Rainfall
Exmoor’s prevailing winds are from the Atlantic, bringing with them relatively warm and moist air. When the winds meet the edge of the high plateau they are forced upwards into cooler air as there is an unbroken ridge of hills on Exmoor’s western side. This causes condensation of the moisture, forming clouds. If the temperature drops greatly and there is much moisture, the tiny water droplets that form the clouds aggregate together, forming larger droplets that fall as rain. This is known as orographic rainfall. If these droplets form high in the atmosphere and fall through thick cloud they can form heavier rain. When the atmosphere where the droplets form is below freezing, the moisture can fall as hail or snow. The snow may turn to sleet where the temperature nearer the ground is above freezing. It is no surprise, therefore, that it is the high ground in the west of Britain that receives most rainfall. Exmoor, however, cannot be described as mountainous and other areas such as the Lake District, Snowdonia and even Dartmoor receive higher rainfall.
The air forced up by the western edge of Exmoor descends on the lee side of the plateau. By this time it has less moisture in it and places in the lee are in what is known as the ‘rain shadow’ of Exmoor. So places to the north east of Exmoor are relatively dry. The low lying areas of the Vale of Porlock, Minehead and the northern edge of the Brendon Hills are also relatively warm and often places to which Exmoor people retire to escape the rigours of the moorland climate.
Not all of Exmoor’s rainfall is orographic, or due to its height. The great Exmoor flood of 1952, which caused so much damage and loss of life at Lynmouth, was caused by the meeting of two air masses moving in different directions. This is known as convergence. In such cases the air in each front can only go up and the moisture from both air masses condenses. Rain can also occur from the differential heating of the ground by the sun, particularly where one side of a hill is in the sun and the other in the shade. This causes a plume of warm air to rise, usually creating large billowy clouds. A similar effect is created near the sea where the warming land causes hot air to rise, drawing in cooler air from over the sea and creating a sea breeze. In some conditions, particularly where there are isolated hills such as at Holdstone Down, the air cools to form lens shaped clouds that hover over the hill as the air at that point is dropping and warming again. In this case a reverse can occur on a clear evening when the land cools rapidly and cool air sinks down the hillside, drawing in warmer, moister air from the sea. An eddy forms in the lee of the summit and the reduction in pressure can form this ‘lenticular’ cloud. It is no surprise that there are many types of cloud and rain on Exmoor.
Temperate zones in Europe get more of their rain in winter due to low pressure zones moving across the Altantic driven by the prevailing westerlies. October through to January is the wettest period on Exmoor. Then, in February, colder, drier air masses often come from the Continent.