At the present time very little can be claimed to be known as to the effects of weather upon farm crops. The obscurity of the subject, in spite of its immense importance to a great national industry, may be ascribed partly to the inherent complexity of the problem which it presents, and more especially to the lack of quantitative data obtained either under experimental or under industrial conditions, by the study of which accurate knowledge alone can be acquired. Of the industrial applications of such knowledge it is unnecessary to speak here in detail. It is sufficient to indicate that the present system of Life Insurance, which safeguards the economic stability of many thousands of families, and occupies the activities of many of the greatest financial corporations, was made possible by the studies of statistics of human mortality by the mathematicians of the eighteenth and nineteenth centuries, and that on the basis of adequate knowledge similar economic stability with its attendant security of capital should be within the reach of the industrial farmer. The inherent complexity of the relationships which it is sought to elucidate, between the yields of farm crops, and the previous weather which largely controls those yields, arises primarily from the complexity of the problem of specifying the weather itself. Meteorologists have, however, gradually devised a number of instrumental observations, which although far from specifying the total environment of the growing plant, as understood by the plant physiologist, do nevertheless give a sufficiently detailed account of the general environmental conditions of the growing crop, in so far as these vary from season to season. It is probable indeed that almost all the weather influences to which crop variations are due could be expressed in terms of the instrumental observations of a modern meteorological station. The actual difficulty of calculating the crop variations from given instrumental records is, however, immense; only an attack of the most preliminary kind upon the general problem can be attempted in this paper. The complete aim of agricultural meteorology should, however, be emphasised, for it is only by its substantial achievement that other causes of crop variation can be freed from much obscurity.