Fri., Apr. 27 notes

Friday Apr. 27, 2007

The quizzes were returned in class today. Computer generated grade summaries will be distributed in class on Monday.

The final exam for this class is scheduled for Friday May 4, 2 - 4 pm in ILC 150, one week from today! A final exam study outline is now available online. Note that about half of the questions on the final exam will come from this semester's quizzes and an old final exam (see the Study Outline for details). A 2 hour "marathon" review is scheduled for next Thursday afternoon (May 3) from 2-4 pm in EDUC 353.

If you would like to take the final exam with the T Th class on Tuesday May 8 (8 am - 10 am in ILC 150) you may do so. You will need to add your name to the signup sheet in class next week.

There will be 5 basic questions on the final exam about hurricanes. Here is a detailed summary or review of the material that we will be covering.

You'll find the following sketch comparing a middle latitude storm and a hurricane on p. 141 in the photocopied notes.

Some of the similarities and differences (mostly differences) between middle latitude storms and hurricanes are noted in the following table

Differences	Similarities	Differences
Found at middle latitudes (30 to 60 latitude) Can form over land or water	The term cyclone refers to winds spinning around low pressure. Both storms have low pressure centers. (the low pressure becomes high pressure at the top of a hurricane)	Found in the tropics (5 to 20 latitude) Only form over warm ocean water
Movement is from west to east	Upper level divergence can lower the surface pressure and cause both types of storms to intensity	Movement is from east to west
Warm and cold air masses brought together by converging winds		Warm moist air mass only
Storm winds intensify with altitude		Storm winds weaken with altitude
Strongest storms usually occur in the winter		Strongest storms late summer to fall
generally larger than hurricanes		usually smaller than middle latitude storms

Hurricanes form over warm ocean water. The warm layer of water must be fairly deep to contain enough energy to fuel a hurricane and in order that mixing doesn't bring cold water up to the ocean surface. The atmosphere must be unstable so that thunderstorms can develop. Hurricanes will only form when there is very little or no vertical wind shear (changing wind direction or speed with altitude). Hurricanes don't form at the equator because there is no Coriolis force there.

The figure above shows the relative frequency of tropical cyclone development in different parts of the world. The name hurricane, cyclone, and typhoon all refer to the same type of storm (tropical cyclone is a general name that can be used anywhere). In most years the ocean off the coast of SE Asia is the world's most active hurricane zone. Hurricanes are very rare off the east and west coasts of South America.

Note that more tropical cyclones form off the west coast of the US than off the east coast. The west coast hurricanes don't generally get the same attention, because they move away from the coast and usually don't present a threat to the US. The moisture from these storms will sometimes be pulled up into the southwestern US where it can lead to heavy rain and flooding.

This figure shows when hurricanes are most common in the Atlantic. Hurricane season in the Atlantic officially runs from June 1 through to November 30. The peak of hurricane season is in September. In 2005, an unusually active hurricane season in the Atlantic, hurricanes continued through December and into January 2006. Hurricane season in the Pacific begins two weeks earlier on May 15 and runs through Nov. 30.

Some kind of meteorological process that produces low level convergence is needed to initiate a hurricane. One possibility, and the one that fuels most of the strong N. Atlantic hurricanes, is an "easterly wave." This is just a "wiggle" in the wind flow pattern. Easterly waves often form over Africa or just off the African coast and then travel toward the west across the N. Atlantic. Winds converge as they approach the wave and then divergence once they are past it. The convergence will cause air to rise and thunderstorms to begin to develop.

In some ways winds blowing through an easterly wave resembles traffic on a multi lane highway. Traffic will back up as it approaches a section of the highway with a closed lane. Once through the "bottleneck" traffic will begin to flow more freely.

In an average year, in the N. Atlantic, there will be 10 named storms (tropical storms or hurricanes) that develop during hurricane season. 2005 was, of course, a very unusual year. There were 28 named storms in the N. Atlantic in 2005. That beat the previous record of 21 names storms that had been set in 1933. Of the 28 named storms, 15 developed into hurricanes.

Winds blowing over mountains on the west coast of Mexico will sometimes form a surface low on the downwind side of the mountains. Note there are generally a few more tropical storms and hurricanes in the E. Pacific than in the N. Atlantic. They generally move away from the US coast, though the Hawaiian Islands are sometimes affected.

A 20 minute segment from a NOVA program (PBS network) on hurricanes was shown at the end of class. A film crew was on board a NOAA reconnaissance plane as it flew into the narrow eye of hurricane GILBERT. Gilbert set the record low sea level pressure reading for the Atlantic ocean (888 mb). That record stood until the 2005 hurricane season when WILMA set a new record of 882 mb. The world record low sea level pressure, 870 mb, was set in a SE Asian typhoon in 1979.

The following comments were made during the video tape.