Long winter night, clear sky, dry air and absence of winds leads to quick radiation of heat from the earth’s surface, as well as from the lower layers of the atmosphere.
This results in the cooling of the air near the earth’s surface.
The upper layers which lose their heat not so quickly are comparatively warm.
Hence the normal condition, in which temperature decreases with increasing height, is reversed.
The cooler air is nearer the earth and the warmer air is aloft.
In other words, temperature increases with increasing height temporarily or locally.
This phenomenon is termed as inversion of temperature.
Sometimes the cold and dense air remains near the surface for number of days.
So the phenomenon of inversion of temperature is also seen for days together.
The phenomenon of inversion of temperature is especially observed in intermontane valleys.
During winters the mountain slopes cool very rapidly due to the quick radiation of heat.
The air resting above them also becomes cold and its density increases.
Hence, it moves down the slopes and settles down in the valleys.
This air pushes the comparatively warmer air of valleys upwards and leads to the phenomenon of inversion of temperature.
Sometimes the temperature falls below freezing point in the valleys leading even to the occurrence of frost.
In contrast, the higher slopes remain comparatively warmer.
That is why mulberry planters of the Suwa Basin of Japan and the apple growers of Himachal Pardesh avoid the lower slopes of the mountains to escape winters frost.