March 27, 1998
March 27, 1998 |
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Introduction to Hurricanes
As much of the class are Florida residents, many will find our next 3 lectures on hurricanes particularly interesting and informative.
Hurricane development
Hurricanes develop in warm tropical waters. The minimum water temperature required for hurricane development is about 79 F. Hurricanes are most often considered mesoscale meteorological events, however, the larger hurricanes can affect synoptic-sized regions of several hundred miles.
Hurricanes begin their lives as small areas of tropical thunderstorms. In the tropics, the sun is high and strong practically all year long, hence the oceans in the tropics are quite warm relative to cooler mid-latitude regions. When the same type of convection that occurs over land with the development of air mass thunderstorms occurs over tropical oceans, the mechanisms necessary for vertical motion exist as the air above the water, particularly during the nighttime, is heated by the water so insta bility develops and vertical motion begins. Since the air above the water is so moist, the air that is being lifted is extremely juicy and moist and condensation develops very rapidly.
This is the very beginning stage of our hurricane: a region of thunderstorms over very warm, tropical waters. As the convective conditions persist and vertical motion continues, air rushes in to replace the air that has risen. The barometer begins t o fall in the region of maximum vertical motion and a low pressure center begins to develop. As the pressure continues to fall, air rushes in to replace the rising air in the low pressure center faster and faster. As the air reaches the low pressure cen ter, it is forced upward in the unstable environment and condensation and cumulonimbus type cloud development persist.
Remember back to earlier in the semester when we discussed condensation and evaporation. We said that latent heats were associated with evaporation and condensation. When water condenses, water molecules are going from the higher energy vapor phase to the lower energy liquid phase. Energy is not created or destroyed, so as the water vapor in our developing hurricane condenses, there is a release of latent heat. This release of latent heat helps to further fuel the blossoming hurricane.
As the air rises faster at the center of our low, air needs to rush in to replace it faster and faster. The pressure gradient increases, and the wind speeds increase. Now we need to reflect on our discussion of the Coriolis Force. The wind is being directed toward the center of the hurricane by the pressure gradient force, however, it also experiences an acceleration to the right of its direction of motion by the Coriolis Force.
Remember, the Coriolis Force (which is the magnitude at which the winds are accelerated by the force of the Earth's rotation) is a direct function of the velocity of the wind. In other words, the faster the wind blows, the more it experiences a deflec tion to the right. Picture then how it is that hurricanes "wrap up" so symetrically. As our hurricane increases in strength, it wraps up tighter and tigher.
Now think back to our lecture on vorticity when we discussed the Conservation of Angular Momentum. As something rotates faster, its concentration of mass gets closer to its center of rotation. Therefore, as hurricanes increase in strength (and their strength is a function of their top wind speeds), they "contract" and actually decrease in diameter.
Now let's talk about something complicated that happens in the middle of the hurricane called the eye. We have a tremendous amount of moisture being lifted into the atmosphere, which is being cooled and condensed into clouds and heavy convectiv e-type precipitation. Once the moist air cools and condenses, it then releases latent heat of condensation. Therefore, there is a cumulative heating that is going on in the middle of the hurricane! What does this mean in terms of vertical motion ? Since there is heating taking place, our lapse rate will increase and in the vicinity we are discussing, stability will develop and sinking motion will actually take place in the very center of the storm! As air sinks, it compresses, heats, and its relative humidity decreases (remember the saturation ratio?) and the air dries out and clouds dissipate. This calm area is known as the eye of the hurricane and it is a very interesting aspect of hurricanes. The lowest pressures are recorded in the eye of the hurricane.
Immediately outside the eye is the eye wall. This is the region of the hurricane where the most powerful and destructive winds are located. Unsuspecting victims (animals and people) who emerge outside during the eye are often fooled into belie ving the storm has passed. However, the eye wall on the other side of the eye approaches and the winds become as fierce and destructive as before.
Classification of hurricane development
In their earliest stages, when a developing hurricane is a mere region of tropical thunderstorm activity, it is referred to as a tropical disturbance or sometimes a wave of low pressure. When the system has begun to take on a discernible rotation and organization and the pressures show marked drops, the system then becomes classified as a tropical depression. As the storm continues to develop and its pressure continues to drop, it reaches tropical storm status once its sus tained winds reach 39 mph. At this point, the storm is given a name. Upon reaching sustained winds of 74 mph, the storm becomes a hurricane and keeps the same name.
Estimating the strength of hurricanes - the Safir-Simpson Scale
The Safir-Simpson scale is a means of classifying the strength of a hurricane based on its maximum sustained winds. The Safir-Simpson Scale is from 1 to 5 with 5 being the strongest. The scale is defined as follows:
| Category | Maximum sustained winds (mph) | Typical damage assessment |
|---|---|---|
| 1 | 74-95 | Minor structural damage. Downed trees and powerlines. |
| 2 | 96-115 | Roofs blown off. Major trees toppled. Structural damage to weaker buildings. |
| 3 | 116-135 | Widespread tree and powerline falls. Structural damage to many buildings. |
| 4 | 136-155 | Major devestation. Structural damage to all but the strongest of buildings. Few, if any, trees and powerlines survive. |
| 5 | Greather than 155 | Catastrophic devestation. Widespread severe devestation to all buildings. |
Remember back to when we talked about the destructive potential of the wind. We showed that the force of the wind was proportional to its velocity squared. Therefore, there is a very big difference in the destructive power of a Category 1 hurr icane versus Category 2 and between Category 2 and Category 3, etc. Category 5 hurricanes are simply catastrophic. Since 1935, only 2 Category 5 hurricanes have hit the United States: Camille in 1969 and the Labor Day Hurricane of 1935. Camille ravished the Alabama and Mississippi coastlines while the Labor Day Storm of 1935 devestated the Florida Keys. The loss of life in each storm was major.
The destructive players involved in a hurricane
In addition to very strong and powerful winds, there are many other dangerous elements that exist in hurricanes including but not limited to:
Once the storm passes, the hurricane leaves many painful reminders in its wake. Electricity and running water are typically out for long periods of time. Another, somewhat gruesome, aspect that often occurs with landfalling hurricanes is the prepondr ance of poisonous snakes, reptiles, and insects that are flooded from their hiding places and often strike unsuspecting victims.
Another danger are powerlines that are downed that can sometimes all of a sudden go live when power is restored which can electrocute unsuspecting people and animals.
More on hurricane dynamics
Hurricanes will not form within 5 degrees latitude of the equator. Remember that the Coriolis Force is also a function of the sine of latitude, where the value of the Coriolis Force is zero at the equator. The rotational forces are too weak in the im mediate vicinity of the equator for hurricane development.
Hurricanes are considered warm core storm systems. There energy comes from sun-driven convection and the release of latent heat by abundant amounts of condensation. This is different from mid-latitude synoptic scale cyclones which we st udied last week which are driven by east-west temperature gradients, or more informally, the interaction between warm and cold air masses.
In the doldrums, (between the equator and 30 degrees latitude), the predominant wind direction is from east to west, and therefore, this is the typical track that hurricanes follow. However, once a hurricane pushes past 30 degrees latitude, it will often get caught up in the southwesterlies and take a turn toward the northeast. The overall movement of hurricanes however, can be very erratic and predicting their next move provides meteorologists with a very great challenge.
In Monday's (3/31) lecture, we will take a closer look at the devestation that is associated with hurricanes. In addition, we will learn about some of the more powerful hurricanes that have hit the United States this century and of some of the devesta tion they caused and the lessons they taught.