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Mother Nature has a lot of ways to blow off steam. In the mid-latitudes there are low pressure systems, thunderstorms, tornadoes, and cold fronts. In the tropical Atlantic, however, there is only one type of system, called at various stages an easterly wave, depression, tropical storm, or hurricane (called typhoon or cyclone elsewhere). These tropical systems give physical meaning to the term ‘blowing off steam.’
Global Temperature Balance
If the ancients had thought about it, they would have realized that if the world were flat, it would be heated evenly and there would be no variation in temperature from north to south. The fact that the sun is overhead near the equator, and at an oblique angle at high latitudes is responsible for all weather. Without the atmosphere to continually even things out, the poles would get very cold and the equator would get extremely hot.
The planet Mercury, for example, with no atmosphere, is around 800 degrees near the daytime equator and -280 degrees at the nighttime areas of the pole (temperatures are in Fahrenheit). On the earth that difference is just from 100 to minus 60, because the atmosphere continually transports heat poleward.
Atmospheric Energy Balance
The basic mechanism of energy trasfer in the mid-latitudes is accomplished by a conversion of potential energy to kinetic energy and dissipation of that energy back to heat by friction. Here’s the scenario (simplified, but not much).
- Cold air from the poles meets warm air from the tropics in the mid-latitudes.
- Cold air next to warm is a high potential energy state.
- Storms mix the air so that the warm air ends up on top of the cold — a lower potential energy state.
- The potential energy lost becomes kinetic energy (wind).
- The storms move with a northward (in the northern hemisphere) component, and the wind is dissipated into the ground by friction.
- The kinetic energy becomes heat energy and the northern latitudes are warmed.
Tropical systems give literal meaning to the term ‘blow off steam.’ Again the energy is supplied by the heating of the sun. If the tropical ocean and atmosphere get hot enough, and if a couple of other conditions are met, a tropical storm or hurricane can form. Again we’re looking at a conversion of potential energy, this time the potential energy locked up in water vapor molecules (scientifically known as latent heat), to kinetic energy (wind), and back to heat via friction, usually at higher latitudes.
When you boil water in a pot, some of the water molecules are ripped from the surface of the water and rise into the air as gaseous water vapor. If the air cannot hold all the water molecules, some of them condense into water droplets — steam. The same process drives hurricanes.
The heat of the sun, or of the atmosphere which has been heated by the sun, rips water molecules from the surface of the sea. When this happens, the heat supplied is stored in the water molecules. If there are more water vapor molecules than the atmosphere can accommodate, which in common meteorological language means the humidity reaches 100%, some of the water vapor condenses into water droplets — a cloud forms. The cloud is the equivalent of steam from a heated kettle, and the condensation process releases the heat stored in the water molecules. Mother Nature finds it expedient to convert this heat energy into kinetic energy of wind, and then to dissipate it into the ground at higher latitudes.
These are the theoretical considerations of hurricane formation: Mainly latent heat energy converted into kinetic energy. As a practical matter, there are intermediate steps. After all, we don’t get a storm every time the humidity reaches 100%.
The Hurricane in Practice
The tropical circulation features easterly winds up to about 25 degrees latitude. These winds circle the globe, and if they just blew in a straight line, there would be no hurricanes. But nature dislikes wind blowing in a straight line almost as much as it dislikes a vacuum. In fact, nothing goes in an absolutely straight line for very long.
Consider a car going at constant speed on a straight road. Even the best driver will veer a little right or left on occasion. In the flow of tropical easterly winds, there are random perturbations — a little veering this way or that on occasion. This gets the process of tropical storm formation going. If the perturbation is large enough it becomes a tropical wave — just a minor curl in the flow. Under the right circumstances, the curl can become a depression, a tropical storm, and then a hurricane.
The Right Circumstances
Mother Nature, as we said, dislikes straight lines. That’s why weather maps are dotted with high and low pressure systems around which wind flows clockwise and counterclockwise respectively. Easterly waves — significant perturbations in the easterly flow of the tropics — are a common occurrence. For one to spin up into a hurricane takes several favorable factors: warm ocean water, at least 80 degrees Fahrenheit and the warmer the better; high water vapor content in the mid-levels of the atmosphere; an absence of vertical shear, which means the wind doesn’t vary much in direction or speed with height.
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