Pedagoguery

Originally, I was going to write in this column about space tethers and their uses. In light of the recent events in the Gulf of Mexico, however, and at the request of a subscriber, I will defer that discussion and instead write about a more earthly phenomenon: Hurricanes.

Hurricanes arise from a number of factors. Across the globe, many of those factors are common, but individual geographic circumstances influence their formation in various areas. I am going to concentrate on the tropical Atlantic, since that is the source of the hurricanes that affect the United States.

During the northern summer, a hot westerly wind flows across the Sahara and into the Central Atlantic. That, combined with the intense tropical sun, heats the surface water to in excess of 80ºF. The hot, dry wind flowing over the surface of the water eagerly sucks up moisture and heats further. Like any hot air mass, it rises, creating a local low pressure area. As the air rises, it cools, and as it cools, the moisture it contains condenses out, forming clouds. The rising air also draws in air from surrounding areas, which likewise absorbs moisture, is heated, and rises. Eventually, you get one of the common tropical thunderstorms that constantly form in the area, or if it gets big enough, a tropical depression. Tropical depressions are so named because they form in the tropics, and they are an area of low, or depressed, pressure. If the storm stays in the tropics, nothing much else will happen.

Sometimes, however, the prevailing winds will nudge the tropical depression northwards. When that happens, things change drastically. The reason why is that the Earth is a spinning globe.

The Earth has a circumference of about 24,000 miles, and it spins on its axis once every 24 hours. As a result, an object at the equator has a speed of 1000 MPH relative to an object at the pole. Objects at lower middle latitudes are traveling faster than an object at the poles, but slower than an object at the equator. As a result, when an object at the equator moves northward, it will be deflected eastward because of the difference in relative velocity. Likewise, an object at the pole moving southward will be deflected westward. To illustrate this, imagine a cannon at the equator. Let's say the cannon fires a cannon ball northward, and it hits at the 45th parallel. When it hits, it will be moving sideways relative to the ground by 500 mph. This phenomenon is called the Coriolis force, and it works the same in the Southern hemisphere, except all of the directions would be reversed.

So, if a tropical depression moves north of the equator, then it will start to turn counter-clockwise due to the effects of the Coriolis force. The effect isn't large overall – no more than a few tens of miles per hour for a storm a few hundred miles wide. However, as long as the storm stays over warm waters, the slight effect can continue to build, spinning the storm faster and faster. As soon as the sustained wind speeds near the center reach 39 mph, the storm has become a tropical storm. This is when it earns a name.

Tropical storms will continue to strengthen as long as they remain over warm waters. Heat is the engine that drives storms of this sort. In addition to the direct heating of the water, when moisture carried upwards condenses, it also releases heat. This is called the latent heat of vaporization, and it represents the heat absorbed when a liquid turns into a gas, or released when the gas condenses back into a liquid. This heat further feeds the storm and keeps the pressure at the center, now called the eye, low, guaranteeing that adjacent air masses will be drawn in to continue the cycle. The longer the storm remains over warm waters, the stronger it will grow, and the faster its sustained winds will become. When the sustained wind speed reaches 74 mph, the storm is officially a hurricane.

There is one other phenomenon you frequently hear associated with hurricanes, and that is the storm surge. The storm surge has two components. The first, and smaller, component is the pressure surge. The dramatically lower pressure under the eye sucks up the water directly under it, causing a low dome of water at that spot. The larger component, however, is wind-driven. As air masses spiral inward toward the eye, the push water ahead of them, adding to the dome in the center. In a strong hurricane like Katrina, the storm surge easily top 20 feet, causing as much if not more damage than the wind.

When the hurricane finally moves over land, it quickly weakens. Deprived of a constant supply of warm, wet air, the central pressure starts to rise, and the existing moisture is dumped out as rain. A strong hurricane can still penetrate a fair ways inland, but the farther inland it goes, the weaker are its winds, and the more it resembles a strong thunderstorm.

Next issue, I will talk about the amazing possibilities offered by space tethers.