HURRICANES (for more and current info, visit the National Hurricane Center or The U.S. Geological Survey or the LSU Hurricane Center)

Our most powerful weather systems are hurricanes. They go by the following names:

• hurricanes: Atlantic, Caribbean, Gulf of Mexico, and eastern Pacific
• tropical cyclones: most of S. Pacific & Indian Oceans
• typhoons: western N. Pacific
• baguios: Philippines
• willy-willy and cyclones: Australia

A hurricane can release enough energy in one day to supply U.S. electrical needs for 6 months.

The following is a selection of terms that one should know in order to understand the nature of hurricanes.

• Cyclone - low pressure zone with air rising in a counterclockwise spiral.
• Anticyclone - high pressure zone with air descending in a clockwise spiral.
• Convection - the movement of air upward, due to thermal qualities or updrafts.
• Bermuda High - an anticyclone that, from early summer until mid-autumn, locates over the mid-Atlantic Ocean and southeastern U.S. See adjacent figure "Why it's dry in Carolinas."
• Easterly tradewinds - south of the Bermuda high, these winds normally flow southeast to northwestward. These may reach as high as 40,000 ft or more and travel over great expanses of the Atlantic Ocean into the Gulf of Mexico.
• Easterly (=tropical) waves - low pressure areas that normally originate over equatorial Africa and move westward in the easterly tradewinds. They are oriented north-south, are about 20° latitude long, and are spaced about every 15° longitude. Associated clouds are easily spotted on the 6 o'clock news aerial photos by their northeast to southwest orientation on the west of the wave and southeast to northwest on the east. Sometimes they are the result of a polar trough separating at its tip, but they still move in the easterlies.
• Westerlies - winds of the temperate zone that blow from northwest to southeast.
• Polar trough - an area of low pressure embedded in the westerlies that may be pushed south.
• Intertropical convergence zone (ITCZ) (=equatorial trough, doldrums) - a low pressure zone located around the equator between the easterly tradewinds of the northern and southern hemisphere. It varies from being almost undetectable to being a zone of intense storms. Its location varies from day to day, but its seasonal variation is from the vicinity of the equator in February to about 12° N latitude in August.

POSSIBLE HURRICANE STARTER MECHANISMS:
The following are three ways cyclonic wind circulation may begin that may lead to a hurricane.

• Polar trough - In early summer, the Bermuda high slips south and weakens. This may allow a polar trough to move farther south. If the southern tip of the trough gets trapped in the prevailing easterlies of the tropics, it may break loose and develop into an easterly wave. It is already a low pressure cyclone, so if the right circulation increases, it may become a hurricane.
• Easterly wave - Here we are discussing those that originate off the coast of Africa and travel westward. Many of these change little in form and simply dissipate. Occasionally, however, one develops greater cyclonic circulation and may become a hurricane.
• ITCZ - an area of circulation may develop at the periphery, break loose and move into the easterly tradewind belt and intensify.

STEPS THAT OCCUR AFTER A VORTEX IS FORMED (More Information)

1. Initial vortex creates a region into which air from the surroundings flow, accelerating the convection already occurring. If the air has become warm and moist from flowing over tropical waters, it adds energy to the system.
2. As the warm, moist air rises in the cyclone, it cools and water vapor and heat energy are released. The heat energy drives the wind system and the vertical circulation acquires greater organization.
3. If high altitude winds of the proper speed blow over the vortex, they draw the air, now drier and with decreased energy, upward more quickly and distribute it in a anticyclonic fashion away from the vortex. This pumping action
   1. continues to decrease the pressure in the cyclone and
   2. prevents the cyclone from being filled with cool air and thus deteriorating. It is possible only if a convective chimney has developed.
4. MOVEMENT. The cyclone continues to move northwestward in the easterlies on a curved course. When it arrives at 30-35°N latitude (the horse latitudes), the general area where it encounters the westerlies, the hurricane may stall (and, if the water is warm, pick up energy) or begin to move northeastward. This is called the point of recurvature. Before this point, hurricane linear movement may be at about 30 mph; after, up to 60 mph.

WHAT MIGHT STOP A HURRICANE FROM FORMING?

1. Wind circulation never gets organized.
2. Not enough energy in the system to carry its development.
3. Other meteorological events that bump them off.
4. The convective chimney does not form.
5. Jet stream or other strong wind shears off the top.
6. Two sticks crossed with salt poured in the center.
7. You have adequate canned goods, stored water, gas tanks filled, windows boarded, and a precise plan of action ready to implement.

