Weather wise

IT SEEMS that if you really want to avoid being struck by lightning, you should head out to sea. A new Nasa satellite which is able to observe the intensity and frequency of electrical storms has found that between 85 and 90 per cent of lightning strokes occur on land - which forms only one third of the Earth's surface.

The Lightning Imaging Sensor has also given the first accurate measurement of how much electrical activity there is going on over the Earth in total; the satellite, which has been operating for three months, found that on average there are 2,000 thunderstorms at any one time, producing, worldwide, about 100 lightning strokes a second. In intense storms, inter-cloud and intra-cloud lightning strokes may outnumber cloud-to-ground strokes by 30-to-1.

So why are thunderstorms so much more common over land? The generator that produces the electrical fields necessary for a thunderstorm is the upward convection of air, and convection tends to be greater over land than over the sea. This is because the land tends to warm up far faster than open water, and it is only when the surface becomes much warmer than the overlying air that convection can occur.

Convection causes movement and collisions of ice particles within a thundercloud, and it is this friction that is believed to generate the electrical charges that turn into lightning, just as friction between your hair and a comb can generate an electrical charge.

Updrafts and gravity separate the charged particles, with negative ones sinking to the bottom of the cloud and positive charges rising to the top. This creates enormous electrical potential within clouds, and between clouds and the ground, that can suddenly discharge as lightning.