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The Anatomy Of A Thunderstorm
A distant rumble and very dark cloud on the horizon may be the first announcement that a thunderstorm is on its way. As the storm nears, the skies will be lit up with intense lightening strikes in a fork or a sheet pattern. When the storm cell is overhead, there will be a downpour of heavy rain; the frequency of the lightning and the peals of thunder will rise. But just what are thunderstorms and why do they happen?
Even the smallest child quickly learns to identify a thunderstorm from the loud peals of thunder and the flashes of lightening – it is a very distinctive meteorological event. They can be frightening to children (and some adults) because of the very loud noise and the intensity of the lightening discharges, or strikes. These indicators give a clue about the immense energy locked up in a thunder storm.
Meteorologists tell us that right now, some 1800 thunderstorms are taking place around the globe and they produce approximately 100 lightening strikes per second (8 million or so each day). About 86 Americans will be killed this year through being struck by lightning (typically whilst swimming, hiking or golfing). Lightning strikes are responsible for considerable structural damage and are the cause of many destructive wildfires.
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Thunderstorms tend to occur on hot, sultry days when the humidity is high. As warmth from the sun heats the earth, air overlying the near earth surface heats up. As it does so, the density of the air decreases and the air rises, taking with it the moisture it contains. This process forms an updraft of warm, moist air which rises into the atmosphere until cooled by the surrounding air to form a cloud – in this case a cumulus cloud. The cloud, of course, is formed from the water vapour that rose with the warm air. The height of the cloud is such that the surrounding air temperature is low enough for ice crystals to form. The cloud will continue to develop as more moist air feeds into it until it reaches a saturation point, where the force of the rising air is no longer sufficient to support the weight of the moisture.
Once the mass of the cumulus cloud has increased past the point where the rising air can’t support the moisture, rain starts to fall. Cooler air starts to descend in order to rebalance the pressure difference caused at ground level by the rising air. This process creates a downdraft which accelerates the rainfall process and the cloud becomes known as a cumulonimbus cloud (having both up- and downdraft activity).
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Building Up The Electricity
The rising and falling air columns cause the ice crystals to become electrostatically charged through collisions and a polarity develops with the top of the cloud at positive potential. This generates an enormous capacitive charge across the cloud. Now we are close to understanding what are thunderstorms and how they occur. The only thing left to explain is the lightening discharge and the thunder.
The charging process that occurs within the cumulonimbus cloud can generate voltages of between tens and hundreds of millions of volts (the battery in your car is usually 12 volts). The earth is normally at a slightly negative potential to the atmosphere, but this polarity is (locally) reversed as a thundercloud passes overhead because of charge induction by the enormous negative charge at the cloud base (lowest part). Current will always flow from a high potential to a lower potential unless it is prevented by an insulator (this is the case in a capacitor, for instance, which is a device used in electronics to store charge) and air is a very good insulator. However, with the energy locked into a thunderstorm, the insulative properties of air finally break down, as components within air itself become ionized, allowing some of the energy in the thundercloud to be dissipated to earth as a lightning strike (for precise details of the mechanism, refer to reference 3).
But what of the thunder? Well, as the (local) insulative properties of air break down, the surrounding air in the immediate vicinity of the lightning strike is heated to 100 000 °C, causing rapid expansion of the component gases, and this generates a shock wave that is audible as the characteristic rumble and crack of a thunderclap.
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- Met Office: http://www.metoffice.gov.uk/weather/uk/advice/lightning.html
- Windows to the universe: http://www.windows2universe.org/earth/Atmosphere/tstorm/tstorm_formation.html
- National Weather Service Forecast Office: http://www.wrh.noaa.gov/fgz/science/lightnin.php?wfo=fgz