Severe Thunderstorms

Thunderstorm Development

When you hear thunder, you have a thunderstorm nearby. As obvious as it sounds, that is the definition of a thunderstorm: no thunder – no thunderstorm. If only everything else were so simple. Mind you, the weather services came up with some conditions. For example, you must have heard the thunder within the last 10-15 minutes. Now it’s official.

Ever wondered how far away the thunderstorm is? Because light travels faster than sound does, you can determine the distance by counting the seconds between a lightning flash and the arrival of its associated thunderclap. A difference of three seconds represents approximately one kilometer.

How does a thunderstorm develop? There are several ways, but generally it requires air to lift, to cool and water vapor to condense.

The most common type is the afternoon heat thunderstorm. Remember the hot car park? A heated surface warms the air above it. The air rises and cools in the process. At a certain temperature (dew point) the water vapor condenses and forms a tiny puffy cloud (cumulus). If the conditions are right, the cloud continuous to grow until it hits an invisible ceiling – the tropopause (see Chapter Three, ‘Air’). Generally the temperature drops as the cloud rises, until it reaches the tropopause, where it heats up and forms a strong inversion. When the rising air in the growing cloud has the same temperature as the tropopause, however, it stops moving upward. As a consequence, the cloud spreads underneath the inversion and forms the characteristic anvil of a cumulonimbus cloud. The flat top is above the freezing level where water droplets freeze. The ice crystals give the cloud’s top a feathery and ragged look.

Forced lifting creates other types of thunderstorm. When air is forced up the slopes of a mountain range the same process begins and an orographic thunderstorm develops. Cold air masses from polar regions wedge underneath warmer air and force it upwards. This is the characteristic of a cold front, and if thunderstorms develop along the front they are named frontal thunderstorms.

cumulonimbus_cloudA cumulonimbus cloud contains very strong updrafts and turbulence. Water droplets and ice crystals collide and form rain, hail or snow (precipitation). The strong updrafts keep the precipitation within the cloud until it becomes too heavy. Then the heavy water droplets, ice balls or snowflakes fall out of the cloud. The weatherman calls this a shower.

This is also the end in the life cycle of a thunderstorm. The falling precipitation creates heavy downdrafts. They overpower the updrafts eventually and dry out the cloud from the top. In addition, the surface is now wet and cool and no longer able to supply the cloud with warm air. The thunderstorm dies.

Observation and lightning detectors give the weatherman a good idea where the thunderstorms occur, how many there are at any given moment and in which direction they are drifting. Nevertheless, forecasting a heat thunderstorm is a thankless task. The meteorologist knows when the conditions for the development are favorable and predicts thunderstorms for the next day. But ask the forecaster whether your town will get one and he may answer, ‘Beats me.’

Dangerous Thunderstorms

A country the size of the US or Australia experiences approximately 10,000 thunderstorms each year. Most of the time, they are nothing but a nuisance. From a distance, you may even enjoy the spectacular cumulonimbus cloud and the bonus ‘fireworks’ of lightning and thunder. Occasionally, however, a severe thunderstorm can be as destructive as a tropical cyclone or a tornado. A single severe thunderstorm unleashes its fury over an area of about 8km. Sometimes, especially along a weather front, thunderstorms come in company and line up for more than 150km.

Annually, their destructive power results in economic losses in the US of around A$2 billion. In Australia, thunderstorms are more damaging than cyclones, floods or bushfires. Of course, there are exceptional years when the competition takes first place.

Thunderstorms just about have it all. They present you with a variety of hazards and all are potentially destructive. Amongst them are:

  • Strong winds inflict the first damage. Severe thunderstorms deal out gusts exceeding 80km/h, often before the cloud arrives. In 1991 a thunderstorm crossed the North Shore of Sydney, Australia, with wind gusts of up to 230km/h, causing injury to 100 people and leaving a damage bill of around A$680 million. The storm damaged 10,000 buildings and in excess of 50,000 trees.
  • Vertical air movement just underneath the cloud can produce even stronger gusts. Downburst or microburst, the term depends on the size of the outflow, quite often result in gusts of up to 240km/h. Mention microburst in the presence of an airline pilot and you see a sudden change in his or her facial expression. An aircraft, large or small, is very vulnerable to shifts in wind speed and direction during take-off or landing – a ‘sitting duck’ so to speak. Microbursts are strong enough to drive an aircraft into the ground and have been the cause of several major accidents.
  • Strong updrafts (updraughts) keep hailstones within the cloud where they grow until they are heavy enough to overcome the vertical air movement. Hail causes major damage to agricultural crops every year. Occasionally, the hailstones can reach a size that damages buildings and cars. Sydney was again the target of a severe weather event in 1999: hailstones the size of tennis balls damaged 22,000 homes and 63,000 cars. Even aircraft at the international airport weren’t spared. The insurance companies claim that the total damage bill exceeded A$2.3 billion.
  • Flash floods. Thunderstorms can dump an immense amount of rain on a small area. The soil is soon soaked and the drainage system is incapable of removing the excess water. Dangerous flash floods develop very quickly. Dangerous flash floods develop very quickly and are potential killers.
  • Lightning kills and injures people. People usually underestimate the danger of lightning because it selects its victims at random. You think it’s very unlucky to be struck by lightning? Not so. The odds are much shorter than winning the major prize in a lotto game. Lightning is also a major cause of bushfires.

Severe thunderstorms occasionally develop tornadoes. The destructive winds in the narrow funnel destroy everything in its path. See the following paragraphs for more information.

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