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BY Wijke Ruiter

Thinking about thunderstorms there's an immediate association with long, lazy and hot summer days. Though thunderstorms are one of nature's most violent events, at the end of a sultry day, they mostly get a warm welcome: refreshing the air and leaving a reborn earth behind.

These storms develop pure out of heat and occur locally. 
The atmosphere simply awaits to moment that  the temperature of the air right above the ground is so high that is rises very quickly, there's an updraft. Colder air, falling from above, takes its place, that's the downdraft. Rising air cools down and the water vapour condenses. Clouds will appear, those typical cumulus clouds, which keep on growing, as long as the surrounding air is cooler than the warm rising air.
As the cloud grows higher and higher the condensed water will freeze into ice crystals. The cumulonimbus with its anvil shape top arises. 
Actually this is the normal process for showers to develop. These showers will grow out to thunderstorms when the differences in temperature between the various levels in the atmosphere is high enough.  
The updraft and the downdraft are so turbulent then, that the water and ice particles in the cloud are separated and split again and again. This process builds up electrical energy. At the ground the violent up- and downdraft are felt as gusts of wind.

The updraft can become stronger when there's a jetstream at higher altitude. Jetstreams are strong winds at about 9 - 10 kilometer (30.000 - 33.000 feet) high in the atmosphere, with a force of 65 mph or more.
These winds give an impulse to the updraft and to the connecting downdraft: the thunderstorm becomes heavier.

These jetstreams can also be the source of a band of thunderstorms not developing by heat. Its a law of nature that where the flow is high the pressure is low. So there's low pressure in these jetstreams which forces an uplift of  the air at the ground, creating a up- and downdraft. Thunderstorms are developing then, even when the temperature is below 20 C.

Thunderstorms are always accompanied by heavy rain or hail and wind gusts. And lightening of course. The electric energy, built up in the cloud, discharges.In the cloud ice particles and hail whirl in the turbulence. Hail becomes heavier and falls down to the bottom of the cloud as ice particles are taken by the updraft. As they collide ice particles give their electrons to the hail. So the ice particles at the top of the cloud have a positive charge, as the hail at the bottom of the cloud has a negative charge. These electrons also repel the electrons near the ground, leaving the ground also positive. 
And here's the scenario for electrical discharges - a flow of electrons - from bottom to the top within the cloud as well as from the cloud to the earth below. A lighting strike splices the air. In the most marvellous forms and branching. The lightening strike causes a rapid heating of the surrounding air, that'll suddenly expand with loud soundings: as thunder.

In scientific literature thunderstorms are divided in three classes:

  • ordinary cells; as described above

  • multicellular systems

  • supercells 

Ordinary cells are single thunderstorms, developing by heat and short-lived. Multicellular systems are more organized. In summertime they often develop above France -central Massive - where very warm and moist air from the southeast bumps into relatively dry and cold air from the west. A perfect hotbed for those complex  thunderstorm systems.
With a south to southeasterly flow they can easily reach Holland or the south east of the UK. Surviving even nights cooling down: they cover the warm ground, so that it stay warm,  and radiate heat into the higher atmosphere at the top, so that the tops of the clouds stay cool. The difference in temperatures, needful for surviving, is guaranteed.
But also at the "prefronts" of a low these systems can grow.
Weather report Wednesday July 4 2001: heavy thunderstorms at the British Isles took  people by surprise. Especially in West Scotland and Wales In no time flooding everywhere, brooklets turned into currents of mud. Many people had to be evacuated." 
Supercells practically never occur in our regions; they bring alive another phenomenon: the tornado. 
Sometimes tornado's do make their appearance: 
witness the weather report august 21 2000: "This morning a sudden tornado visited East Anglia. Heavy thunderstorms brought so much hail that the landscape got a wintry character. No damage reports so far"

Thunderstorms often give opportunity to the most wonderful pictures.
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