Wed., Apr. 23, 2008

Today's "before class" music, J'ai ete au Zydeco, came from Cajun country in SW Louisianna,  from the Best of BeauSoleil CD.  Here are the words in Cajun French and in English.

The Quiz #4 Study Guide is now finished.  Note that a very little bit of extra material was added to the material that was on the preliminary version of the study guide.

Don't forget about the detailed hurricane summary, where you'll find everything you need to know about hurricanes for next week's quiz.


This is a summary of the steps leading up to hurricane formation.  First some sort of weather process will cause surface winds to converge and form a cluster of thunderstorms.  When meteorologist spots a cluster of thunderstorms on a satellite photograph, a tropical disturbance (see the list of stages of development below), they begin to keep an eye on it, as this might eventually develop into a tropical storm or a hurricane.

Cloud formation warms the air.  High pressure forms and winds begin to diverge at the top of the hurricane.  The diverging winds lower the surface pressure at the bottom center of the storm.  Surface winds spiral in toward the surface low pressure.  Once meterologists notice rotating clouds the tropical disturbance is upgraded to a tropical depression.

In order to be called a tropical storm the storm must organize a little more, and winds must increase to 35 knots.  The storm receives a name at this point.  Finally when winds exceed 75 MPH (easier to remember than 65 knots or 74 MPH) the storm becomes a hurricane.


Hurricane intensification is shown in picture form below.

Basically the lower the surface pressure at the center of the hurricane, the stronger the storm and the faster the surface winds will blow. 
The next figure shows how the surface low pressure and the speed of the spinning winds are related.

The world record low sea level pressure reading, 870 mb, was set by Typooon Tip off the SE Asia coast.  Sustained winds were 190 MPH.  Three 2005 Atlantic hurricanes: Wilma, Rita, and Katrina had pressures in the 880 mb to 900 mb range and winds ranging from 170 to 190 MPH.

A crossectional view of a mature hurricane and a picture like you might see on a satellite photograph. 

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 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 a simplified version of the Saffir-Simpson scale.  Pressure decreases by 20 mb, wind speeds increase by 20 MPH, and the height of the storm surge increases 5 feet for every increase in Saffir Simpson Scale rating.  You don't need to remember all the numbers.  Just remember that there are 5 categories on the scale, category 1 is the weakest.  Hurricane winds must be over 75 MPH for the storm to be called a hurricane. 


The following figure shows how storm surges develop


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 and the flooding they can cause.

The bottom portion of the hurricane summary
contains some information about the 2005 hurricane season, an unusually active year (this is not material you need to remember for the quiz).

3 of the 10 most intense Atlantic hurricanes ( Wilma, Rita, and Katrina) occurred in 2005 (you'll find the top 10 listed on p. 146a in the photocopied classnotes, reproduced below).  Wilma became the most intense hurricane every in the Atlantic, beating out Hurricane Gilbert (1988) which was featured in the video tape segment shown in class on Monday.

On average there are about 10 named storms (tropical storms and hurricanes) in the Atlantic per year.  Before 2005 the record was 21 storms.  There were 28 storms in 2005 which blew the old record out of the water.

Katrina was the third most intense hurricane to hit the mainland US and easily became the most costly natural disaster in US history. 

Fortunately none of the 2005 came close to becoming the deadliest hurricane in US history. That distinction belongs to the 1900 Galveston hurricane. 






Strong winds and big waves from hurricanes are a hazard to ships at sea.
Strong winds and the storm surge cause the most damage along a coastline.
Hurricanes weaken rapidly when they move onshore (they are cut off from their moisture supply and friction slows the winds).
Hurricanes however can produce very large amounts of rainfall, a foot or two of rain over a period of a day or two.  This can lead to catastrophic flooding.

Somewhere between 10,000 and 20,000 people were killed in Central America from mudslides and flooding associated with Hurricane Mitch in 1998.

We ended the section on hurricanes by watching a short segment on Hurricane Camille (1969).  The video showed the 3-story tall Richelieu Apartments before Camille and after Camille had made landfall (the apartment building was completely demolished by a 20 foot storm surge).
We had time to take the first of ten easy steps toward understanding why winds blow the way they do.  The first step was on a handout distributed in class (steps 2-6 were included on the Wednesday handout, steps 7-10 will be distributed in class on Friday).

Upper level winds spinning around high and low pressure in the northern and southern hemispheres are shown in the first set of four pictures.  The first thing to notice is that upper level winds blow parallel to the contours.  We will see that 2 forces, the pressure gradient force (PGF) and the Coriolis force (CF) are what causes the winds to blow this way.  Eventually you will be able to draw the directions of the forces for each of the four upper level winds examples.  Here is an example of what you will be able to do.  Note I am saying will be able to do and not should be able to do.

The four drawings at the bottom of the page show surface winds blowing around high and low pressure in the southern hemisphere.  These winds blow across the contour lines slightly, always toward low pressure.  The frictional force is what causes this to occur.  He is an example of what you will be able to say about surface winds blowing around low pressure in the southern hemisphere.