This concept was proposed by a German meteorologist R. Scherhag in 1948. According to him the changes in the direction of winds at all levels in the atmosphere are directly related to the temperature changes in the air above the friction layer. But the phenomenon of upper-air monsoon is not as localised in the free air as it is on the surface because it is mostly controlled by the annual oscillation of temperature.
The continents, in particular, are more affected by this monsoon although it appears to be a general phenomenon over the entire earth.
According to Pierre Pedelaborde a distinct monsoonal circulation exists in upper atmosphere at an altitude of about 21 km (lower stratosphere). In winter the westerly’s blow around the cold polar depression almost in all latitudes. In the summer, on the other hand, there is gradual fall in temperature and pressure from the pole to equator ward which allows the easterly winds to blow around the warm northern anticyclones.
Another monsoon circulation is observed in the upper parts of the troposphere which lacks complete reversal of zonal flow over the earth. The monsoon circulation in the free troposphere is quite opposed to the thermal monsoon of the surface. It is borne out by the existence of a depression over the surface winter anticyclones formed over the Indian sub-continent. Similarly, the hot-season low over the Punjab is surmounted by an anticyclone in the upper troposphere.
Although both these monsoons follow the same rhythm, the processes governing them are entirely different. The monsoon of the free atmosphere influences the monsoon circulation in the surface layers by superimposing a strong and very different type of circulation above it (Lai, D.S. 1986. pp. 1 18-119).