Such qualitative observations, though interesting and suggestive, are not wholly satisfactory. The same may be said of the study of air currents by aid of smoke from tall chimneys. The eddy about such columns may extend to a considerable height above them, and the wake is farreaching. The experiments would therefore best be made from high open-work towers above plane country or a broad sheet of water.
A better method perhaps would be to liberate a pilot balloon, or discharge a bomb giving a bright compact cloud, and to trace its path by means of two cameras, as it floats from point to point in the aërial current. The instruments, if suitably stationed, would give the continuous space history of the floating object; that is, its actual path and the speed at each part thereof, or, in other words, the magnitude and direction of the velocity at each point. But, of course, this method would not reveal the wind’s history at any given fixed point, as recorded by the anemograph above described.
Fig. 58.—Records of Wind Speed Obtained by Langley.
Fig. 58 is a typical wind-speed record obtained by Langley in January, 1893, by means of a very light cup anemometer mounted eleven feet above the north tower of the Smithsonian Institution, and 153 feet from the ground. The abscissæ represent time in minutes, the ordinates wind speed in miles per hour. The records were taken in cloudy weather and in a south-southeast wind. Other records were taken during the month of February, showing like deviations from the mean, though at times more pronounced; for Dr. Langley noted that “the higher the absolute velocity of the wind, the greater the relative fluctuations which occur in it.”
It will be observed from this record that, when the average speed was about twelve miles an hour, the extreme fluctuation was rarely one third greater or less than that, and on the average varied hardly one sixth. It must be further added that the air on approaching the anemometer had traversed a mile of the lower residential section of the city, then crossed the body of the Smithsonian building, which itself is half as high as the tower. It should be expected, therefore, that this wind was, other things equal, naturally more turbulent than if flowing in from a level plain. This surmise is justified by the more extensive records of wind speeds shown in meteorological records taken respectively in clear and in obstructed places. On the other hand, even in level places where no obstruction is visible for several miles, the wind, though it may be steady at one time, can at another time be gustier than that shown in Langley’s record, according to the state of the weather; for the gusts are not all due to neighboring obstacles, but may be transmitted from afar, even from the depths of the atmosphere.
Assuming the wind speed at any instant to vary by one sixth of the mean, its impactual pressure will then vary by thirty-six per cent of the pressure of the mean wind, remembering that the pressure varies as the square of the speed. This fluctuation of the impactual pressure tallies fairly well with that found by Professor Marvin at the top of Mount Washington, in 1890, by means of a pressure plate.[76] He found the variation to be approximately thirty-five per cent of the mean pressure. Professor Hazen, however, reports but little variation in the wind speed in the free atmosphere well above the earth. In several balloon ascensions he suspended from the basket a lead weight by means of a cord to which was looped the thread of a toy balloon. He found that the little balloon sometimes moved ahead if the weight sometimes followed it, but that in general the relative motion was very feeble, thus indicating that the fluctuations of the velocity in the depth of the atmosphere at those times were very slight.[77] However this be for such distances from the earth and its protuberances, the fluctuations of wind speed found at meteorological stations sufficiently resemble those reported by Dr. Langley. As corroborative evidence, the reader may be referred to the wind records published in the Interim Report for 1909, of the British Advisory Committee for Aëronautics.
Without the material evidence of commotion in the atmosphere, a moment’s reflection will make clear that such turmoil must exist, even over a vast, smooth plain, especially in bright weather, and more particularly over bare ground in dry weather. For it is well known that clear, dry air transmits radiation with very slight absorption, when the sun is well toward the zenith, and hence that the temperature in the depth of the atmosphere is but little changed from moment to moment, due to the passage of sunlight. At the earth’s surface, however, the air by contact with heating or cooling soil may change temperature rapidly. The direct sunlight falling perpendicularly upon a perfectly absorbent material transmits nearly two calories of heat per minute to each square centimeter of the receiving surface. It would, therefore, under favorable circumstances, elevate by nearly two degrees C. per minute a layer of water one centimeter deep, or a layer of air something over a hundred feet thick, if all the heat falling on the assumed surface were communicated to the neighboring air stratum. In practice, a large percentage of the incident sunlight is reflected and radiated by the soil, into sidereal space without heating the air. But every one per cent of it caught up by the air in contact with the earth is sufficient to heat a layer roughly one foot thick one degree per minute. Hence, unless the heated air streamed upward continually, the layer next the earth would quickly be raised to a very abnormal temperature, which would result in a violent uprush. The gradual ascension of the surface air may take place in large or small columns, or in both kinds at once. In either case, the composition of the ascensional motion with the general movement of the wind due to barometric gradient must cause gustiness and marked irregularity of speed and direction.
Various causes have been assigned for the gustiness of the winds. Ferrel and many other writers assume that the air, especially near the earth, is full of small vortices rotating about axes of various inclination. These whirls, on passing squarely across a weather vane, cause it to point one way for a moment, then presently the opposite way, while if they cross obliquely they cause a like sudden veering of the vane, but less extensive.
Helmholtz has proved that in the atmosphere strata of different densities come at regular intervals to be contiguous one above the other, and thus to beget conditions favorable to the formation of aërial waves, sometimes so large as to set the lower regions of air into violent commotion and thereby generate the so-called gusty weather. He has summarized as follows some of the important conclusions of his dynamic analysis.[78]