The new air world

by Willis L. Moore

The word “High” is written inside the isobar marked 30.6, located in southern Oregon, and the same word is written inside the isobar marked 30.4, located on the South Atlantic coast, and also inside the isobar 30.04, which traverses Nova Scotia. These are the regions of great air pressure. The word “Low” is written at the center of the area inclosed by the isobar 29.6, which is situated in the State of Iowa. The latter is the region of least pressure. Sometimes there are several such regions shown on the weather map.

Why the Wind Blows. Under the pull of gravity the atmosphere presses downward and outward, thus causing it to flow from the several regions of great pressure towards regions of less pressure. Observe the arrows, which fly with the wind, and it will be seen how generally this law is obeyed. The velocity with which the wind moves from the High toward the Low depends on differences in air pressure, modified in the lower stratum by the friction offered in passing over surfaces of varying degrees of roughness, the speed being greater over a water surface with the same difference in air pressure than over a level unwooded prairie, and greater over the open prairie than over an irregular wooded area. To illustrate:

If the barometer were 30.5 at Bismark, Dakota, and 29.5 at Chicago, it would press upon the earth with a force of about seventy pounds greater per square foot at the first place than at the second. This difference in pressure would cause the air to flow from Bismark towards Chicago so rapidly that after allowing for the resistance due to friction on the earth there would remain a velocity of some fifty miles per hour, and Lake Michigan would experience a severe “Northwester”; and if the wind continued from the same direction for twenty-four hours a mighty sea would beat upon the eastern shore of the lake, and mariners and marine property would be at the mercy of a destructive tempest unless the Weather Bureau forecaster were alert and gave warning as soon as he saw such a juxtaposition of pressure distribution in the process of formation.

We will give careful attention to this chart, for when its details are understood, others will be easily read.

The chart shows a winter storm central in Iowa on December 15, 1893. The word “Low” marks the storm center. It is the one place in all the United States where the barometer reading is the lowest. The heavy black lines, oval and nearly concentric, about the Low, show the gradation of air pressure as it increases quite uniformly in all directions from the center of the storm outward.

The arrows fly with the wind, and, as will be seen, almost without exception are moving towards the Low, or storm center, clearly demonstrating the effect of gravity in causing the air to flow from the several regions marked “High”, where the air is abnormally heavy, toward the Low, where the air is lighter. As the velocity of water flowing down an inclined plane depends both upon the slope of the plane and the roughness of its surface, so the velocity of the wind, as it flows along the surface of the earth towards the storm center, depends on the amount of the depression of the barometer at the center and the resistance offered by surfaces of varying degrees of roughness.

Storms and Cold Waves Simply Great Eddies in the Atmosphere. Now picture in your mind that all the air inside the 30.2 isobar, as it flows inward, is rotating about the Low in a direction contrary to the movements of the hands of a watch, and you have a fair conception of an immense atmospheric eddy. Have you ever watched the placid waters of a deep-flowing brook and observed that where the waters encountered a projecting rock little eddies formed and went spinning down the stream? Well, our storms are somewhat similar eddies in the atmosphere, more or less perfect, that are carried along by the general easterly movement of the atmosphere in the middle latitudes of both hemispheres. But they are not deep eddies; the Low marks the center of an atmospheric circulation of vast horizontal extent as compared with its thickness or extension in a vertical direction. Thus a storm area extends from Washington, D. C., to Denver, Colorado, and yet extends upward only about six miles. The whole disk of whirling air, six miles thick and two thousand miles in diameter, is called a cyclone, or low-pressure area. It is important that a proper understanding be had of this fundamental idea, since the weather experienced from day to day depends almost wholly upon the movement of these migrating cyclones, or areas of low pressure, and the anti-cyclones, or areas of high pressure.

The temperature readings are omitted from each station, but the average temperature of each quadrant of the Low is shown by the large black figures. The greatest difference in temperature is seen to be between the southeast and the northwest sections. This is due in part to the fact that in the southeast quadrant the air is drawn northward from warmer latitudes, and in the northwest quadrant it is drawn southward from colder latitudes, and to the further fact that winds blowing into the west side of a Low have a downward component of motion, and those blowing in on the front, or east side, have an upward component.

One should gain a clear idea of the difference between the movements of the air in the cyclone and the movement of the cyclone itself, or its translation from place to place; how the wind must blow into the front of the storm in a direction partly or wholly contrary to the movement of the storm itself, and into the rear of the storm as it passes away; how the wind increases in velocity as it spirally gyrates about the center and approaches nearer and nearer the region where it must ascend; how the higher layers of air move spirally away from the center and thus cause an accumulation of air about and over the outer periphery of the Low, which in turn presses downward and impels the surface air inward. This whole complex system of motion moves eastward. Think of the sun drifting in space, while at the same time each of the planets maintains its respective orbit, and it will help one to visualize the phenomena of a migrating cyclone or anti-cyclone.