Hurricanes will be the final topic that we cover this semester.

On average, hurricanes kill 20 people per year in the United States and cause about $5 billion of damage.  As the table below indicates though there are exceptional years (such as 2005) where the death and damage totals greately exceed these average values (data are from www.economics.noaa.gov)

Year
 Deaths
 Total Damage
(billion $ ))
2000
 0
 < 1
2001
 24
 6.5 B
2002
 51
 1.7 B
2003
 14
 2.3 B
2004
 34
 22.9 B
2005
 1016
 107.5 B
2006
 0
 < 1
2007
 1
 < 1
2008
 11
 7.9 B
2009
 2
 < 1
2010
 0
 < 1

2005 was, of course, the year hurricane Katrina hit New Orleans.  Three of the ten strongest hurricanes ever observed in the N. Atlantic occurred in 2005 (Wilma was the strongest and the new record holder, Rita was 4th and Katrina 6th strongest).  The deadliest hurricane in US history is the 1900 Galveston hurricane which caused 6000 - 12,000 deaths.  The Great Hurricane of 1780 killed over 20,000 people in the Lesser Antilles.  Historic rainfall amounts (75 inches perhaps in some locations) and flooding associated with Hurricane Mitch killed over 19,000 people in Honduras, Guatemala, and Nicaragua in 1998.

A good place to begin is to compare hurricanes (tropical cyclones) with middle latitude storms (extratropical cyclones).  These are the two largest types of storm systems found on the earth.

Satellite photographs and sketches of the two types of storm system are shown below.








Next we'll list some of the similarites (first table below) and differences (second table; the left column applies to middle latitude storms, the right most column to hurricanes) between these storms.


Similarities
both types of storms have low pressure centers
(the term cyclone refers to winds blowing around low pressure)
upper level divergence is what causes both types of storms to intensify
(intensification means the surface low pressure gets even lower)


Differences (the order may differ from that given in class)
1. Middle latitude storms are bigger,
perhaps 1000 miles in diameter (half the US)
 1. Hurricanes are smaller,
100s of miles in diameter (fill the Gulf of Mexico)
2. Formation can occur over land or water
 2. Can only form over warm ocean water
weaken rapidly when they move over land or cold water
3. Form at middle (30o to 60o) latitudes
3. Form in the sub tropics, 5o to 20o latitude
4. Prevailing westerlies move these storms
from west to east
 4. Trade winds move hurricanes
from east to west
5. Storm season: winter to early spring
 5. Storm season: late summer to fall
(when ocean water is warmest)
6. Air masses of different temperatures collide along fronts
 6. Single warm moist air mass
7. All types of precipitation: rain, snow, sleet freezing rain
 7. Mostly just lots (a foot or more) of rain
8. Only an occasional storm gets a name
("The Perfect Storm", "Storm of the Century", etc.)
 8. Tropical storms & hurricanes gets names




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 generic 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.

Hurricanes form between 5 and 20 degrees latitude, over warm ocean water, north and south of the equator.  The warm layer of water must be fairly deep to contain enough energy to fuel a hurricane and so that turbulence and mixing don'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 Coriolis force is what gives hurricanes their spin and it causes hurricanes to spin in opposite directions in the northern and southern hemispheres).

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 much attention, because they move away from the coast and usually don't present a threat to the US (except occasionally to the state of Hawaii).  The moisture from these storms will sometimes be pulled up into the southwestern US where it can lead to heavy rain and flooding.




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 even 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.  Here's a little bit better sketch than the one on p. 142 in the photocopied ClassNotes.




In some ways winds blowing through an easterly wave resembles traffic on a multi-lane highway.  Traffic will slow down and start to bunch up as it approaches an obstruction.  This is like the convergence that occurs when air flows into an easterly wave.  Once through the "bottleneck" traffic will begin to flow more freely.  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 diverge once they are past it .  The convergence will cause air to rise and thunderstorms to begin to develop. 



Normal hurricane activity in the Pacific Normal hurricane activity in the Atlantic
16 tropical storms per year
8 reach hurricane strength
0 hit the US coastline		 10 tropical storms per year
6 reach hurricane strength
2 hit the US coastline

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, if you remember,  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.

This is a reasonably important figure (I showed it in class while a video was playing; we'll come back to it again next Monday).  It tries to explain how a cluster of thunderstorms can organize and intensify into a hurricane.

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

2.   Rising air expands, cools, and thunderstorm clouds form.  The release of latent heat during condensation warms the atmosphere.  The core of a hurricane is warmer than the air around it.

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 at the top edges of 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 (you can sometimes see this on satellite photographs of hurricanes).

4.   The upper level divergence will cause the surface pressure at the center of the hurricane 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.

 Here's another view of hurricane development and intensification



In the figure at left the moderate divergence found at upper levels is stronger than the weak surface convergence.  Divergence is removing more air than is being added by 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 has strengthened to moderate levels.  The upper level divergence has also strengthened.  The upper level divergence is still stronger than the surface convergence so the surface low pressure will decrease even more and the storm will intensify.

In the right figure the surface low pressure has decreased enough that the strong surface convergence now balances the strong 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 tropical disturbance is just a localized cluster of thunderstorms that a meterologist might see on a satellite photograph.  But this would merit observation because of the potential for further development.  Signs of rotation would be evidence of organization and the developing storm would be called a tropical depression.

In order to be called a tropical storm the storm must strenthen 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.  You don't need to remember all these names, just try to remember the information highlighted above.


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.


Hurricane Katrina making landfall on Aug. 29, 2005. (source)


The Saffir Simpson Scale is used to rate hurricane intensity (just as the Fujita Scale is used for tornadoes).  The scale runs from 1 to 5.  Remember that a hurricane must have winds of 74 MPH or above to be considered a hurricane.  Category 3,4, and 5 hurricanes are considered "major hurricanes" (in other parts of the world the term super typhoon is used for category 4 or 5 typhoons).

Here's an easy-to-remember version of the scale

Pressure decreases by 20 mb, wind speeds increase by 20 MPH, and the storm surge increases by 5 feet with every change in level on the scale.

The storm surge listed above is a rise in ocean level when a hurricane makes landfall.  This causes the most damage and the greatest number of fatalities near a coast.



The converging surface winds associated with a hurricane sweep surface water in toward the center of a hurricane and cause it to pile up.  The water sinks and, in deeper water, returns to where it came from.  This gets harder and harder to do as the hurricane approaches shore and the ocean gets shallower.    So the piled up water gets deeper and the return flow current gets stronger.

The National Weather Service has developed the SLOSH computer model that tries to predict the height and extant of a hurricane storm surge (SLOSH stands for Sea, Lake, and Overland Surges from Hurricanes).  You can see some animations of SLOSH predictions run for hurricanes of historical interest (including the Galveston 1900) hurricane at a National Hurricane Center website (http://www.nhc.noaa.gov/surge)