STORM SYSTEMS.

Auckland Star, Volume LXIV, Issue 180, 2 August 1933, Page 9

STORM SYSTEMS.

RULE OF THE WIND. SPEEDS AND PRESSURES. CYCLONES AND ANTI-CYCLONES (By H.M.V.) Wishing to convey an idea of the strength of wind in a storm, writers occasionally refer to a wind as being of "cyclonic force."/ This is entirely incorrect. The term "cyclonic" is meant to describe the type of storm system, and in no way describee the height of the wind in the disturbance. Rather does it indicate a system of winds in a cyclonic depression, the'various wind directions in the storm system being circular, usually "backing" in this country as the storm progresses. This means that 'at the commencement of the disturbance j the wind is likely to be north-east, and | gradually moving by north to west and south as the cyclone passes. Occasionally the wind "veers" in a cyclone by east to south, and the improvement that follows the passage of the storm is then only temporary, this being an indication that a "secondary" cyclone is closely following. The word "cyclone" was formerly only applied to the rare but violent storms of the tropics. Every large storm of the atmosphere is now called a cyclone, because it is found that the air moves round and in towards a central area. A cyclone is a large disc of nearly horizontally moving air circulating spirally round a central area over which the barometric pressure varies from one fifth of an inch to a 6 much as three inches below that at its border. The direction in which the wind circulates is the same as that in which the earth's surface would appear to rotate-in each hemisphere if we stood several miles directly above the pole and looked downwards. Cyclonic storms range in diameter from 20 to as much as 3000 i miles.

The Tornado. A tornado, on the other hand, consists of a narrow column of air varying in width from 20ft to 1400 ft, which is rotating with immense velocity (up to 500 miles per hour) round a central shaft, up which it is also ascending with a speed in some cases reaching 100 miles an hour.

While cyclones are comparatively rare in the tropics, they are very prevalent in higher latitudes, though fortunately as a rule in much milder form. North and south of latitude 35 degrees, continuous streams of small cyclones travel along the borders of the large permanent areas of low pressure or polar cyclones which surround either pole. Sometimes, one of these streams passes over us, in which case we experience wet and stormy weather. The areas in which cyclones move are frequently occupied hy large, high barometric pressure systems, from which the air flows quietly outwards to feed the cyclones. These systems are termed "anticyclones." While the weather in a cyclone is cloudy and rainy, conditions in the anti-cyclone are just the reverse, the weather being fine and clear. This is due to the fact that a cyclone is composed of ascending damp air, while the air in an anti-cyclone is descending, and is dry and crisp. The location of a cyclone in the southern hemisphere can be roughlj gauged in the following manner:—

If we stand with our backs to the wind, the centre of a cyclone is on our right. The reverse is the case in the northern hemisphere, the storm centre then being on our left. . This is known as "Buys-Ballot's law." Warm Winds Over Mountains. A peculiar wind arises in connection with the motion of cyclones over mountain ranges, called in Switzerland the "foelin," and in America the "ehinook." In New Zealand it is locally known as a "hot nor'-wester," and it is found to occur everywhere on the lee side of mountain ranges running athwart the paths in which cyclones travel. This wind is uncommonly hot and dry, and melts the suiow on the Alps in one night more than the sun shining for several weeks. Some countries experience very heavy rainfalls during the passage of cyclones. I On June 12, 1876, occurred the heaviest

rainfall ever measured in the world, viz., ( 40 inches in 24 hours, at Chirapunji, Assam. The following are the world's record low barometer readings: —Reikjavik, Ice- ' land, February 4, 1824, 27.25 inches. At 1 a.m. on January 10, 1913, the remarkably low barometer reading of 26.96 inches was recorded by the , aneroidbarometer on the British steamer Manchester Inventor. This is probably the lowest barometer reading ever made on the North Atlantic. The highest barometer reading on record was registered at Semipalatinsk, Siberia, on December 16, 1877, when, the instrument recorded 31.72 inches. The rate at which cyclones progress over the earth varies in different countries, as the following table shows:— United States .... '28 miles per hour., North Atlantic ... 18 miles per hour. Europe 16 miles per hour. West Indies 14 miles per hour. Bay of Bengal and China Sea ...... 8 miles per hour.