Friday Dec. 4, 2009

I'm always excited when I stumble upon a new group like The Starlings.  They'll be playing at Plush next Monday night.


Here are the results of the informal survey conducted in class last Monday.  Almost everyone felt that it would be a good idea to put at least 2 questions from each of this semester's quizzes and at least 10 questions from the old final exam on this semester's final exam.  A majority of the students (60 for and 34 against) felt that putting a few hurricane questions on the final would be OK.  Students were generally against (35 for versus 56 against) the idea of putting the "NATS 101 Finale" on the exam.  I'll follow your wishes.

We'll finish up hurricanes today.

Here's another illustration of hurricane intensification




In the figure at left the upper level divergence is stronger than the surface convergence.  The surface low pressure will decrease.  The decrease in surface pressure will cause the converging surface winds to blow faster.

In the middle picture, the surface low pressure is lower, the surface convergence is stronger.  The upper level divergence has also been strengthened a little bit.  The upper level divergence is still stronger than the surface convergence so the surface low pressure will decrease even more.

In the right figure the surface low pressure has decreased enough that the surface convergence now balances the upper level divergence.  The storm won't strengthen any more.

Generally speaking the lower the surface pressure at the center of a hurricane the stronger the storm and the faster the surface winds will blow.


This figure tries to show the relationship between surface pressure and surface wind speed.  The world record low sea level pressure reading, 870 mb, was set by Typooon Tip off the SE Asia coast in 1979.  Sustained winds in that storm 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 (top) and a picture like you might see on a satellite photograph (below). 

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.  Generally speaking the smaller the eye, the stronger the storm.

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 layer clouds.

Hurricanes are, of course, very destructive.


The Saffir-Simpson scale is used to rate hurricane intensity (just as the Fujita scale is used with tornadoes).
A simplified version of the Saffir-Simpson scale is shown above.  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 a storm surge develops as a hurricane comes onshore.

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.

We watched another short video at the end of class.  It mostly described the arrival of Hurricane Camille along the Mississippi coast in 1969.  Camille is the 2nd most intense hurricane to hit the US (see p. 146a in the photocopied
ClassNotes)

The storm surge probably causes the most hurricane damage along a coastline.  Further out at sea strong winds and high seas are the biggest hazard.  Once a hurricane moves onshore it weakens very rapidly (friction slows the winds and the hurricane is cut off from its supply of moisture).  However very heavy rains, thunderstorms and tornadoes can remain a threat over a large area for days to come.
The following information was displayed during a short video segment shown at the end of class.  This is just for informational purposes, you don't need to remember all these details.

The 2005 hurricane season (the year Hurricane Katrina hit New Orleans) was unusual in many respects.  

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

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 storms came close to becoming the deadliest hurricane in US history. That distinction belongs to the 1900 Galveston hurricane.