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help in the orientation before the final heading into Kunming. During the winter it is sometimes possible to judge general location when there is an overcast by the rise and fall of the cloud tops over valleys and ridges. Frequently there are breaks in the clouds above significant rivers and lakes, but during the spring or summer, when there is considerable vertical development of clouds, the tops may seem to be at the same altitude the entire route.

3. Weather on the "hump" route can be conveniently broken up into four seasons: spring, summer, fall and winter. The spring season will refer generally to the months of February, March and April. The summer season includes the months May through September. Fall includes October and November and winter consists of the months December and January. The periods are not of equal length, but are so defined for the similarity of the weather during each. A particular date does not mark a definite change in the weather and characteristics described as typical of any period are likely to be found near the border of an adjacent period.

[[underlined]] II - SPRING SEASON WEATHER [[/underlined]]

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

a. During this season, strong southwest winds and increasing cloudiness as well as strong up and down drafts cause the worst flying weather of the year. There is considerable thunderstorm activity and clouds build up to heights of twenty-five and thirty thousand feet. Average cloud tops are above 20,000 feet during the late afternoon, with the highest cloud tops to the north and the lowest to the south over Myitkyina and the Hukon Valley. The freezing level remains fairly close to 15,000 feet. Icing above this level in clouds is frequently severe. The heaviest icing conditions are not in clouds at a temperature close to minus four degrees centigrade. Clear ice may be encountered at any temperature between plus two and minus ten degrees centigrade. Rime ice is generally more predominant at temperatures below minus ten, but at a temperature of minus four degrees centigrade both clear and rime ice may be severe. The more turbulent the air, the greater the chance of picking up either clear ice or severe rime ice. However, severe icing conditions on the route are usually limited to certain levels or areas. Experienced pilots know this and are prepared to make a 180 degree turn at a moments notice and enter the clouds again at a different point, usually at a higher altitude where temperatures are below-20 degrees centigrade. The southwest to west winds as strong as 100 milers per hour cause updrafts over the ridges and downdrafts over the valleys. Downdrafts have frequently been reported to cause planes to drop at the rate of 2,000 feet per minute and occasionally as much as 3,000 feet per minute. These drafts are strongest over the Salween and Mekong Rivers. Knowledge of downdraft intensity is most important when flying the route from east to west. The updrafts over the easterly ridges is small compared to the downdrafts over the valleys and it is possible for a plane to lost two or three thousand feet of altitude over the valleys before it reaches the updrafts over the western ridges. In the past pilots have found the best policy to be a flight level above the turbulence. Flying from west to east, the strong updrafts are first encountered and act

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(Page 2, ICW-ATC Cir #18, 26 March, 1944)

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