CHAPTER XXIX

CYCLONES AND ANTI-CYCLONES

The criticism of the weather in the meteorological column of our daily newspapers invariably speaks of "cyclones." It is, therefore, advisable to give as plain an explanation of these as possible. Cyclones are "storm-winds." Their nature has to be carefully studied by meteorologists, who are industriously at work to ascertain some scientific basis for the atmospheric movements.

What is the cause of the spiral movement in storm-winds? In their centre the depression of the barometer is lowest, because the atmosphere there is lightest. As the walls of the spiral are approached, the barometer rises.

Dr. Aitken has ingeniously hit upon an experiment to illustrate a spiral in air. All that is necessary is a good fire, a free-going chimney, and a wet cloth. The cloth is hung up in front of the fire, and pretty near it, so that steam rises readily from its surface; and, when there are no air-currents in the room, the steam will rise vertically, keeping close to the cloth. But if the room has a window in the wall, at right angles to the fireplace, so as to cause the air coming from it to make a cross-current past the fire, then a cyclone will be formed, and the vapour from the cloth will be seen circling round. When the cyclone is well formed, all the vapour is collected into the centre of the cyclone, and forms a white pillar extending from the cloth to the chimney. This experiment shows that no cyclone can form without some tangential motion in the air entering the area of low-pressure.

Now to illustrate the spiral approach. Fill with water a cylindrical glass vessel, say 15 inches in diameter and 6 inches deep. Have an orifice with a plug a little from the centre of the bottom. Remove the plug, the water runs out, passing round the vessel in a vortex form. But, as the passage between the orifice (or centre of the cyclone) and the temporary division is narrower than in any other place, the water has to pass this part much more quickly than at any other place. And this curious result is observed: the top of the cyclone no longer remains over the orifice, but _travels_ in the direction of the water which is moving most speedily. Similar to this is the cyclone in the atmosphere; its centre also moves in the direction of the quickest flowing wind that enters it.

Dr. Aitken is of opinion that, in forecasting storms, too little attention has been paid to the _anti-cyclones_. They do more than simply follow and fill up the depression made by the cyclones. They initiate and keep up their own circulation, and collect the materials with which the cyclones produce their effect. Neither could work efficiently without the other.

Suppose a large area on the earth over which the air is still in bright sunshine. After a time, when the air gets heated and charged with vapour, columns of air would begin to ascend in a disorderly fashion. But suppose an anti-cyclone is blowing at one side of this area. When the upper air descends to the earth, it spreads outwards in all directions; but the earth's rotation interferes and changes the radial into a spiral motion. The anti-cyclonic winds will prevent the formation of local cyclones, and drive all the moist, hot air to its circumference, just above the earth. The anti-cyclone forces its air tangentially into the cyclone, and gives it its direction and velocity of rotation, also the direction and rate of travel of the centre of depression. The earth's rotation is the original source of the rotatory movements, but both intensify the initial motion.

Accordingly, the cyclone must travel in the direction of the strongest winds blowing into it, just as the vortex in the vessel with the eccentric orifice travelled in the direction of the quickest moving water. This is verified by a study of the synoptic charts of the Meteorological Office.

The sun's heat has always been looked upon as the main source of the energy of our winds, but some account must also be taken of the effects of cold. It is well known that the mean pressure over Continental areas is high during winter and low during summer. As the sun's rays during summer give rise to the cyclonic conditions, so the cooling of the earth during winter gives rise to anti-cyclonic conditions. It is found during the winter months in several parts of the Continent that as the temperature falls the pressure rises, producing anti-cyclones over the cold area; whereas, when the temperature begins to rise, the pressure falls, and cyclones are attracted to the warming area.

Small natural cyclones are often seen on dusty roads, the whirling column having a core of dusty air, and the centre of the vortex travelling along the road, tossing up the dust in a very disagreeable way to pedestrians. Sometimes such a cyclone will toss up dry leaves to a height of four or five feet. They are very common; but it is only when dust, leaves, or other light material is present that they are visible to the eye.