Posted by Larry Hoover on September 12, 2004, at 9:42:16
In reply to Hurricanes, posted by sb417 on September 10, 2004, at 1:46:40
> I have forgotten almost all of the Earth Science I studied years ago in middle school. I have to re-educate myself. I forgot what causes hurricanes. Why are there so many this year, and why are they in such close succession?
Hurricanes form in the tropics, crudely that area between the Tropics of Cancer and Capricorn (23.5 degrees N and S latitude, respectively). The tropics are the limits of the region where the sun can be directly overhead, and are crude limits on the ocean regions where the water warms to the greatest extent.
Within that zone are the winds of tropical convergence, those winds that the sailors of old called the trade winds. There are places (as off the western coast of Africa) where those winds tend to create the perfect conditions for a hurricane to develop.
It all starts with the formation of a thunderstorm. A thunderstorm is a chain reaction, where rising moist air cools, and the water condenses out. The condensation is a physical change of state, and with that change of state, the latent heat of vapourization is released, heating the rising air (diabatic heating). This increases the tendency for that air to rise, and creates a low pressure below it. This pulls in more moist air, and so on.
If that thunderstorm forms at a greater latitude than about 8 degrees, the Coriolis effect starts the storm spinning. Air flow into the low pressure area doesn't go straight into the middle, it is deflected by the rotation to a region just outside the geometric centre of the tropical disturbance. Once sustained wind speeds reach 37 km (23 miles) per hour, the tropical disturbance is called a tropical depression. As winds increase to 63 km (39 miles) per hour, the cyclone is called a tropical storm and receives a name.
These storms can only form and intensify (to hurricane strength) over warm water. The warmer the water, the greater the supply of heat energy to create the chain reaction that sustains the storm.
A tropical storm is really not much different in concept than is a low pressure storm over land. At some point, though, a tropical storm can become something quite different, a hurricane.
The influx of warm moist air at the bottom of the storm would be sufficient to create a chaotic wind structure near the centre of the storm, except for the Coriolis effect. Hurricanes are self-organizing storms, and the mechanism of eye formation and so on is poorly understood. In any case, this is what makes a hurricane unique....
The rising air at or near the eye-wall surges upwards at an immense rate of speed, drawing air behind it so fast that lateral wind speeds can exceed 150 m.p.h. The greatest lateral windspeeds are a surface phenomenon. Just like a chimney drawing smoke, the moist air reaching the centre is drawn upwards very efficiently. The condensation of the water releases energy of a similar magnitude to that released in an atomic bomb. At the top of the "chimney" the pressure is so great that the air deflects outwards at a great rate, producing the characteristic flat top seen in satellite photography.
In the eye, you get a zone of immensely reduced air pressure, as a result of the vacuum-like effect of the chimney all around it. Despite the low pressure, this air is actually falling down from above, and as such it is generally less humid than would typically be seen at ground level. The weather in the eye can be quite gorgeous, if it weren't for the surrounding devastating winds.
The low pressure is so powerful that the hurricane actually lifts the surface of the ocean. The very same effect as what you do when you suck on a straw, but over an immense area. The weight of that water is many millions of tonnes. The storm surge is caused by this bulge of water being drawn over a land area. It can be 20 or more feet higher than the normal surface elevation of the ocean. Of course, it is disturbed by waves, magnified by sub-surface ocean terrain, and so on. That's why you don't want to be near the point of landfall. The storm surge dissipates a huge amount of energy, and water is much denser than is air. It smashes things.
Once over land, the hurricane is starved for energy (in a relative sense), and chaotic winds can break up the organization of the storm. It reverts to something like a typical low pressure cyclonic storm, but with higher precipitation levels than are normally sustained by land-based storms.
Why are there so many storms this year? Well, there aren't really more named storms, they are just of unusual likelihood to reach hurricane status. This may be related to the stalling of the northern jetstream over central North America this summer. This reduced the tendency of the tropics to export heat to the mid-latitudes (leading to a virtually failed summer over much of Canada, and more energy stored in the Atlantic, Caribbean and Gulf of Mexico waters). Hurricanes are just Mother Nature's way of consuming that heat, and the hurricanes are eating it up with great pleasure.
Lar
poster:Larry Hoover
thread:389071
URL: http://www.dr-bob.org/babble/social/20040907/msgs/389947.html