• Anatomy of a Hurricane

STAGES OF TROPICAL CYCLONES

STAGE	SURFACE CIRCULATION	CLOSED ISOBARS	WIND SPEED (MPH)
Tropical Disturbance	absent - slightly	0-1	some-22
Tropical Depression	some	1+	23-38
Tropical Storm	well developed	several	39-73
Hurricane	very well developed	more	74+

SAFFIR/SIMPSON HURRICANE SCALE RANGES

HURRICANE SCALE NUMBER (CATEGORY)	CENTRAL BAROMETRIC PRESSURE (INCHES)	WINDS (MPH)	STORM SURGE (FT)	DAMAGE
1	>28.94	74-95	4-5	Minimal
2	28.50-28.93	96-110	6-8	Moderate
3	27.91-28.49	111-130	9-12	Extensive
4	27.17-27.90	131-155	13-18	Extreme
5	<27.17	>155	>18	Catastrophic

FREQUENCY OF RETURNS FOR HURRICANES WITHIN 80 MILES OF NEW ORLEANS

CATEGORY	FREQUENCY OF RETURN
1	8 years
2	19 years
3	32 years
4	70 years
5	180 years

FUJITA SCALE FOR DAMAGING WIND (USED FOR TORNADOES)

SCALE	MPH	DAMAGING WIND
F 0	40-72	Light damage
F 1	73-112	Moderate damage
F 2	113-157	Considerable damage
F 3	158-206	Severe damage
F 4	207-260	Devastating damage
F 5	261-318	Incredible damage

WAYS HURRICANES CAUSE DAMAGE:

1. Winds. Pressure against structures mounts disproportionately with wind velocity.

   Example: a 20 knot wind increases atmospheric pressure by 2 lb/sq ft. A 200 knot wind increases atmospheric pressure by 225 lb/sq ft.

   We noted that the speed of wind in a hurricane is derived from the storm’s travels over warm water, where it picks up energy. Normally, the power (=energy) of the storm is dissipated once it hits land. Hurricane Danny, which crossed southeast Louisiana on July 18, 1997, was the first hurricane to actually increase in power once it crossed the state’s barrier islands. This was because it was the first storm to travel across this section of our coastal wetlands since most of the marsh became open water, thus presenting the storm not with friction and terra firma to slow it, but with shallow, warm water from which it gained more strength.

2. Storm surge. This is a "bulge" of water being pushed in front of the eye of the hurricane. This is on top of increased tide. For Hurricane Camille, the surge was 25 ft!

   The next time you visit Gulfport, MS, go by the marina just east of the junction of US 49 at the coastal highway. Beside the boat ramps, there is a pole that marks the storm surges from several storms that have affected that area. It is quite impressive. Note the black tape bands on the pole.
   
   At about eye level is Hurricane Juan (1985), then Hurricane Betsy (1965), then the unnamed hurricane of 1947, then, 25 ft above your head, is Hurricane Camille (1969). This pole gives you an excellent idea of the enormous amount of water that a hurricane can push ahead of its eye.

   Based on information obtained by the U.S. Army Corps of Engineers in the Terrebonne estuary after Hurricane Andrew, a general rule-of-thumb is that for every three miles of healthy coastal marsh that a hurricane crosses, one foot of its storm surge is dissipated. Since Hurricane Camille emerged directly from the Gulf of Mexico to hit the Mississippi Gulf Coast, it had a 25 ft storm surge. Had it crossed a healthy marsh some 75 mi wide, it may have had no surge. This is critical for coastal Louisiana when we consider the speed at which we are losing our “storm buffer.”

3. Wave activity. There may be 5-10 ft waves on top of the surge. Water weighs 1700 lb/cu yd, so the pounding waves can be deadly.

4. Misplaced currents. These occur because of the dynamic movement of water during the storm; existing currents are simply pushed closer to the shore. These will erode beaches and foundations.

5. Secondary weather. These arise from the hurricane and include rain (flooding) and tornadoes.

How do we determine the impact of a hurricane?

1. Deadliest Hurricanes in the U.S.
2. Costliest Hurricanes in the U.S.
3. Most Intense Hurricanes in the U.S.

Hurricane tracks.
When a hurricane has completed its destructive course, a plot of its travels becomes its hurricane track. These have been plotted for most hurricanes and look like spilled spaghetti in the Gulf of Mexico (see the slides we show).

Typhoon Tracks for the Western Pacific
Hurricane Tracks for the Eastern Pacific
Hurricane Tracks for the Atlantic and Gulf of Mexico
Hurricane Tracks for the Louisiana Area
U.S. Hurricane Strikes
Hurricane Strikes by State

Critical Path Hurricane for New Orleans.
The worst case scenario hurricane (based on a Category 3 storm) for New Orleans will basically follow a path whereby the eye, traveling from the southeast, passes between the Mississippi River and the mouth of Lake Pontchartrain from the Gulf. Since hurricanes have a counterclockwise wind motion, this path would result in the storm pushing water into Lakes Pontchartrain, Borgne, Catherine, and Maurepas. As the eye nears the center of the lake, the storm would kick to the northeast, thus causing the counterclockwise winds to push the copious amounts of water south and over the levees into Greater New Orleans. It is projected that such a storm would put about 18 ft of water throughout Greater New Orleans.

