The subtropical westerly jet stream moves towards the north of the Himalayas with the apparent movement of the sun in _________.
a: Winters
b: Advancing monsoon
c: Retreating monsoon
d: Summers

Hint: Narrow bands of very strong wind in the upper atmosphere are called jet streams. The direction of wind in jet streams is from west to east. Sometimes their flow shifts to north and south.

Complete Answer: The jet stream is a high altitude wind. It blows horizontally through the upper part of the atmosphere. Generally its direction is from west to east at the height of 20000 to 50000 feet. The place where air masses of different temperatures meet produce jet streams. Generally jet streams are formed according to the surface temperature. The velocity of the wind in the jet streams increases with the greater difference in the temperatures. The extension of the jet streams is from 20 degree latitude to the poles.
There are three types of jet streams namely subtropical jet stream, tropical easterly jet stream and polar night jet stream.
Tropical easterly jet streams take place near the tropopause. Tropical easterly jet stream contains a deep layer of warm air to the northern part of the jet stream and colder air to the southern part over Indian Ocean.
Polar night jet streams are present in the upper part of the stratosphere over the poles.
Subtropical jet streams are mostly developed in winters and early seasons of spring.
The winter monsoon is related to subtropical westerly jet streams. These streams blow between 20 degree to 35 degree latitude. Subtropical westerly jet streams blow south to the Himalaya mountain throughout the year except in the summer season. The subtropical westerly jet stream moves towards the north of the Himalayas with the apparent movement of the sun in summers.
Option d is the correct answer.

Note: Jet streams play a significant role in the coming and going of the monsoon. The intensity of the high pressure centre at the pole increases during winters which intensify the formation of jet streams.