Thunderstorms are local storms accompanied by lightning and thunder and a variety of weather phenomena, such as heavy rain, hail or - in winter - snow, high winds and sudden temperature changes. Thunderstorms originate when intense heating causes a parcel of moist air to rise from the earth's surface into upper levels of the atmosphere, a process called
convection. Thunderstorms are therefore also known as convective storms.
At any given moment, it is estimated there are 2000 thunderstorms in progress around the world. They occur most frequently in the tropics but are also common in the mid-latitudes.
Thunderstorms need an ample supply of moisture, preferably in the lower and mid-levels of the atmosphere, as they are mainly powered by latent heat released as water vapour condenses.
Thunderstorms need unstable air, a temperature profile with warm air near the ground and cold air aloft. When an air parcel is given an initial push upwards it will continue rising without additional force. Thus thunderstorms are more likely in the spring and summer than in the fall and winter. The sun warms the ground, which warms the air near the ground. In spring the air aloft retains its winter cold and thus will be more unstable than in the fall when the air aloft retains its summer warmth.
Thunderstorms need a source of lift. This can be (1) differential heating when air near the ground is warmer than in upper levels, (2) orographical effects when air has to rise to pass a mountain ridge, (3) frontal boundaries when air masses of a different temperature clash, (4) drylines when air masses with differing humidity but similar temperatures clash and (5) Land/Sea breezes. Thunderstorms can get started by even faint air boundaries and thus sometimes seem to pop up out of the blue sky.
Thunderstorms are often accompanied by severe weather and lightning is among the biggest weather killers. However, less then one percent of all thunderstorms produce hail bigger than the size of a golf ball and/or strong downburst winds. Only a small fraction of severe storms actually produce tornadoes or waterspouts.
No place in Europe is completely immune from the threats of thunderstorms. Severe weather can strike at any place, and at any time. Thunder and lightning occur simultaneously but thunder is heard later than lightning is seen, as light travels faster than sound. A good measure of distance from a storm is 1 mi (1.6 km) for every 5 seconds between flash and thunder.
Related features and links:
Hailstorms in Britain
The Lifted Index - Thunderstorm Probability
The Lifted Index (forecast maps)
Where Lightning Strikes - Congo Under Fire
Sferics - The Observed Lightnings Now!