Monsoon

The interplay of jet streams and monsoon winds is well illustrated in the Indian monsoon. During the early summer months, increased solar heating begins to heat the Indian subcontinent, which would tend to set up a monsoon circulation cell between southern Asia and the Indian Ocean. However, the subtropical jet stream occupies its winter position at about 30° north latitude, south of the Himalayan Mountains. As long as the subtropical jet blows over India, it inhibits the development of summer monsoon. As summer progresses, the subtropical jet slides northward. The extremely high Himalayas present an obstacle for the jet; it must "jump over" the mountains and reform over central Asia. When it finally does so, a summer monsoon cell develops, supported by the tropical jet stream overhead. The transition can be very fast—the Indian monsoon has a reputation for appearing suddenly as soon as the subtropical jet is out of the way. The retarding effect of the subtropical jet delays the Indian monsoon by up to one month compared with the rest of Asia.

During the Indian summer monsoon, winds blow from the southwest, bringing moist air on shore. As the air is forced to rise over the foothills of the Himalayas, it causes constant and frequently heavy rains. The town of Cherrapunji, India, located on the Himalayan slopes, receives an annual rainfall of over 36 ft (11 m), making it one of the wettest places on Earth.

The summer monsoon ends as the highlands of Tibet begin to cool in the fall. The cooling atmosphere over Tibet allows the subtropical jet to reappear south of the Himalayas, bringing the rainy summer monsoon to an end. The circulation shifts to a winter monsoon cell, with sinking air over India and surface winds that blow out to sea. The resulting winter weather is dry.