Tropical Maritime

The source region for this type of air mass is the waters of the Atlantic Ocean between the Azores and Bermuda. The air usually reaches the British Isles on south-westerly winds, commonly in the warm sector of a depression or around the periphery of an anticyclone located over central Europe.

The air is warm and moist in its lower layers and during the passage over the cooler waters the air becomes stable and close to saturation. The air mass is modified quickly during its passage across the British Isles and its characteristics vary considerable from place to place depending on exposure to the moist south-westerly flow.

In the western areas of the British Isles, the tropical maritime air is stable and saturated in its lowest layers. As a result, the weather is characterised by much low cloud, drizzle and hill fog. During the winter months the air may reach eastern Britain with very similar characteristics providing the ground is cold, but for much of the year insolation is sufficient to warm the air appreciably.

Visibility in such conditions is more difficult to quantify. When tropical maritime air reaches our western shores it is already near to saturation and any uplift produces low cloud and fog which can reduce visibility to near zero. Following a short land track however, and providing the air has dried out appreciably, visibility can become excellent because the air is inherently clear and aerosol free. Following a lengthy land track, haze particles increase in number and are trapped by the stable air reducing visibility to the moderate category.