Storms

Storms are atmospheric disturbances accompanied by continuons variety in the direction and force of the wind, and generally covering an elliptical or circular area. The overlapping of several storms sometimes occurs, the resultant storm spreading over an area of very irregular shape. The normal tendency to the circular form is, however, of great use in predictions of storms at sea and in weather forecasts. The area visited by a storm may vary from about 1,000 to 5,000 square miles. The direction in which a storm travels is probably controlled by the generally prevailing winds, with modifications introduced by irregularity of the conntries over which it passes, and by the occurrence of large expanses of water. In Europe a storm always has some easterly component, although it may vary between N.E. and S.E. On a few occasions when a storm has started in a westerly direction it has ultimately changed and followed an eastern course. In the Mediterranean Sea the storms are changeable in direction and confined to small areas; in the West Indies and in India they often trace out a parabolic path. The want of a sufficient number of meteorological observatories has prevented the deduction and verification of the laws governing the behaviour of storms, and the connection of the storms in different parts of the world is but very imperfectly understood. On the approach of a storm the barometer falls, clouds form, and the temperature rises, till rain announces the proximity of the storm's nucleus. The rain increases till the storm's centre has passed by, when its violence subsides, the temperature is lowered, and the clouds are dispersed. The rate at which storms proceed varies in different parts of the earth. An average of seventeen miles an hour has been deduced from European storms, twenty-eight for those in North America; while around India the speed is as low as nine miles an hour, and often much less. Certain storms have been known whose velocity was over seventy miles an hour, but such high speed is very rare. It was noted that low pressures precede storms, and it has been found that in the northern hemisphere winds circle round the points of lowest pressure in a direction contrary to the hands of a clock, while in the southern hemisphere their direction is clockwise. As well as this circular direction, however, there is some slight radial inclination towards the centre. The circular direction in which the winds blow has been proved to be connected with the rotation of the earth on its axis, and hence their origin may be compared with that of the trade-winds. The violence of the storm is naturally greater where the wind is strongest, and the violence of the wind depends on the difference of pressures of the places between which the wind is blowing. If there is great difference of pressure between near places, there is great force of wind, and the storm is intensified. Hence the arrangement of lines of equal pressure (or isobars) at any time determines the wind and weather at that time, and since the direction of the wind enormously affects the character of the season, it is at once seen how important a factor is the knowledge of the distribution of barometrical pressure in weather forecasts.