A very strong and steady westerly wind\xa0blowing just below the tropopause.These are a narrow belt of high altitude (above 12,000 m) westerly winds in the troposphere.\xa0Their speed varies from about 110 km/h in summer to about 184 km/h in winter.A number of separate jet streams have been identified. The most constant are the mid-latitude and the sub tropical jet stream.These jet streams are located approximately over 27°-30° north latitude, therefore, they are known as subtropical westerly jet streams. Over India, these jet streams blow south of the\xa0Himalayas, all through the year except in summer. The western cyclonic disturbances experienced in the north and north-western parts of the country are brought in by this westerly flow.