps from your finger to the metal knob.

In 1752 Benjamin Franklin showed that lightning is an electrical spark. In the nearly 250 years since Franklin, atmospheric physicists have been unable to explain the exact mechanism of lightning. 

Lighting Types: Four of the more common types of lightning include: (A) intracloud (60 percent of all lightning), (B) intercloud, (C) cloud-to-ground, and (D) ground-to-cloud (also called triggered lightning).

Intracloud discharge is veiled by the cloud mass and may lead to what is sometimes called sheet lightning. If such lightning occurs at very high elevations, and at some distance away, the sound is refracted aloft and may not be heard from the ground. This type of lightning can be impressive, but it is harmless as far as the earth's surface is concerned.

Intercloud lightning strikes from cloud to cloud, usually at higher altitudes. These spectacular zigzag bolts can be seen quite readily as they breach at times considerable distances between clouds. They do not pose danger to observers on the ground, however, discharges of this type may become hazardous to aircraft.

Cloud-to-ground lightning, which hurtles from the dark skies toward the earth. It may be reassuring to know that by the time we hear the crashing thunder the immediate danger has already passed. The intense heat generated by such lightning bolts, and by its return stroke, can split trees and may turn sandy surfaces into a glassy crust.

Ground-to-cloud lightning (also known as Triggered Lightning), which moves from ground features toward a cloud. Lightning behavior of this kind was observed in the 1930s in connection with electrical discharges that apparently emanated from the tip of the Empire State Building in New York. It was difficult to demonstrate clearly that the lightning bolts really originated in that way. It was not until much later that studies, made by the Swiss Mt. San Salvatore Lightning Observatory, revealed that such lightning is quite frequent under certain conditions. Two metal towers had been erected for this study. Of the more than 100 lightning strokes recorded, about 80 percent originated from the tips of these towers. As this research indicates, favorable places for triggering ground-to-cloud lightning, are high mountain peaks, tall television towers, radio towers, and skyscrapers. Triggered lightning, as this phenomenon is also called, apparently starts with a stepped leader, as in cloud-to-ground lightning, but no return stroke from the cloud takes place.

Theories for the development of lightning: 1) Friction from falling droplets generate charge separation. According to this theory, charge separation occurs in much the same way as when you walk across a carpet. Raindrops and ice particles collide as the heavier particles fall past smaller suspended water droplets and ice crystals in the cloud. Friction transfers electrons to the heavier particles. 2) Charged particles are produced from above by cosmic rays and below by interactions with the ground. This theory suggests that cosmic rays, bombarding the cloud from outer space, produce ions at the top of the cloud. Other ions form on the ground as winds blow over sharp objects at the Earth's surface. Convection transports these ions through the cloud. Neither theory is entirely accurate, some combination of the mechanisms is responsible for lightning.


some concepts:
What Causes Thunderstorms? 
Development of a Thunderstorm
Theories for the development of lightning
Lighting Types




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