The New International Encyclopædia/Storm

STORM (AS. storm, OHG. sturm, Ger. Sturm, storm; probably connected with Gk. ὁρμή, hormē, attack, Skt. sar, to flow, hasten). Any unusual, severe, or destructive atmospheric phenomenon, such as a windstorm, sandstorm, tornado, typhoon, or 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 feature; a thunderstorm in which the thunder is impressive and the lightning is destructive; a blizzard, in which the combined 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 ‘lows.’ 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.

Fig. 1. CHART SHOWING STORM-PATHS.

Storms are often classified as attending areas of low pressure (cyclonic storms); or as attending areas of high pressure (anti-cyclonic). 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 does the lighter moist air. For both reasons, therefore, the lighter is raised up by the denser air, and overflows toward the pole. Now a 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 respectively, so that instead of meeting at the centre they whirl around it in an inflowing spiral curve. By reason of this circulation an outward centrifugal pressure is produced, and the barometric pressure 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 connected with the generation and maintenance of storms are set forth without technical mathematics by Prof. William Ferrel in his Popular 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 subject by all students during the past century will be found in the Lehrbuch der Meteorologie, by Prof. Dr. Julius Hann (Leipzig, 1901), of which an English translation is promised in the near future.

Fig. 2. TRACKS OF CENTRES OF LOW AREAS, JANUARY, 1901.

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 reference to their relations to the topography of the continents and the general distribution of the barometric pressure, temperature, and moisture, and especially to the so-called general circulation 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 showing 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 direction of motion of storm centres along the lines of greatest frequency, so far as now 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 general 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 rapidly 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 be 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.

The great irregularities in individual storm paths may be appreciated by studying the accompanying chart (Fig. 2) showing the tracks of all the centres of low pressure which passed over the United States during January, 1901. The storms in that month were unusually severe over the North Atlantic Ocean. Of the thirteen tracks that are here charted four moved with a velocity of over one thousand miles daily; one of them at a velocity of less than five hundred miles daily; and one was stationary for one day. Further details in regard to these storms are given in the text of the Monthly Weather Review for the month in question. More than one-half of the cyclonic storms that pass over the United States have been developed somewhere in the Pacific Ocean. Perhaps one-quarter originated within the United States. The others are first perceived off the coasts of the West Indies or on the Gulf of Mexico. The most severe storms are the hurricanes that begin in the tropical portion of the Atlantic, move westward and northward into the South Atlantic or Gulf States, then turn toward the northeast and disappear while still moving toward Europe. The paths pursued by general cyclonic storms are apparently determined by the so-called general circulation of the atmosphere, but are modified considerably by the formation of cloud and rain or snow attending the storm.

The most extensive condensed collection of data relative to American storms is found in the “Contributions to Meteorology,” by Professor Loomis, as revised and published in the Memoirs of the National Academy of Sciences, vols. iii., iv., and v. (Washington, 1885, 1887, and 1889). The details of current storm phenomena are published regularly by the Weather Bureau in the Monthly Weather Review. The physical-mathematical theories founded by Espy and Ferrel have been further developed in numerous memoirs, some of which will be found in a collection of translations published by the Smithsonian Institution in 1891. The current literature is contained in the successive volumes of the Meteorologische Zeitschrift (Berlin). A general summary of our knowledge of the mechanics of storms is given by Prof. F. H. Bigelow in his Report on International Cloud Work (Washington, 1900).