STORM (AS. storm, 011G. storm, Ger. Sturm, storm; probably connected with Gk. Oppii, attack, Skt. sac, to flow, hasten). Any unusual, severe, or destructive atmospheric phenomenon, such as a windstorm, sandstorm, tornado, ty phoon, o• hurricane, in which the wind is the destructive agent; a rainstorm, hailstorm, or snowstorm, monsoon, cloudburst, or flood, in which the precipitation is the prominent fea ture; a thunderstorm in which the thunder is impressive and the lightning is de structive; a blizzard, in which the com bined cold wind and snow is the prominent feature. All these storms attend the flow of air from areas of high to those of low barometric pressure, or so-called storm centres or low s.' In general the winds blow around and in toward these low areas, thereby producing still lower barometric pressures near the centre. The lower air, being forced to rise above the earth's surface, expands, cools, and precipitates its excess of moisture, thereby giving us rain, snow, or hail.
Storms are often classified as attending areas of low pressure (eyelonie storms) : or as attend ing areas of high pressure (anti-cyclonie). In all cases the flow of the air is primarily due to differences of density: the denser air is impelled to the earth's surface and is also pushed toward the equator by gravity. But the centrifugal force due to the diurnal rotation of the earth also pushes the denser air toward the equator harder than it doe the lighter moist air. For both reasons, therefore. the lighter is raised up by the denser air, and overflows toward the pole. NOW :1 body on the earth's surface and in motion relative to it, while at the same time rotating with it, will appear to an observer on the earth to be deflected toward the right hand as it moves forward in the Northern Hemisphere, but to the left hand in the Southern Hemisphere. By virtue of this deflection the winds that are blowing toward a region of low pressure acquire, each for itself, a deflection toward the right or the left respec tively, so that instead of meeting at the centre they whirl around it in an inflowing spiral curve. By reason of this circulation an outward centrif ugal pressure is produced, and the barometric in the central region is much smaller than it would be if the winds flowed directly to the centre, without any spiral circulation. In extensive storm areas this general tendency of the lower winds to circulate around a centre may exist over a region a thousand miles or even more in diameter.
The mechanical and thermal problems con nected with the and maintenance of storms are set without technical mathe matics by Prof. William Ferrel in his Popu lar Treatise on the Winds (1st ed., New York, 1889). They are discussed most elaborately in a technical manner by Prof. F. H. Bigelow in his Report on International Cloud Work (published as vol, ii. of the Report of the Chief of the
Weather Bureau for 1898). A full presentation of the work that has been done upon this sub ject by all students during the past century will be found in the Lehrbuch der Meteorologic, by Prof. Dr. Julius Hann (Leipzig, 1901), of which an English translation is promised in the near future.
As far as possible the paths pursued by storm centres during past years are platted upon charts, then they are classified according to their general characteristics and studied with refer ence to their relations to the topography of the continents and the general distribution of llie barometric pressure, temperature, and moisture, and especially to the so-called general circula tion of the atmosphere. The frequency of storms and the general types of storm paths are, of course, best known for the continental portions of the Northern Hemisphere, but there is also sufficient data for the oceanic regions to justify an attempt at presenting the accompanying sketch chart of the Northern Hemisphere show ing the general character of the storm paths and the locations where they most frequently occur. (See Fig. 1.) This chart shows by its numerous lines and arrow heads the occurrence and the di rection of motion of storm centres along the lines of greatest frequency, so far as 1101V known. The Arctic region. Northern Africa, and Central Asia must be considered as blanks; we have no daily maps for these regions, and only know that gen eral cyclonic storms are infrequent in Africa and Asia. The chart shows that storm centres move slowly westward when within the tropics and also slowly toward the pole. but move more rap idly eastward between the tropics and the Arctic Circle, as well as more rapidly northward. The zone of greatest storm frequency lies between latitudes 45° and 55°. In general the path of any observed storm may he predicted on the basis of a simple study of this chart of storm tracks. But individual temporary departures from average conditions are so great that in actual weather forecasts it is necessary to allow these general maps of types and averages to have only a very slight influence upon the work. It is, in fact, always necessary to consider what the special individual storm has been doing during the preceding few days, and to what extent it is normal, and to what extent it is abnormal, as to the direction and velocity of its motion, and the rate of its increase or decrease in intensity. This can only be done by a careful comparison of several successive weather maps. As these maps are made up at least twice a day and sometimes more frequently, the forecaster is in a position to say how fast the storm is moving and whether it is growing more intense or rapidly dying away.