The information contained in this paper is intended to show the reliance which a pilot or air navigator can put on his expected winds in different parts of the world, in different seasons and at different heights. From the outset it must be realized that the figures given are subjective, and, in some regions at any rate, may need modification as facilities for obtaining upper air information are improved. However, it is not thought that such modification would be great and it would certainly not alter the broad pattern which is shown in the data presented here.