Mon., Dec. 04 notes

Monday Dec. 04, 2006

The graded quizzes and up-to-date grade summaries were handed out in class while the course evaluation was being administered. Check the grade summaries carefully to be sure they are correct.

The final exam for this section of the class is this coming Friday, Dec. 8, from 2 to 4 pm in ILC 150. You may take the final with the other section of the class (Thursday, Dec. 14 from 8 am to 10 am in ILC 15) if you wish. However you must let the instructor know ahead of time. A final exam study outline is available online. Note that a review will be held on Thursday afternoon (Dec. 7) from 2 to 4 pm in EDUC 353.

We learned last week the first step in hurricane formation is a meteorological process of some kind (such as an easterly wave) that can cause surface winds to converge. If the atmosphere is unstable, the rising air motion created by the convergence can cause a cluster of thunderstorms to begin to form.

The next figure (redrawn after class for clarity) shows what can happen next.

1. The converging winds pick up heat and moisture from the ocean. These are the two mains sources of energy for the hurricane.

2.   Rising air cools and thunderstorm clouds form. The release of latent heat during condensation warms the atmosphere. The core of a hurricane is warm.

3.   Pressure decreases more slowly with increasing altitude in the warm core of the hurricane. The result is that pressure at the top center of the hurricane is higher than the pressure further out from the hurricane (pressure at the top center is still lower than the pressure at the bottom center of the hurricane). Upper levels winds diverge and spiral outward from the top center of the hurricane.

4.   The upper level divergence causes the surface pressure to decrease. The speed of the converging surface winds increases and the storm intensifies. The converging winds pick up additional heat and moisture which warms the core of the hurricane even more. The upper level high pressure and the upper level divergence increase. The increased divergence lowers the surface pressure even more.

Stages of storm development. The developing storm receives a name when it reaches tropical storm strength. To be considered a hurricane the winds must be 74 MPH or greater (75 MPH might be easier to remember). Note the weather map symbols for northern hemisphere tropical storms and hurricanes (counterclockwise rotation)

A crossectional view of a mature hurricane and a picture like you might see on a satellite photograph. These figures were changed slightly after class.

Sinking air in the very center of a hurricane produces the clear skies of the eye, a hurricane's most distinctive feature. The eye is typically a few 10s of miles across, though it may only be a few miles across in the strongest hurricanes.

A ring of strong thunderstorms, the eye wall, surrounds the eye. This is where the hurricane's strongest winds are found.

Additional concentric rings of thunderstorms are found as you move outward from the center of the hurricane. These are called rain bands. These usually aren't visible until you get to the outer edge of the hurricane because they are covered by high altitude clouds.

Here is an easy to remember version of the Saffir Simpson scale used to classify hurricane strength and damage potential.

If you remember that winds must be 75 MPH or higher in order for a tropical cyclone to be a hurricane. In this easy to remember scale the winds increase by 20 MPH as you move up the scale.

Pressures decrease in 20 mb increments (start at 1000 mb and go down from there) and the height of the storm surge increases by 5 feet. It is thought that parts of the Louisianna and Mississippi coasts were hit with a 30 ft. storm surge as Hurricane Katrina moved onshore in 2005.

Out at sea, the converging surface winds create surface currents in the ocean that transport water toward the center of the hurricane. The rise in ocean level is probably only a few feet, though the waves are much larger. A return flow develops underwater that carries the water back to where it came from.

As the hurricane approaches shore, the ocean becomes shallower. The return flow must pass through a more restricted space. A rise in ocean level will increase the underwater pressure and the return flow will speed up. More pressure and an even faster return flow is needed as the hurricane gets near the coast.

Here is a link to the storm surge website (from the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Labororatory). It has an interesting animation showing output from the SLOSH model used to predict hurricane storm surges.

Hurricanes can, of course, be very destructive. Out at sea the main hazards are the strong winds and the large waves. The Perfect Storm by Sebastian Junger describes the sinking of the Andrea Gail in a strong hurricane like storm in October 1991. The exact fate of the fishing ship is not known but it may have been turned end over end by a large wave (pitch poled). Large waves can also flood a ship and begin to fill it with water.

Along a coast the greatest threat is from the hurricane winds and the storm surge. Large waves are superimposed on the storm surge.

The hurricanes winds slow quickly as it moves onshore, though tornadoes may form. The biggest threat is from flooding. Hurricanes can easily drop a foot or more of rain on an area as they pass through.

Another short segment of video tape was shown just before the end of class. The video showed the Richelieu appartments which were completely flattened by a storm surge from Hurricane Camille in 1969.

This is an example of the grade summaries handed out in class today. You should first check to be sure all of your grades have been entered into the computer correctly.

The first average, without any quiz scores dropped, is the one that must be 90.0% or above in order to not have to take the final exam. The second average (with the lowest quiz score dropped) is usually higher than the first grade. This is the grade you currently have in this class if you do have to take the final exam. There are a few situations where the second grade is lower than the first grade. The computer will select the higher of the two grades at the end of the semester when final grades are being determined.