Wind and air masses

Wind is the movement of air from areas of high atmospheric pressure to areas of low pressure. The differences in pressure are caused by differences in temperature on the Earth’s surface. Prevailing winds are arranged in a series of belts around the globe, resulting from the amount of heating each part of the globe is receiving from the sun and the speed the Earth is rotating at that point. In mid latitudes such as in the British isles the winds generally move from west to east. You only have to look at the trees on top of Exmoor’s hedgebanks to see that most are leaning away from the main winds, which are from the south west.

Windblown hedgebank trees on Broadmead

There is a permanent area of high pressure in the Atlantic around the Azores and that tends to direct winds towards Exmoor. In winter the ‘high’ recedes southwards, so that the ‘lows’ that form out in the Atlantic tend to track across southern Britain rather than to the north of Scotland as they do in summer. Although the most common winds in England are south-westerlies, winds from other directions are quite frequent and long spells of easterly or north-easterly winds are not unusual. Winds over the British Isles can be a resource and a hazard. Hundreds of wind turbines have been installed in exposed parts of the British Isles, adding electricity to the National Grid, and exploiting this wind resource.

As Atlantic depressions pass by the UK, the wind typically starts to blow from the south or south-west, but later comes from the west or north-west as the depression moves away. The range of directions between south-west and north-west accounts for the majority of occasions and the strongest winds nearly always blow from these directions.

Easterly winds in winter bring the coldest weather. This is because they come from Russia and have not come over the sea. They are generally dry winds and it is the northerly winds coming down the Atlantic that bring cold, moist air producing showers in summer and snow in winter. Much air that comes from the Arctic sweeps out into the Atlantic and turns around to return northwards. This produces the typical dull, overcast, drizzly or showery Exmoor weather with south-westerly winds. The warmest air usually comes from the south with winds starting in North Africa and passing over France. They are usually dry but may meet up with moist south-westerly winds, creating thunderstorms in summer.

There is a close relationship between surface isobars (lines joining points of equal air pressure) and wind speed and direction over open, level terrain. However, in upland areas such as Exmoor, local topography also has an effect, with winds tending to be aligned along valleys. Over land, the roughness of the ground causes a decrease in the mean wind speed compared with that which occurs over the sea, with the size of the decrease depending on the nature of the terrain. Local funnelling may occur and the wind can gust to about the same speed as in open country. It is this gustiness that causes damage to buildings and trees on really windy days.

In general, wind speed increases with height, with the strongest winds being observed over the summits of hills and mountains. The highest gust recorded at a low-level site: 103 knots (118 m.p.h.) was in Cornwall in December 1979. South-west England is one of the more exposed areas of the UK, with average wind speeds only greater in western Scotland. The strongest winds are associated with the passage of deep depressions close to or across the British Isles. The frequency and strength of depressions is greatest in the winter half of the year and this is when mean speeds and gusts are strongest. Inland areas have lower speeds, which also decrease to the north-east. Thus the Scilly Isles have an average of 24 days of gale per year, Plymouth has 16, Yeovilton has 7 and Bristol has 4. Wind speeds also tend to increase with increasing altitude. The highest parts of Exmoor have speeds similar to those on the Cornish coast.

There are local variations with warm and cool sea breezes, plus valley winds and föhn winds in the lee of upland areas. The sea breezes can create diurnal variations, with higher wind speeds in the afternoon, due to heating from the ground. Places sheltered by hills, or which are in extensively wooded areas have lower wind speeds and fewer days of gale. Coastal parts of Exmoor have a relatively high frequency of NE winds, reflecting the shelter afforded by the high plateau and sea cliffs. Visitors to Exmoor often assume that the coast is not the best place to go in stormy weather. The opposite is often the case, with the Coast Path offering sheltered walks. The A39 coast road, however, is another matter. Exmoor roads are generally sheltered by the high hedgebanks but fenced and unfenced parts of the A39 between Lynmouth and Porlock are subject to high cross winds and it is not uncommon for high-sided vehicles to be blown over.