Within recent years the whole subject of meteorology has undergone a revolutionary change in outlook and method, and undoubtedly the fuller understanding of atmospheric processes which the new outlook has placed at our disposal will, in due course, shed much light on the study of climates. The revolution began with the work of Professor Bjerknes in Norway during the war of 1914 to 1918, and has been powerfully accelerated by the demands for accurate weather forecasting for civil and service flying since then. Stated briefly the new outlook consists in the recognition of the fact that the general circulation of the atmosphere produces in certain places large masses of air with characteristic and well-defined physical conditions, especially of temperature and humidity. The strongly marked individual peculiarities of these air masses are acquired by sufficiently long residence in one place for the lower layers to acquire the physical characteristics of the surface on which they rest, and from these lower layers there takes place a steady and progressive transmission of characters to greater heights. The process may be completed in a few days, but the stay may often be longer than this. Eventually the air body becomes characterized by a considerable degree of uniformity of conditions horizontally and a clearly marked transition of characteristics vertically. The prerequisite condition for the assumption of such characteristics is that of stagnant or outward spreading air, for under these circumstances there can be no disturbance by the introduction of alien elements by air from outside. Thus the sources of air masses are the great permanent or semi-permanent anticyclones of the earth’s circulation. Conversely, areas of low pressure are regions of convergent air masses, which, if they are derived from source-regions with widely different temperatures or humidities may, on meeting, be strongly contrasted in these respects. Such boundary zones between air masses are often very narrow and precisely defined; the transition from one to the other is abrupt, and strong disturbances are likely to develop where they are in contact, giving rise to ‘fronts’.