17th April 2006
Thunderstorm
Source: Excerpt from The Book " Weather "
Distribution: Worldwide, except Antarctica; common in the tropics. Height: 2000 to 35,000 feet. Cause: Powerful convection assisted by atmospheric instability. Associated Weather: Heavy rain or hail, strong winds. Hazard Warning: Lightning, wind, hail, and tornadoes; severe turbulence in cloud. The magnificent anvil-shaped cloud of the mature thunderstorm has long been an object of awe and fascination because of its capacity to unleash devastating rain, wind, hail, and even tornadoes, as well as awesome displays of lightning and thunder.
Thunderstorms occur under varied conditions, but are most common in spring and summer in tropical and subtropical zones. Air-mass storms tend to occur in the late afternoon or evening when heating of the ground has reached its peak. Storms that result from frontal systems can occur at any time, but ground heating will tend to intensify their development.
Each day, approximately 40,000 thunderstorms occur throughout the world. The most thunderstorm-prone area is the southeastern United States, with some parts of Florida experiencing thunderstorms on an average of 100 days a year.
A typical thunderstorm may last up to two hours, although it will generally be at its mature stage for only about 15 to 30 minutes. After this, it will start to dissipate, often leaving only a few wisps of high-level cloud behind.
Storm Alert
There are a number of ways that the weather-watcher can tell if there are storms in the area. If the terrain is reasonably flat and the sky is not obscured by low-level stratus, a towering cumulonimbus cloud may be visible up to 200 miles (320 km) away. The direction in which it is moving can sometimes be determined by observing the shape of its anvil. (See Cumulonimbus Incus)
Normally, there are long and short parts to the anvil, with the long section spreading out in the direction in which the upper-level winds are blowing. This is generally the best indication of the movement of the storm. Surface wind is not a good indicator because thunderstorms are affected by the speed and direction of the wind at all levels of the troposphere.
In some cases, when the sky is covered with many different types of cloud, or the terrain is mountainous, a thundercloud may be hidden from view. However, storms up to 100 miles (160 km) away can be detected by using a radio receiver. Tune the receiver into an area on the dial where no transmissions are taking place, and then turn up the volume.
If there is an active thunderstorm around, you will hear distinctive bursts of static, produced by the storm's lightning. An increase in the volume of the static indicates that the storm is getting closer. If an active thunderstorm is less than about 20 miles (32 km) away, you should be able to hear it. Because light and sound travel at different speeds, you can approximate the storm's distance by counting the interval between a flash of lightning and the associated sound of thunder. As a rough guide, every 5 second interval is equal to 1 mile (3 seconds is equal to 1 km) between you and the storm. If the interval between the lightning flash and the thunder decreases, the storm is getting closer, with simultaneous lightning and thunder indicating that you are directly beneath it.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
