Altitude in Feet Annual Average Wind Velocity, Feet per Second 656 23.15 1,800 32.10 3,280 35. 8,190 41. 11,440 50.8 17,680 81.7 20,970 89. 31,100 117.5 Average Wind Velocities, Altitude in Feet Feet per Second Summer Winter 656 to 3,280 24.55 28.80 3,280 to 9,810 26.85 48.17 9,810 to 16,400 34.65 71.00 16,400 to 22,950 62.60 161.5 22,950 to 29,500 77.00 177.0 [Illustration: DIURNAL TEMPERATURES AT DIFFERENT HEIGHTS] These results are shown in a more striking way by the chart. At a five or six mile height, double-barreled hurricanes at speeds exceeding 200 miles per hour are not merely possible; they are part of the regular order of things, during the winter months. The winds of the upper air, though vastly more powerful, are far less irregular than those near the surface: and the directions of prevailing winds are changed. If 50% of the winds, at a given location on the surface, are from the southwest, then at as moderate an elevation as even 1000 feet, the prevailing direction will cease to be from southwest; it may become from west-southwest; and the proportion of total winds coming from this direction will not be 50%. These factors are represented in meteorological papers by what is known as the _wind rose_. From the samples shown, we may note that 40% of the surface winds at Mount Weather are from the northwest; while at some elevation not stated the most prevalent of the winds (22% of the total) are westerly. The direction of prevalence has changed through one-eighth of the possible circle, and in a _counter-clockwise_ direction. This is contrary to the usual variation described by the so-called Broun's Law, which asserts that as we ascend the direction of prevalence rotates around the circle like the hands of a watch; being, say, from northwest at the surface, from north at some elevation, from northeast at a still higher elevation, and so on. At a great height, the change in direction may become total: that is, the high altitude winds blow in the exactly opposite direction to that of the surface winds. In the temperate regions, most of the high altitude winds are from the west: in the tropics, the surface winds blow _toward_ the west and toward the equator; being northeasterly in the northern hemisphere and southeasterly in the southern: and there are undoubtedly equally prevalent high-altitude counter-trades. [Illustration: THE WIND ROSE FOR MOUNT WEATHER, VA. (From the _Bulletin_ of the Mount Weather Observatory, II, 6)]