Polar Maritime

Polar maritime air flows towards the British Isles on north-westerly winds from the Arctic regions of both Greenland and northern Canada. A typical synoptic situation for such an airflow would have a low pressure centred near Iceland.

This air starts very cold and dry and during its long journey across the comparatively warm waters of the North Atlantic its temperature rises rapidly. The air temperature rises rapidly, allowing it to become unstable to a great depth and causing the moisture content to rise significantly.

Polar maritime air is perhaps the most familiar air mass. The instability produces showers over the sea, and in the exposed west and north of the British Isles these showers are frequent. In winter months when convection is most vigorous over the sea, hail and thunder are common in exposed hilly areas. In eastern Britain, where moisture input and surface heating are reduced, showers are less frequent except when troughs of low pressure pass. In the summer months, land temperatures are higher than sea temperatures and the heaviest showers occur over eastern England.

Large variations in shower activity can happen on a diurnal basis as well as on a day-to-day timescale, largely due to subtle changes in both the stability and moisture content of the air.