The Science Guys
Science Guys > March 2002
Why are tornadoes occurring earlier in the year in West Tennessee?
Tornadoes seem mighty, but they are also delicate storms which occur within minutes, and die as quickly. Predicting tornadoes is an uncertain science, with much still to be learned.
About 75% of all tornadoes occur in the US, because we live where arctic air masses collide with warm air from the Gulf. Each spring these large parcels of air do a deadly do-si-do, pushing and shoving against each other, producing some of the world’s most violent weather. Convection, the mixing of air in a circular motion, occurs along the "frontal zone." Here very severe thunderstorms form, but most of these don’t spawn tornadoes.
Tornadic thunderstorms require special conditions, but above all lots of heat energy. The air hugging the ground must be humid and warm. Who can forget Jackson’s awful tornadoes three years ago? The winter of ’99 was warm, with the temperatures the day before the tornadoes reaching well into the 70’s. This is why tornadoes in the South usually occur in the early evening after a warm day.
Above the humid, warm air, there must be another layer of warm, dry air, and highest of all, there must be a layer of cold, dry air. This condition is called a "temperature inversion," and is unusual because the cold, dense air is above the warm, less dense air. The wind velocities of the layers are different, creating a "wind shear." This wind shear provides impetus for air column rotation.
A disturbance in the upper atmosphere can provoke the moist, warm air at the bottom to "punch through" the cool air above it. The moist, warm air spirals as it ascends, assisted by the wind shear of the different layers, and as the air column rises the water vapor condenses to form water droplets. (Tornadoes are real clouds filled with water droplets; the term "funnel cloud" is accurate.)
It’s the condensation of the water droplets that really stokes the tornado’s engine. As a substance changes from a gas to a liquid, "latent heat" is released. The heat liberated when vapor becomes liquid is huge, 540 calories per gram. This heat causes the air column to continue rising. As it rises, it cools, more droplets form, releasing more heat which intensifies the motion of the column. This combined with the wind shear increases the velocity of the spiraling air column and if continued long enough, a tornado is formed.
This explanation, although simplified, tells us why we have lately suffered tornadoes early in the year. Native Jacksonians know our winters have been warmer recently. The warm Gulf air is getting here sooner, often in January. When you’ve had lots of warm winter days and a cold front is approaching, watch out for the weather. You might see a funnel cloud. But we hope not.