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over the "hump". Any altitude where the temperature is between plus two and minus ten degrees centigrade is a potential region of severe ice in clouds. During most of this period the three to four day cloud outbreaks can be flown over at eighteen to twenty thousand feet. However, there may be overcasts up to twenty-five and thirty thousand feet during the latter part of January.

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

[[underlined]] a. [[/underlined]] December and January in the Brahmaputra valley are the fog months of the year. There are an average twenty heavy fog days per month. Rainfall drops off to a low average of two rainy days per month. Bad weather associated with cold fronts breaking on the "hump" may cause severe icing conditions over the valley fields, when the icing level is at 11,000 to 12,000 feet and the clouds may extend up to 20,000 feet. The lowest ground fog visibilities occur between mid-night and mid-morning. There may be visibilities of a half mile and less all during this period. On an average fog morning, low visibilities last from four to eight AM and during occasional foggy spells may last as late as nine and ten o'clock. Frequently ground fog visibilities very considerably due to light winds aloft which keep the thin layer of ground fog moving. The forecaster can do fairly well in predicting fog by noticing carefully the cloud cover, amount of moisture in the air and amount of fog on the preceding day. Luckily, rain and fog are seldom combined. Heavy fog may set in after rain stops and after the overcast clears in the evening, but when there is rain, visibilities during the winter are seldom less than four or five miles. During a few days of these two months all valley fields may record visibilities less than a half mile, but usually at least one field in the valley is open. An exception to rule of good visibilities during winter rain occurred on January 25 and 31, 1944, when there were ceilings as low as 100 feet and visibilities as low as one mile in rain at several valley fields, off and on. During these two days at least one of the valley fields was open for landings all the time.

[[underlined]] b. [[/underlined]] On the morning of January 1, 1944, a typical heavy fog morning, all valley fields were closed except Tezpur. On the following morning, Tozpur was still open except for one hour and during that hour Mohanbari weather station recorded a visibility of two miles. It is a rare occurrence when all valley fields are closed due to fog and the recent development of weather reports distributed within fifteen to twenty minutes after the hour will solve any other fog problems. When the fog is heavy, Sookerating and Jorhat are usually closed. As the fog first sets in and as it begins to break, Dinjan, Chabua, and Mohanbari may be alternately open and closed as the patches of fog shift with the upper winds. Tezpur remains open more than any other station in the valley, and Dergaon is frequently open when Jorhat is closed.

[[underlined]] c. [[/underlined]] February in the valley is a transition month between winter and spring. There is an increase from the low winter average to an average ten to fifteen rainy days. Foggy days decrease from more than twenty to an average six. The number of thunderstorms increases slightly from one day in January to an average three in February.

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(Page 7, ICW-ATC Cir #18, 26 March, 1944)

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