Explanation: Mechanism of Weather in Summer Season

Surface Pressure and Winds

• The wind circulation over the subcontinent experiences a complete reversal when the sun moves northwards at both the lower and upper altitudes
• By the middle of July, the low-pressure belt near the surface [known as the Inter-Tropical Convergence Zone (ITCZ)] had shifted northwards, nearly parallel to the Himalayas between 20° and 25° north latitude
• The westerly jet stream had left the Indian subcontinent at this time
• The northward shift of the equatorial trough (ITCZ) and the withdrawal of the westerly jet stream from across the North Indian Plain are intertwined
• ITCZ is a low-pressure zone that attracts marine tropical air mass from the southern hemisphere, which rushes to the low-pressure area in a generally south-westerly direction after crossing the equator
• The southwest monsoon is the term used to describe this wet air circulation
• The troposphere is the only place where pressure and winds are formed
• In June, an easterly jet stream passes across the southern tip of the Peninsula
• It is restricted to 15oN latitudes in August and up to 22o N latitudes in September
• In the upper atmosphere, easterlies generally do not extend north of 30o N latitude

Easterly Jet Stream and Tropical Cyclones

• Tropical depressions are steered into India by the easterly jet stream
• These depressions influence the distribution of monsoon rainfall throughout the Indian subcontinent
• The areas of maximum rain in India are the tracks of these depressions
• The regularity with which these depressions pass across India and their direction and severity all affect the rainfall pattern during the southwest monsoon season

Source: Class 11th NCERT