Inversion of Temperature
Under normal conditions, the temperature of the atmosphere falls with altitude. But there are some special conditions under which the atmospheric temperature increases instead of decreasing with height. This rise of temperature with height is known as inversion of temperature. It is clear that in case of inversion of temperature, the air near the earth’s surface is cold while higher above it is warm. Conditions which favour inversion of temperature are long night, clear sky, stable weather, dry air and ice cover. It can occur in a number of situations but it is most characteristic of mountain valleys. During long winter nights, the air on higher slopes cools down quickly and becomes dense. It moves down the slope and settles down on the valley bottom by pushing up the comparatively warmer air.
Sometimes, the temperature of the air at the valley bottom falls below freezing point, whereas the air at higher altitude remains comparatively warm.This is known as ‘Air Drainage Temperature Inversion’. As a result of temperature inversion in the valleys, the trees are frost-bitten along the lower slopes, whereas those at higher levels are free from it. Air pollutants such as dust particles and smoke do not disperse in the valley bottoms. It is because of these reasons, houses and farms in inter montane valleys are generally situated along the upper slopes, avoiding the cold and foggy valley bottoms. For example, mulberry planters in the Suwa Basin of Japan and apple growers in the mountain states of the Himalayas avoid lower slopes.
Similarly, the hotels at holiday resorts in the Himalayas are built on the upper slopes. In areas where there is intense cooling due to rapid radiation from the earth’s surface air close to the surface becomes very cold while air at higher elevation may still remain warmer. This type of inversion is common in plain areas during winter season and is called radiation inversion. Inversion of temperature also occurs in area where warmer air blows over a colder surface such as snow covered area. This is known as advection inversion. Another type of temperature inversion when a warm air mass rises over a cold air mass is known as frontal inversion. This type of inversion occurs in the mid latitudes where cold polar air mixes with warm subtropical air masses. Lots of fog is generated when cold and warm air masses meet and lead to inversion of temperature.