The vertical sun-rays shift toward north of Equator in this season. The direction of winds changes and becomes opposite to that in the winter. The main factors of climate are the following:
(i) Position of Equatorial Trough.
It is an area of low pressure which is usually stationed in north India and Pakistan. It is in fact the Zone of Contact of summer. It is also called Inter Tropical Convergence Zone.
Upto the middle of July it remains around 25°N. It is an area of low pressure. It attracts monsoon winds from the south to north.
(ii) Jet Stream.
By this time the southern jet stream shifts towards north. The scholars believe that the effect of the northern shift of this trough appears in the attraction of south-western monsoons towards the equatorial trough.
It is also understood that the sudden rush of the monsoons towards the north is due to the combined, effect of the trough and the jet stream.
The south-westerlies which are attracted from the south towards this trough are called south-western monsoons. These monsoon winds provide heavy rainfall to India.
(iii) The Formation of Two Depressions on one another.
In June and July, an area of low pressure (Depression) is created in the north-west part of Indian subcontinent. The cause is the intense heat in this season. Hence this depression is called Thermal Depression.
Due to the north-ward shift of the jet stream another depression is formed in the upper air. It is called Dynamic Depression. Whenever a dynamic depression is created above the thermal depression of the surface an intense attraction is produced and they attract the monsoon winds with great force.
This is why the monsoon winds on being vigorously pushed provide heavy rainfall. This phenomenon is called Burst of Monsoon. As long as the attraction is due to only one depression, the incursion of monsoon is ordinary. How this phenomenon of two depressions takes place is still not clear.
(iii) Equatorial Westerlies.
Some scholars think that there are westerly winds in the Equatorial Trough. When the sun shifts towards north in the summer season, this Equatorial Trough also shifts towards north.
These westerly winds, according to the Ferrel’s law become South Westerlies. These are considered to be the part of southwest monsoons.
(iv) The Cyclones.
These are low pressure areas of hot tropical zone. In the way in which the jet stream from the west enters the upper air, an easterly jet is formed in summer season. This pushes tropical cyclone towards India.
This increases rainfall in a substantive way. It has been found that the highest rainfall takes place in the route of this cyclone.
On the intensity and the frequency of these cyclones depend the amount of rainfall and the dry gap interval between them.
(v) The Effect of the Radiation Window.
The Tibetan highland covers an area of 4-5 million sq km and its average height is 4000 m. When the vertical rays of the sun shift towards north in the summer season this Tibetan Highland becomes very hot.
There is also a lot of radiation on this highland because the air density is low. It appears as if this area has become a Radiation Window. The air above it begins to move in a clockwise direction due to the intense radiation.
In this way two currents of winds emanate from this window in opposite directions. One air current moves towards equator.
It replaces the wind which crosses the equator at the surface. These winds in India are called Easterly Jet Stream and that moving towards the Pole is called Westerly Jet Stream. The Easterly Jet Stream pushes the cyclones in India.