What have we done to prevent this?
We can't stop the storm, so we have to protect ourselves otherwise. The current plan is to encircle the region with 17 ft high levees (the high level plan). There was a plan proposed by the Corps of Engineers to place control structures at the entrance of the Lake (see slides) that could be closed as a storm appoaches (the Environmental Impact Study was finished in 1974). This was not implemented because environmentalists did not want to see the lake ecosystem interrupted in such a way that marine larvae (such as crabs, shrimp, menhaden, etc.) could not move into and out of the lake.

So, when does one evacuate?
This is the hardest question to answer, and it should definitely be an individual decision. Many say they will never evacuate. Here is my rule of thumb.

By June 1st of each year, I am prepared for the hurricane season. I update my belongings inventory (I always pay my flood insurance on time), have at least 10 five gallon bottles of water in my garage, 20 or so butane cylinders for my butane lights and stove, at least 2 large butane tanks for my barbeque grill, lots of batteries and several good flashlights, plenty of matches, lots of canned foods (many of which I would only eat in a crunch - most of it goes to charity after the season), an axe in my attic (in Hurricane Betsy, a number of people drowned when they crawled in their attics to avoid rising water and became trapped and drowned), a couple of snake bite kits, extra medications that we need, and the like. Once a storm enters the Gulf, my cars have full tanks, I monitor the forecast at least hourly, I constantly tell my family members what I am thinking, I begin to put yard things away, and the like. If it really looks bad, just before I evacuate I fill my bathtubs and all containers in the house, get together the things I simply must save and keep them in a place where they can easily be packed in the cars (yes, I have an official list of what I can’t live without so that I don’t have to think about it; I review it at the beginning of the season), call my out-of-town family to tell them what we are thinking and planning, and further batten down the hatches.

If I am absolutely certain that a hurricane will be a Category I when it hits land, I don’t leave (but I am ready to if needed). Even at a moderate to low Category II, I would be hesitant to leave. I would be comfortable if such a storm was approaching from due south, i.e., targeted to cross the coast near Grand Isle.

My family is on stand-by any time a hurricane is heading toward the Gulf of Mexico. If it is a Category II when it enters, I assume it will escalate so I begin preparing to leave. (see finals stages above). When do I abosolutely, definitely, always leave? Anytime we have a Category III or above (or a lesser that appears to be escalating to that level) nearing the middle of the Gulf and heading toward New Orleans on a critical path, I’m gone before it hits the center. It is just plain dumb to wait too long and not be able to get out, and it is just plain dumb to stay. Life is not a gamble. By the way, I also don’t run in front of Big Mack trucks or stick my head in a tiger’s mouth. One of my best friends stayed in his house during a direct hit by Hurricane Andrew. His roof was completely torn off, and he told me that for every minute for many, many hours he expected to die and to see his family die. It was the most terrifying time of his life and he will never make that mistake again.

What is the biggest present fear?
There has not been a major storm in the immediate Greater New Orleans area since Hurricane Betsy in 1965. The population has increased markedly, with most residents not really knowing the power of a storm. If a hurricane approaches New Orleans, many would stay here thinking that a hurricane is no big deal. Also, evacuation would be tough with this many people.

Our recent Hurricane Andrew (1992) may have educated the masses. Witness, though that one of the City of New Orleans' four escape routes, I-10 West, was not available due to construction!

Damage Scenes from Hurricane Camille, August 17, 1969

Damage Scenes from Hurricane Andrew

• Importance of Barrier Islands
• Hurricane Andrew story
• Impact on coastal wetlands
• Impact on forested wetlands
• Impact on Wildlife

• Anatomy of a Thunderstorm
• Thunderstorms
• Storm seasons '89: mapping the risk
• New Orleans Pea Soup (fog and what causes it)

PAST WEATHER CODE

Code Number	Past Weather
0	Clear or few clouds
1	Partly cloudy (scattered) or variable sky
2	Cloudy (broken) or overcast
3	Sand storm or dust storm, or drifting or blowing snow
4	Fog, or smoke, or thick dust haze
5	Drizzle
6	Rain
7	Snow, or rain and snow mixed, or ice pellets (sleet)
8	Shower(s)
9	Thunderstorm, with or without precipitation

SKY COVERAGE CODE

Code Number	Sky Coverage (low and/or middle clouds)
0	No clouds
1	Less than one tenth or one tenth
2	Two tenths or three tenths
3	Four tenths
4	Five tenths
5	Six tenths
6	Seven tenths or eight tenths
7	Nine tenths or overcast with openings
8	Completely overcast
9	Sky obscured

Course Document List