[Illustration: DIURNAL TEMPERATURES AT DIFFERENT HEIGHTS (From Rotch's _The Conquest of the Air_)]
Every locality has its so-called "prevailing winds." Considering the compass as having eight points, one of those points may describe as many as 40% of all the winds at a given place. The direction of prevalence varies with the season. The range of wind velocities is also a matter of local peculiarity. In Paris, the wind speed exceeds thirty-four miles per hour on only sixty-eight days in the average year, and exceeds fifty-four miles on only fifteen days. Observations at Boston show that the velocity of the wind exceeds twenty miles per hour on half the days in winter and on only one-sixth the days in summer. Our largest present dirigible balloons have independent speeds of about thirty-four miles per hour and are therefore available (at some degree of effectiveness) for nearly ten months of the year, in the vicinity of Paris. In a region of low wind velocities--like western Washington, in this country--they would be available a much greater proportion of the time. To make the dirigible able to at least move nearly every day in the average year--in Paris--it must be given a speed of about fifty-five miles per hour.
Figures as to wind velocity mean little to one unaccustomed to using them. A five-mile breeze is just "pleasant." Twelve miles means a brisk gale. Thirty miles is a high wind: fifty miles a serious storm (these are the winds the aviator constantly meets): one hundred miles is perhaps about the maximum hurricane velocity.
As we ascend from the surface of the earth, the wind velocity steadily increases; and the excess velocity of winter winds over summer winds is as steadily augmented. Thus, Professor Rotch found the following variations:
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.