The new air world

by Willis L. Moore

1. A cyclone, the center of which is to the north or northwest;

2. An isotherm of 70° or over extending from the southeast well up into the center of the cyclone, and then passing outward toward the southwest, all inside the southeast quadrant of the Low;

3. Excessive humidity;

4. Time of year March 15 to June 15.

Fig. 17.—Tornado Cloud.

If any one of the four foregoing conditions be absent, tornadoes are not liable to occur. The reason why spring and early summer is the time when tornadoes are most frequent is because the earth and a thin stratum of air immediately next the earth are heated up rapidly with the gaining heat of the sun’s rays in the spring, while the air a short distance aloft still retains much of the cold of winter. At this time cyclonic action may bring together air masses of widely different temperatures, especially when the upper layers on the west side of the Low are drawn down and commingled with the hot and humid surface winds of the southeast quadrant.

Tornadoes Not Increasing. The writer does not indorse the theory that the number of these storms is increasing; that the breaking of the virgin soil of the prairie, the planting or the cutting away of the forests, the drainage of land surfaces by tiles, the stringing of thousands of miles of wire, or the laying of iron and steel rails have materially altered the climate or contributed to the frequency or the intensity of storms. To be sure, as population becomes more dense greater destruction will ensue with the same number of storms.

Difficult to Forecast Tornadoes. It is not possible for the forecaster to warn the exact cities and towns that will be struck by tornadoes without unduly alarming many places that will wholly escape injury. What we know is that tornadoes are almost wholly confined to the southeast quadrant of a cyclone, and that when the thermal, hygrometric, and time conditions are favorable, a region about one or two hundred miles square will be sacrificed by a number of these atmospheric twisters. One of the most destructive tornadoes of record devastated St. Louis in the afternoon of May 27, 1896. The abnormal heat and humidity of a rather small and weak cyclone centered in eastern Kansas on the morning weather map of that day, caused the Weather Bureau to distribute tornado forecasts at 10 A.M. throughout all of Missouri. The schools of St. Louis were dismissed and the children sent home on receipt of the warning, and although some eight or ten separate tornadoes touched various parts of the State and the people were prepared for their coming, so many people were terrorized by the warning in communities that were not harmed, that the writer, then Chief of the Weather Bureau, at once issued orders forbidding the specific forecasting of tornadoes in the future. Under tornadic conditions the forecast is for “conditions favorable for severe local storms.”

Freaks of the Tornado. The writer was in St. Louis the day after the storm and spent much time in examining the wreckage. He was impressed with the fact that some buildings were burst outward and that all four walls fell away from their bases, indicating that the tornado cloud must have lifted and dropped down over them in such a way that the partial vacuum that is created by the rotating cloud through centrifugal force so reduced the pressure of the air on the outside of the houses that the normal pressure of fifteen pounds per square inch exploded them. He saw bricks in a plastered wall that were neatly cleaned of all plaster by the expansion of the air inside the brick, as the air pressure from the outside was reduced. He saw a two by four pine scantling shot through five eighths of solid iron on the Eads Bridge, the pine stick protruding several feet through the iron side of the roadway, exemplifying the old principle of shooting a candle through a board. He saw a six by eight piece of timber driven four feet almost straight down into the hard compact soil, a gardener’s spade shot six inches into the tough body of a tree, a chip driven through the limb of a tree, and wheat straws forced into the body of a tree to the depth of over half an inch. Such was the fearful velocity of the wind as it gyrated about the small center of the tornado,—a velocity exceeding that of any rifle bullet. (See [Figures 17], [18], [19], and [20].)