_Thunderstorms_ occur chiefly in warm climates and during the warm season in temperate climates, but they are by no means unknown in the polar regions. They are characterized by rapidly rising air currents, which may be either incidental to the circulation of a low, or due to local overheating of the lower atmosphere. In the former case they are called “cyclonic thunderstorms,” and in the latter “heat thunderstorms.” This is only a rough classification, however. Some thunderstorms partake of the features of both these types, and, on the other hand, additional classes are distinguished by many authorities. It is a common occurrence for thunderstorm conditions, starting in some small area, to travel across country at a speed of perhaps thirty or forty miles an hour, at the same time spreading out fanwise until the front of the storm is hundreds of miles in length. This front constitutes a “line squall” (so called from the long line or apparent arch of dark cloud that marks its location), and is attended by more or less thunder and lightning, but is not necessarily a continuous thunderstorm. The characteristic wind of a thunderstorm is the squall that rushes out in front of the storm when close at hand. This blast of wind, lightning, hail and torrential rain are all agencies of destruction in severe thunderstorms. [Illustration: IDEAL CROSS SECTION OF A TYPICAL THUNDERSTORM _A_, ascending air; _D_, descending air; _C_, storm collar; _D’_, wind gust; _H_, hail; _T_, thunderheads; _R_, primary rain; _R’_, secondary rain. (W. J. Humphreys.)] Concerning the winds of the globe in general and the remarkable atmospheric interchanges that they involve, Sir Napier Shaw writes: “Of the millions of tons of air which form the atmosphere nearly the whole is moving. The regions of calm at the surface at any one time, taken all together, do not form a large part of the earth’s surface, and above the surface calm regions are still rarer. Let us remember that the motion of the air is always ‘circulation’; air cannot move forward or backward or upward or downward without displacing other air in front of it and being replaced by other air behind it, though the circulation may be quite local and limited in extent, as is frequently the case when warm air rises or cold air sinks. In the course of investigations into the life history of surface air currents in the Meteorological Office we have traced air over long stretches of the surface of the Atlantic. We have found, on one occasion, the shores of Greenland to be fed with air that left the middle of the Atlantic four days previously, while in the course of six days air traveled from Spitsbergen to join the northeast trade wind off the west coast of Africa. On another occasion the air that formed the wind off the south of Ireland was traced back to the north of Africa, but that which blew at the opening of the Channel two days later came from Hudson Bay, via the Azores.”