play in the making of upper air circulation. All of Western and Central Asia remains under the influence of westerly winds along the altitude of - km from west to east. These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan highlands (Figure . ).
These are known as jet streams. Tibetan highlands act as a barrier in the path of these jet streams. As a result, jet streams get bifurcated. One of its branches blows to the north of the Tibetan highlands, while the southern branch blows in an eastward direction, south of the Himalayas.
It has its mean position at °N in February at - mb level. It is believed that this southern branch of the jet stream exercises an important influence on the winter weather in India. Western Cyclonic Disturbance and Tropical Cyclones : The western cyclonic disturbances which enter the Indian subcontinent from the west and the northwest during the winter months, originate over the Mediterranean Sea and are Figure . : Direction of Winds in India in Winter at the Height of - km brought into India by the westerly jet stream.
An increase in the prevailing night temperature generally indicates an advance in the arrival of these cyclones disturbances. Tropical cyclones originate over the Bay of Bengal and the Indian ocean. These tropical cyclones have very high wind velocity and heavy rainfall and hit the Tamil Nadu, Andhra Pradesh and Orissa coast. Most of these cyclones are very destructive due to high wind velocity and torrential rain that accompanies it.
Have you seen their movement in the weather report in the television? Mechanism of Weather in the Summer Season Surface Pressure and Winds : As the summer sets in and the sun shifts northwards, the wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels. By the middle of July, the low pressure belt nearer the surface [termed as Inter Tropical Convergence Zone (ITCZ) ] shifts northwards, roughly parallel to the Himalayas between ° N and °