The new air world

by Willis L. Moore

Thunderstorms. The thunderstorm is caused by cold and heavy air from above breaking through into a lighter and superheated stratum next the earth. Some of them have a horizontal rolling motion which throws forward the cool air in the direction in which the storm is moving. It seldom is more than five or ten miles in width and twenty to thirty miles in length. In general, thunderstorms move from the west toward some eastern point, more often southwest to northeast.

The frequency of thunderstorms is the greatest with ill-defined Lows whose pressure is but little below the normal air pressure of thirty inches. Any depression of the barometer slightly below the level at surrounding stations—such as occurs when a weak High of only thirty inches, or thirty and one tenth inches, breaks up into two or more areas, with slightly lower pressure between them—is fruitful of thunderstorms. A High of but modest intensity advancing eastward into a region of slightly lower pressure and much higher temperature causes thunderstorms along its eastern front. A temperature of 80° on the morning weather map, with a high humidity, seldom can endure beyond the second day without a break and the coming of cooling thunder-showers. Any Low with abnormal heat and humidity in its southeast quadrant is usually attended with numerous thunder squalls in the regions of high temperature and moisture.

Of the thunderstorm days in the United States few occur in the Rocky Mountain regions or in northern New England. The greatest number is in Florida and the Gulf States and thence northward up the Mississippi Valley.

The Moon Has No Influence on the Weather. The moon used to be the farmer’s most valued friend as a forecaster of the weather and as a guide in the planting of crops, but a higher order of intelligence is causing this fallacy to pass away. The moon’s nearness to the earth and the fact that its phases occur in about seven days, which is about twice the period of storm recurrence, in the minds of many have endowed it with potency in the influencing of our weather. Rain may occur on the same day of the week for several weeks in succession, but only occasionally, while the moon is constantly progressing from one phase to another. The few cases that prove the mistaken theory are taken as proof conclusive, while the many cases that do not prove acceptable to the moon forecaster are ignored and not mentioned to his friends nor even acknowledged to himself. One is reluctant to have a belief disproved, no matter how ridiculous it may be. In fact, the more untenable it is, the more tenaciously some adhere to it, as though they were loyally standing by an old friend who had made mistakes, but who still was good at heart. The attraction of the moon, because of its nearness and notwithstanding its small mass, is far more potent in the raising of the tides of the ocean than is the sun, but its attraction on our atmosphere produces a tide of only four thousandths of an inch of the barometer, an influence that is shadowy and without the least influence in causing storms, or changes of any kind in the weather; and there is no possible way in which the moon could influence the germination of seed or the growing of crops.

Equinoctial Storm. As the summer wanes the Lows become more pronounced and the sporadic showers give place to general rain storms along in September. There is no objection to these storms being known as “Equinoctial”, except that any date in the latter half of September is as liable to show a beginning of these storms as is the 21st or the 22d. The equinox simply marks the middle period in the transition from one type of weather to another.

Forecasting from Halos. The halos that sometimes surround the sun or the moon indicate the coming of precipitation to the extent of making manifest the presence in the upper air of large quantities of vapor of water in a congealed state. When the vapor of water cools quietly in the laboratory it frequently forms minute spheres of water, which, strange to relate, may remain liquid all the way down to zero and below; but if touched or jostled they instantly turn to ice, in the form of spiculæ, or needles; they are simply hexagonal slender prisms capped by hexagonal pyramids. These needles rotate or spin about as they fall. The geometrical relations of the facets of the crystals to the axis of rotation and to the line along which they fall are a complex problem in optics. Suffice to say that the observer, looking through a filmy cloud of such crystals, would see in one part of the sky a halo, in another part an arc of light, and in other directions bright spots like the sun, all of them arranged symmetrically with regard to the sun and the observer’s zenith. A lunar halo is a large ring concentric about the moon. A secondary halo surrounds the first. Mock suns or mock moons may appear coincident with solar or lunar halos. The ice prisms through which one sees the phenomena both refract and diffract the light as it passes through the cloud and by partly decomposing the rays render visible a part of their elementary colors. The red is on the inside, next to which is a little yellow or green, with bluish white on the outside. In coronas, which are much smaller, the red is on the outside. A detailed description of these phenomena may be found in Moore’s “Descriptive Meteorology” (Appleton).

Tornadoes. The cyclone has a diameter of a thousand to two thousand miles, the hurricane about one to three hundred and the tornado only one to ten hundred feet. The hurricane is much more destructive than the cyclone, and the tornado is incomparably greater in velocity of gyration and rending force than the hurricane. New England, Florida, and the wide region including the eastern slope of the Rocky Mountains westward to the Pacific are nearly free from the atmospheric convulsions that cause the tornadoes, and they are infrequent in any Atlantic coast State, but numerous in the States bordering on the Mississippi River, and in the eastern halves of Oklahoma, Kansas, and Nebraska. During a year of great frequency of tornadoes, about ninety storms occurred, while during some other years the number has been as low as twenty. The direction generally is toward the northeast. The average rate of movement of the tornado cloud is about twenty-five miles per hour and the width of its destructive path only five hundred to one thousand feet; the time of passage is less than half a minute. It does not come upon one unseen and unheralded. Many times the advancing funnel-shaped clouds may be seen, and they always are accompanied by a great roar which may be heard for miles. Except a tornado cellar, the cellar of a frame house is the safest place. The writer has examined either the wrecks or the records of hundreds of tornadoes and does not know of a single case of a person being killed by a tornado in the cellar of a frame house. If one is in the open and a tornado approaches, never flee to the north or to the east, but rather to the northwest, and one needs to travel but a short distance to pass out of the track of the monster. The tornado always twists counter clockwise, the same as the cyclone in whose southeast quadrant it nearly always occurs. On the southeast side of the path there are indrafts; so that it is safer, unless the track of the oncoming storm is clearly seen to be well to the north of the observer, for one to run toward the northwest. Persons have stood near to the north side of a tornado track during its passage without suffering injury. If a cave, the cellar of a frame house, or a narrow ditch cannot be reached, the best thing to do is to lie flat on the ground as far from buildings and trees as possible.

The tornado is essentially an American storm, doubtless caused by the running together, in the southeast quadrant of a cyclone, of cold northwest currents and warm winds from the southeast, at a time when the latter are saturated with moisture. They are confined almost entirely to the region between the two great mountain systems of the continent, none occurring in the Rocky Mountains and but few east of the Alleghanies. The north and south trend of our mountain systems, quite different from the systems of Europe and Asia, facilitates the coming together of conflicting winds of widely different temperatures in the lower reaches of the atmosphere where there is an abundance of water vapor; no tornadic whirls probably can occur without an abundance of water vapor and the energizing effect of the heat liberated in the whirling cloud as this vapor is suddenly carried aloft and liberated by condensation right in the center of the disturbance. Because of the relation of the trend of its great mountain systems to its oceans, the United States occupies a somewhat unique position meteorologically in the world. Its atmospheric conditions are more active than those of any other continent, which conditions are beneficial to the people of this country.

When to Watch the Weather Map for Tornadoes. The four conditions essential to the formation of tornadoes are as follows: