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Question: Why do tornadoes generally form over tornado alley?
Tornadoes are 'dissipative structures'. All things that use up, or dissipate, energy gradients are dissipative structures, including people, machines and all of society, as well as winds, rivers, and even plants. Plants dissipate energy mostly by transpiration, which is the evaporation of water from the pores in their leaves. This helps plants obtain carbon dioxide, which makes up some of their food.
Energy gradients exist wherever something exists next to an area where that thing is absent or scarce, or you could say, wherever there is a difference in something that could get shared, reducing or mixing that difference. A gallon of gas would dissipate slowly into the air by evaporating, or it could dissipate explosively if it contacts something very hot. As it explodes the temperature dissipates too.
The work people do comes from utilizing some of a dissipatable energy gradient by harnessing it to our purposes. Work is a way of capturing some of a dissipating energy gradient. The energies we use are mostly taken from fossil fuels, coal, oil and natural gas, which are especially rich energy gradients, and so very ready to dissipate, and therefore easy to burn. As these become scarcer, we need to work harder to obtain them, and so we get a net return of less energy for our work, which consists in activities that also dissipate energy.
Tornadoes form as a way to dissipate energy differences in air masses. Like winds generally, they help to dissipate differences in temperature and humidity in air masses, which form when two air masses – cold / dry and warm / moist -- meet during atmospheric circulation. The thunderstorms that give birth to them also dissipate such differences in air masses, but they can't do it fast enough when the differences are very great. When they meet such large energy differences, they give birth to tornadoes, which get the job done faster. Tornado Alley in North America is a place where at certain seasons very different air masses meet, and thunderstorms alone just don't always dissipate the differences in temperature and humidity as fast as a tornado can. Even bigger energy gradients elsewhere give rise to hurricanes and cyclones. The biggest known hurricane-like example would be the activity documented on in the planet Jupiter, a giant storm known as its 'Red Spot'.
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