The Movements and Forces in Tornadoes

Abstract

It has been my privilege to observe several tornadoes at close range and also to examine the effects of others. In the early days in Kansas little whirlwinds would appear over bare or scantily covered ground and move forward for short distances and then disappear as quickly as they appeared. Sometimes several would apppear within a small area and rise with sufficient force to remove our hats and even move heavier objects on the ground. The observation of these led to the more extensive study of tornadoes. Tornadoes are often miscalled cyclones because both are formed under similar conditions and are governed by the same natural laws. The tropical ocean cyclones with which we are most familiar have a swiftly whirling wind around a calm center sometimes hundreds of miles in diameter. They move forward for days along the warm ocean currents such as the Gulf stream and the Japan stream. Tornadoes form on the oceans and on the land. The vortices at the surface of the water and the land are sometimes only a few feet in diameter. They move forward almost always in a northeasterly or easterly course. Several may appear in different places on the same day. Sometimes one will follow another in the same path within a few hours. Ten years ago one moved over Kentucky into Indiana and united with the great one which moved out of Missouri through Illinois into Indiana. Three separate tornadoes united to form the one which did so much damage east of Perry, Kan., many years ago. I watched one approaching the town of Oak Hill, Kan. When within less than a mile it divided into two tornadoes. The larger one moved north, the other one moved eastward within less than a quarter of a mile of me. When mud and water began to fall on me I sought shelter. Tornadoes form on unseasonably warm days when there are no atmospheric currents to disturb the overheated air next to the surface. When a heavy stratum of cold air blows over this overheated air the warm air will rise into the cold air just as bubbles of air rise through water. The warm air will make an opening into the cold stratum and rise until its temperature is reduced to that of the surrounding atmosphere. The warm air beneath will flow from all directions toward this opening and whirl round it just as water will whirl round an opening in the bottom of a vessel. The centrifugal force of the whirling water will keep it from flowing down through the center of the opening and if this is large enough one may thrust his finger through the center without touching the water. In like manner the centrifugal force of the whirling wind will make a partial vacuum in the center of the ascending air. As the earth rotates on its axis the atmosphere rotates with it and the nearer it is to the equator the more rapid the rotation. When a south wind blows toward a tornado this eastward rotation of the atmosphere will draw the wind a little to the east of the tornado. The slower velocity eastward of a north wind will cause it to fall a little to the west of a tornado. In the northern hemisphere these winds will cause the whirl to rotate in the opposite direction from that of the hands of a clock. In the southern hemisphere this whirling movement will rotate in the direction of the hands of a clock.