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Aviation History
1952
1952 - 0401.PDF
15 February 1952 179 ft 30.000 20,000 10,000 5.000 miles O 200 SEVERE FRONT 400 It 30.000 20,000 IO.OOO 5,000 cirrus formed from distant cumulonimbus tops scattered altocumulus ESE. lO kt. ^^^^ra^S^ ENETO kt. & A <s ci c2,^^ miles 6 100 200 WEAK FRONT 300 400 These typical north-south cross sections throughthe intertropical front are reproduced from the Air Ministry's Meteorological Reports No. 10 and with acknowledgement to the Director, Meteorological Office, Air Department, New Zealand. opposite. When, however, the intertropical front is farther from the equator than this, the trade-wind air stream which has crossed the equator is usually deflected so that it has a westerly component. This is because an air stream tends to acquire anticyclonic vorticity when moving away from the equator. The resulting belt of westerly winds between the intertropical front and the equator is some times known as "equatorial westerlies." It is noteworthy that over the western Atlantic the S.E. trade winds of the southern hemisphere extend as easterly winds up the coast of South America, sometimes beyond 10 deg N. The surface winds usually show definite convergence towards the intertropical front, and, although as in higher latitudes there is a greater tendency for the surface wind to blow across the isobars than for the wind at a few thousand feet to do so, it is probable that there is definite convergence towards the intertropical front at least up to 5,000ft above the surface. As indicated earlier the air masses which converge towards the intertropical front over the oceans are moist in the surface layers and relatively dry at higher levels; they also have a lapse-rate near, or slightly exceeding, the saturated adiabatic. When such air masses are slowly lifted by the general low-level convergence in the region of the intertropical front, instability develops and large cumulus clouds form, developing into cumulo-nimbus and giving rise to showers, squalls and thunderstorms. The intertropical front is thus primarily a band of convectional phenomena which varies in intensity and extent with the degree of convergence taking place. The tops of the cumulo-nimbus and cumulus clouds spread out at levels where the lapse-rate is more stable and give rise to extensive sheets of alto-stratus and cirro-stratus cloud. Precipitation may be found falling from the former over wide areas. Several such layers of medium and high cloud may exist. Characteristics—Continental Regions In those regions where the intertropical front forms the summer limit of the penetration of a monsoon current into a continent, its trade \ S.E.trad ;\.\5 W y S.E.trade wind & 5^*5*** // X ' ( [JANUARY, N.E.trade wind trade wind T . , . . INTERTROPICAL FRONT. OCEANIC TYPE MEAN LIMIT OF DEEP MONSOON AIR nil 11 INTERTROPICAL FRONT, CONTINENTAL TYPE Mean positions of the intertropical front and main surface airstreams in the tropics for the conventional summer and winter months.
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