1. Introduction. A detailed paper describing the theory and calculation of equivalent head winds on air routes has been prepared by J. S. Sawyer and is being published as a Meteorological Report. The present paper is a simplified account which draws largely on Mr. Sawyer's paper.

When regular air services over a route are being planned, the wind at the chosen level of flight is an important factor. From the long term point of view, aircraft require to be designed so as to operate economically against adverse winds, if not on all occasions at least on all but a small percentage of occasions; for example, British Overseas Airways Corporation usually specify an 85% regularity of operation as a minimum. Similarly, for short term planning, wind information is necessary for the computations regarding the averages and extremes of fuel consumption, payload, time of flight, and so on. The aircraft operator consequently requires to know the frequency distribution of winds on any given route at any time of the year.