weather make aircraft operations above 20,000 feet difficult and oceasionally hazardous during this weather situation. The difference between this situation and trade weather, as far as winds ars concerned, is most marked in the atmosphere above 30,000 feet. The lower easterlies turn as before to westerlies, but these westerlies are stronger and form part of a very large cyclonic circulation which lies more or less stationary between the Eniwetok-Bikini area and Wake Island. The circulatory system is ‘known as an upper level tropical cyclone, Winds as high as 100 knots at 45,000 feet have been reported over the northern Mar~ shalls during such situations. Once established, an upper level cyclone may remain in situ for ten days before moving out into higher latitudes. Even though there may be temporary clearing for a day or two at some points in the area, the weather remains generally unsettled as long as the cyclone lies between Eniwetok and Wake, There may be a very general deterioration in the weather as the stationary cyclonic circulation intensifies and reaches lower latitudes, Following the movement of the upper level cyclone into higher latitudes, however, the trade situation will re-establish itself and this is often accompanied by sudden and dramatic in~ provements in the weather, C. Vortex. The third category of weather situation, the vortex, is characterized by winds in the lower atmosphere which fluctuate between northeast and southeast in the northern Marshalls and be- 168