FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1919
1919 - 0603.PDF
The former is necessary when planning out standard air routes with their chains of landing grounds, and the latter to enable the navigator to select the best altitude for flight and the probable time table for his journey. Forecasting surface winds is, however, a difficult matter, and it may be that the winds of the upper air will prove as little tractable. It happens, fortunately, how ever, that there has, in the last few years, been a great increase in knowledge of the physics of the atmosphere, due in no small measure in this country to the distinguished work of Shaw, Taylor, Gold and Dobson. We may hope, therefore, that before long the science of meteorology may prove equal even to G the considerable demands Btaion)' of the air navigator and his urgent needs. The ability to predict the upper wind depends upon a knowledge of the pressure and temperature changes in the atmo sphere. A few words may perhaps be appro priately inserted here to indicate the method by which this information is utilised. They will indi cate the need for iurther development work in this direction. A knowledge of the barometric pres sure throughout a given area enables a series of equal pressure lines, or isobars, .to be drawn. The pressure gradient across these lines enables the resulting wind to be calculated ; this is known as the gradient wind. The gradient wind is made up of two parts, the geo- strophic component and the cyclostrophic com ponent ; the former, which is the more important, is due to the earth's rotation and the latter to the curvature of the isobars. The origin and nature of the geostrophic wind cannot be better described than in the following extract from the official " Barometer Manual," which I take the liberty to quote :— " Let us consider the case of an arctic bird, or an aero plane, that starts from some point on the parallel of 84 degs. N., and makes a bee-line for the pole (supposed in sight for the purpose of keeping a straight course), and keeps ' straight on ' beyond it, flying at 60 nautical miles an hour. It will reach the pole by direct line after six hours' flight, and at the end of 12 hours will have done a journey of 720 miles and have got to latitude 84 degs. again, and meanwhile the place from which it started will have come round with the earth and have made the journey of about 1,130 miles, and the pilot who has made a straight course will find himself at home again. If he had marked his trail by dropping bombs at intervals or in some other effective manner, he would have provided conclusive evidence that he never ' set out ' for the pole at all, but after once getting up speed he made off at a great pace to some point about west-north west, gradually slackened his speed and got drifted towards the pole, and when he arrived there turned slowly round and came back to where he started from and arrived at the starting point again from the east-north-east. Whenever anything flies or floats in the air, as airships or winds do, the rotation of the earth has to be reckoned with, and its effect is to turn the course of a body that is left to its own momentum at the rate of 15 degs. x sin A per hour, where X is the latitude. On an earth that does not rotate, a body that is left to its own momentum keeps in a vertical plane and moves along a great circle. If the earth rotates the moving body left to its own momentum is diverted from the great circle at the rate of 15 degs. sin A per hour, to the right in the Northern Hemi sphere and to the left in the Southern. If, on the other hand, it is to be kept in the great circle it has to be pushed from the side on which it would be left behind by the rotation of the earth underneath it. The push must always be at right angles to the direction of the motion, otherwise it would do more than alter the course, it would accelerate or retard Nofc lap dnff n-q\2 • Fig. 9. T+Ti the velocity, and that is not wanted. With a current of air in the free atmosphere we get, so far as we are able to tell, exactly the conditions required; the pressure difference on the two sides of a moving stream of air is always at right angles to its motion, and just provides the push necessary to steer the air with no appreciable effect upon the speed. We get a proper balance, and the air is moved under its own momentum without being diverted from its path along a great circle if the push represented by the pressure gradient y is balanced by a speed of motion v such that y = 2wvp sin A, when A is the latitude, a> the angular velocity of the earth's rotation, p the density of the moving air." This formula allows the geostrophic wind to be calculated. Careful researches along these lines have brought to light a surprising and most fortunate closeness of connection between the geostrophic wind and the actual wind. It is found in fact that the two may without grave error be regarded as substantially identical. Sir Napier Shaw * remarks on this :— " To assume that this balance of wind and pressure in the upper air is an operative principle of atmospheric structure may be thought a hazardous mode of procedure, and it requires the most scrupulous examination, but the proper course seems to be to accept it, at least until the proved exceptions are numerous enough to show that, under the prescribed conditions of motion approximately in a great circle, finite differences of pressure do exist in the air without the compensating velocity in the air currents. It need not be supposed that the balance is always strictly perfect, but only that in ordinary circumstances the accelerating forces operating in the air are so small in relation to the pressures that we measure, that they are beyond our powers of observation." Shaw also points out that in ordinary circumstances there is a deviation of some 20 to 30 degs. between the direction of the surface wind and that of the geostrophic wind, due to surface friction and in the direction which that friction would indicate. Dobson has shown that although the geostrophic velocity may be arrived at within 1,000 ft. of the earth, the calculated direction will not usually be obtained till 2,500 ft. ; it follows, therefore, that if the surface friction effects, always local and uncertain, are to be avoided, as accuracy in air navigation requires that they should, flight at a lower height than 2,000 or 3,000 ft. over land is un desirable. The effect is much less over the sea, and the equivalent heights would be lower. As a general rule, the velocity of the surface wind over the sea is one-third less EquofiOfi to cu.vr A£C r-i ' a'ce-n) if'<«-i' wh«>« ft • Wino-snwa OlT-SOttO Beocon Fig. 10. than it would be were there no surface friction : and over the land two-thirds less than it would otherwise be. In the absence of surface friction these velocities would each be that of the . gradient wind. This loss of velocity of 33 per cent, over the sea involves the loss of about half the wind's kinetic energy ; the missing half is found in the energy of the ocean billows. Fog is so much the worst enemy the air navigator has * " Manual of Meteorology." 603
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
