An area of low pressure at the surface is known as a depression. As we learnt in the lessons on Air Pressure and Convergence & Divergence, the wind blows anticlockwise around a low and there is convergence at the surface. The air rises upwards at the centre of the depression and diverges at high altitude.
As the air rises, it cools and expands, creating cumuliform clouds. So in a depression we can expect:
Europe regularly experiences the polar frontal depression – a type of depression that forms over the North Atlantic Ocean and moves east towards Europe. This depression has two distinct fronts: a warm front followed by a cold front. The two fronts have the typical frontal weather described in the lessons on the Cold Front and the Warm Front. Between the two fronts is tropical maritime air – known as the warm sector.
The warm sector has generally calm weather, possibly with a little stratus. Any rain that was present in the preceding warm front usually decreases to drizzle or stops altogether. The isobars are straight and parallel in the warm sector.
While the whole depression is by definition a low pressure system, the air pressure is higher in the warm sector than the other parts of the depression.
The weather is usually worse closer to the centre of the depression and better as you travel further from the centre.