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[[underlined]] III - MONSOON SEASON WEATHER [[/underlined]]

1. [[underlined]] THE HUMP [[/underlined]]

a. During the month of May the pilot finds that he no longer has to worry about severe icing over the "hump" except in rare instances in thunderstorms. The westerly winds have died down to be replaced by winds of variable direction, fifteen to twenty miles per hour. Clouds follow a typical pattern. Early in the morning a low deck of stratocumulus or altocumulus clouds will form which can be topped at 16,000 to 17,000 feet. This formation may be broken up by the cumulus clouds which build up from 0900 hours IST on to reach 20,000 to 30,000 feet almost every afternoon. On the worst flying days these cumulus clouds will merge in the afternoon causing instrument flying weather the whole route and moderate to severe turbulence in a few scattered thunderstorms on the ridges. The towering cumulus clouds flatten out at night with a marked tendency for the "hump" to clear by midnight. By 0300 another overcast forms and the cycle continues. Most worthy of attention during this season are the afternoon thunderstorms over the first ridge and the main "hump". In these thunderstorms severe turbulence, ice and precipitation may be encountered. Usually it is possible to avoid thunderstorms since they seldom form in a line on the ridges, but it has been occasionally reported that thunderclouds were completely hidden by other clouds associated with them. Out of thunderstorms, rime ice has been the only type of wing ice encountered during the monsoon season. The icing level rises to 17,000 feet during May and reaches the average level of 18,000 feet during June. A noticeable fact is that the icing level may fluctuate from 18,000 feet to as low as 16,000 feet or to as high as 20,000 feet during relatively brief periods of time. The average icing level, however, is 18,000 feet.

2. [[underlined]] BRAHMAPUTRA VALLEY [[/underlined]]

a. During the monsoon season, June through September, in the valley, the thunderstorm pattern persists west of Chabua with the activity falling to a minimum four to eight thunderstorm days in August and increasing to a second maximum of eight to twelve days during the month of September. A violent thunderstorm closed Chabua field of three to four hours the morning of July 18, 1943, between the hours of 0200 and 0600 IST. Surface winds were as high as thirty-five miles per hour, out of the southwest. On the hills to the south and east thunderstorms form almost every evening during this season.

b. The normal daily monsoon season cloud pattern is similar to that over the "hump". Low stratocumulus decks form every morning about 0400 IST and cause rain which lasts anywhere from one to two hours on a good day to seven or eight hours on an average day of moderate to heavy rainfall. Ceilings drop with the rain to as low as 100 foot. Visibilities occasionally drop below two or three miles during brief periods of heavy rainfall. In these cases, landing and take-off visibilities are reduced to zero. The average ceiling on the monsoon mornings is no higher than 500 feet. Usually by noon the cloud overcast has lifted and broken up.

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(Page 4, ICW-ATC Cir #18, 26 March, 1944)

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