Depressions seem to be lining up in the North Atlantic, waiting to bring clouds and rain to Britain. But Spain and Portugal have been getting the worst of Europe's weather.

Look at all those Lows on the weather map all in a queue on the transatlantic ferry to Britain. Areas of low atmospheric pressure are formed when high- level winds take away more air than can quickly be replaced. The resulting partial vacuum leads to a reduction in the weight of air near the ground, and low-level winds flow inwards to restore the balance.

The result is high winds blowing anticlockwise around the centre of the depression (the Coriolis effect produces the direction) and a general rise in the flow of air - emptying at the top and filling at the bottom. Ascending air cools and forms wide areas of cloud and rain.

Spain and Portugal have been seeing the worst consequences of an occluded front, when cold air forces the rapid ascent of warm, leading to fast condensation and heavy rain. On Wednesday night, storms led to flash flooding that killed 27 people, with many more unaccounted for. These were the worst storms in living memory in the area - another entry in the catalogue of this year's extreme weather events.