WEATHER LORE

MOUNTAIN WEATHER

Altitude has a marked impact on all elements of the weather. Take temperature for instance. Some parts of the Alps in Southern Europe, much further south and therefore closer to the equator than us, are permanently covered with snow and ice. Mont Blanc, the ‘white mountain’ and highest mountain in the Alps at 4810 metres or nearly 16000 feet in old money, is stark evidence of this.

Temperature falls by an average of 6.4 degrees Celsius for every rise in altitude of 1000 metres. So even on the equator mountains may be high enough and therefore cold enough to hold snow and ice on their summits.

While temperatures are lower than the surrounding lowlands, precipitation (rain and snow) in upland areas is always higher. This relief rainfall is caused by the air rising over mountains and cooling to produce cloud.

Odd things can happen to winds in mountains. During the day air rises up slopes, stimulated by the heat of the sun, but at night cold, heavy air can descend from upper slopes into valley bottoms. In these circumstances a weird phenomenon called an inversion can occur, as temperatures in the valley plummet way below those on the nearby uplands!

In Britain this effect can see cold air descending into a valley causing a ‘frost hollow’ marked by a pool of fog. In the industrial valleys of Lancashire and Yorkshire this fog was often added to by pollution from the woollen and cotton mills.

Aspect, the way a hillslope faces, can also have an important effect on temperature. In the northern hemisphere south facing slopes get the full intensity of the sun radiation concentrated on a small area. North facing slopes see much less of the sun and are consequently much cooler. In the Alps this can affect the location of villages, and the heights of the tree and snow lines. Even in this country fruit crops are most likely to be grown warmer south facing slopes.

The most predictable thing about mountain weather is its unpredictability! Often very different to the conditions found in nearby sheltered lowlands, temperatures, cloud cover, wind and precipitation can all change in minutes. There’s a great deal of truth in the old adage ‘mountains make their own weather’.