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Thunderstorms, lightning and tornadoes

Thunderstorms

You can recognise an approaching thunderstorm by the appearance in the distance of a blackening sky and ominous, upward-billowing clouds. The storm's arrival is heralded by sudden winds, torrential rain, thunder and lightning - and perhaps the odd tornado thrown in for good measure.

The intensity of a thunderstorm relies on a combination of intense heating of a moisture-laden part of the Earth's surface and an upward movement amplified as the rising air meets a cold front or a mountain range.

In the early stages of a developing thunderstorm, the updraught is powerful enough to keep the water droplets and ice crystals suspended in the cloud. Eventually, the weight of water and ice is so great that rain, or sometimes snow or hail, starts to fall from the leading edge of the advancing cloud. The falling water brings with it the cool, gusty downdraught which spreads ahead of the storm.

The downdraught slowly spreads backwards beneath the storm cell, cutting off the moist updraft that feeds it. Eventually the storm runs out of the moisture that keeps it going.

Thunderstorms are often made up of several individual storm cells, each at different stage of development. These large scale thunderstorm complexes can cover huge areas, and can last anything up to 24 hours.

A supercell thunderstorm is the most dangerous of all. It contains just one giant storm cell, and the updraught at its centre can reach up to 175mph (280kph). The rising air can easily turn into the strong rotational circulation that gives birth to a tornado.

On Earth, at any one time there are likely to be about 2,000 thunderstorms of different intensities, mainly in tropical and subtropical countries. In Britain, thunderstorms occur mainly in summer, but can crop up at any time. Fortunately, less than 1% of all thunderstorms produce tornadoes.

On Jupiter, giant thunderstorms 30 miles (50km) high constantly rage through the atmosphere. The planet's 60,000°C core causes hot gases to rise rapidly into the dense atmosphere, creating the most powerful thunderstorms ever known.

Neptune and Saturn are also surrounded by swirling cloud bands that, like Jupiter, may contain constantly raging thunderstorms.

Lightning

You can't mistake the searing electrical discharges and the shock waves of superheated air they create. They're the sound and light show that accompanies an approaching thunderstorm.

As a thundercloud builds up, electrical charges separate and build up in different parts of the cloud. The result is an electrical potential of 100 million volts - almost half a million times greater than the 240 volts supplied by a domestic socket.

Dry air is a very good electrical insulator, but in a developing thunderstorm, the air becomes saturated and the electrical resistance starts to break down. The brilliant flash of light we call lightning is the result of a huge electrical discharge moving at 30,000 miles per second (50,000kms) within the cloud, or between the cloud and the ground. Each stroke of lightning consists of up to six discharges following the same path. This all happens within a fifth of a second, too fast for the human eye to see more than a single flickering flash.

The rumble of thunder comes from the shock waves caused by the rapid expansion of air heated to 25,000°C along the path of the multiple lightning flash.

On Earth, a flash of lightning contains enough energy to damage buildings and start fires, and more than enough to kill humans and other living things.

On Jupiter, vast depressions (lows) the size of Alaska are the home of the most powerful lightning ever recorded. Each lightning bolt may release as much energy as the power supply of a large industrial city - in a single heavyweight punch.

Tornadoes

At first glance, you might think that a tornado is just a dust devil's big brother. Look again, and you'll see it's much more dangerous. On Earth, a swirling cone of debris and dust capped by a dark, brooding funnel cloud, heralds the imminent arrival of the most violent of all weather systems.

Unlike dust devils, tornadoes - sometimes known as twisters - aren't fine weather phenomena. They're nearly always produced by strong updraughts of warm wet air associated with supercell thunderstorms. The rapidly spiralling air produces the funnel cloud that is the tornadoes calling card.

The wind around the base of an intense tornado can reach 300mph (500kph), with updraughts up to 100mph (160kph). The giant whirling column of air drags up dust and much larger debris from the ground, and the surface winds are powerful enough to blow down trees, electricity lines and buildings. Tornadoes move at an average speed of 30mph (50kph), but can race across the landscape at up to 75mph (120kph).

Tornadoes weaken and slow down as the downdraught of cold air from the dying thunderstorm surrounds the tornado and cuts off its supply of warm, wet air. A weak tornado may travel only 1 mile (1.5km) or so over land and last just a few minutes. An intense tornado can create a 2-hour, 100-mile (160km) trail of damage.

Multi-vortex tornadoes are most dangerous of all: several powerful tornadoes orbiting around each other or around a common centre.

On Earth, one of the most deadly tornadoes ever recorded ripped through Missouri, Illinois and Indiana in March 1925. During its three-and-a-half-hour rampage, its 75mph (120kph) winds killed 695 people, injured 2000 and made another 11,000 homeless.

On Jupiter, Neptune and Saturn, it's likely that the giant thunderstorms raging through their atmospheres create super-tornadoes with wind speeds, updraughts and damage potential far greater than those on Earth.

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