Subject : C1) Doesn't the low pressure in the tropical cyclone center 
cause the storm surge? Contributed by Chris Landsea
 No. Many people assume that the partial vacuum at the center of a 
tropical cyclone allows the ocean to rise up in response, thus causing 
the destructive storm surges as the cyclone makes landfall. However, this 
effect would be, for example, with a 900 mb central pressure tropical 
cyclone, only 1 m (3 ft). The total storm surge for a tropical cyclone of 
this intensity can be from 6 to 10 m (19 to 33 ft), or more. Most (>85%) 
of the storm surge is caused by winds pushing the ocean surface ahead of 
the storm on the right side of the track (left side of the track in the 
Southern Hemisphere).
Since the surface pressure gradient (from the tropical cyclone center to 
the environmental conditions) determines the wind strength, the central 
pressure indirectly does indicate the height of the storm surge, but not 
directly. Note also that individual storm surges are dependent upon the 
coastal topography, angle of incidence of landfall, speed of tropical 
cyclone motion as well as the wind strength.
Subject : C2) Doesn't the friction over land kill tropical cyclones?
(Parts of this section are written by Sim Aberson.)
 No. During landfall, the increased friction over land acts - somewhat 
contradictory - to both decrease the sustained winds and also to increase 
the gusts felt at the surface (Powell and Houston 1996). The sustained (1 
min or longer average) winds are reduced because of the dampening effect 
of larger roughness over land (i.e. bushes, trees and houses over land 
versus a relatively smooth ocean). The gusts are stronger because 
turbulence increases and acts to bring faster winds down to the surface 
in short (a few seconds) bursts.
However, after just a few hours, a tropical cyclone over land will begin 
to weaken rapidly - not because of friction - but because the storm lacks 
the the moisture and heat sources that the ocean provided. This depletion 
of moisture and heat hurts the tropical cyclone's ability to produce 
thunderstorms near the storm center. Without this convection,the storm 
rapidly fills.
An early numerical simulation (Tuleya and Kurihara 1978) had shown that a 
hurricane making landfall over a very moist region (i.e. mainly swamp) so 
that surface evaporation is unchanged, intensification may result. 
However, a more recent study (Tuleya 1994) that has a more realistic 
treatment of surface conditions found that even over a swampy area a 
hurricane would weaken because of limited heat sources. Indeed, nature 
conducted this experiment during Andrew as the hurricane traversed the 
very wet Everglades, Big Cypress and Corkscrew Swamp areas of southwest 
Florida. Andrew weakened dramatically: peak winds decreased about 33% and 
the sea level pressure in the eye filled 19 mb (Powell and Houston 1996).
Subject : C3) Aren't big tropical cyclones also intense tropical 
cyclones? Contributed by Chris Landsea
 No. There is very little association between intensity (either measured 
by maximum sustained winds or by central pressure) and size (either 
measured by radius of 15 m/s [gale force, 30 kts, 35 mph] winds or the 
radius of the outer closed isobar) (Weatherford and Gray 1988). Hurricane 
Andrew is a good example of a very intense tropical cyclone (922 mb 
central pressure and 64 m/s (125 kt, 145 mph) sustained winds at landfall 
in Florida) that was also relatively small (15 m/s winds extended out 
only about 150 km [90 mi] from the center). Weatherford and Gray (1988) 
also showed that changes of both intensity and size are essentially 
independent of one another.
Subject : C4) Has there ever been an attempt or experiment to reduce the 
strength of a hurricane ?
Contributed by Chris Landsea
 The U.S. Government once supported research into methods of hurricane 
modification, known as Project STORMFURY. For a couple decades NOAA and 
its predecessor tried to weaken hurricanes by dropping silver iodide - a 
substance that serves as a effective ice nuclei - into the rainbands of 
the storms. During the STORMFURY years scientists seeded clouds in 
Hurricanes Esther (1961), Beulah (1963), Debbie (1969), and Ginger 
(1971). The experiments took place over the open Atlantic far from land. 
The STORMFURY seeding targeted convective clouds just outside the 
hurricane's eyewall in an attempt to form a new ring of clouds that, it 
was hoped, would compete with the natural circulation of the storm and 
weaken it. The idea was that the silver iodide would enhance the 
thunderstorms of a rainband by causing the supercooled water to freeze, 
thus liberating the latent heat of fusion and helping a rainband to grow 
at the expense of the eyewall. With a weakened convergence to the 
eyewall, the strong inner core winds would also weaken quite a bit. For 
cloud seeding to be successful, the clouds must contain sufficient 
supercooled water (water that has remained liquid at temperatures below 
the freezing point, 0C/32F). Neat idea, but it, in the end, had a fatal 
flaw. Observations made in the 1980s showed that most hurricanes don't 
have enough supercooled water for STORMFURY seeding to work - the 
buoyancy in hurricane convection is fairly small and the updrafts 
correspondingly small compared to the type one would observe in 
mid-latitude continental super or multicells.
In addition, it was found that unseeded hurricanes form natural outer 
eyewalls just as the STORMFURY scientists expected seeded ones to do. 
This phenomenon makes it almost impossible to separate the effect (if 
any) of seeding from natural changes. The few times that they did seed 
and saw a reduction in intensity was undoubtedly due to what is now 
called "concentric eyewall cycles". Thus nature accomplishes what NOAA 
had hoped to do artificially. No wonder that the first few experiments 
were thought to be successes. Because the results of seeding experiments 
were so inconclusive, STORMFURY was discontinued. A special committee of 
the National Academy of Sciences concluded that a more complete 
understanding of the physical processes taking place in hurricanes was 
needed before any additional modification experiments. The primary focus 
of NOAA's Hurricane Research Division today is better physical 
understanding of hurricanes and improvement of forecasts. To learn about 
the STORMFURY project as it was called, read Willoughby et al. (1985).
Subject: C5a) Why don't we try to destroy tropical cyclones by seeding 
them with silver iodide:
Contributed by Chris Landsea
Actually for a couple decades NOAA and its predecessor tried to weaken 
hurricanes by dropping silver iodide - a substance that serves as a 
effective ice nuclei - into the rainbands of the storms. The STORMFURY 
project , as it was called, proposed that the silver iodide would enhance 
the thunderstorms of the rainband by causing the supercooled water to 
freeze, thus liberating the latent heat of fusion and helping the 
rainband to grow at the expense of the eyewall. With a weakened 
convergence to the eyewall, the strong inner core winds would also weaken 
quite a bit. Neat idea, but it, in the end, had a fatal flaw: there just 
isn't much supercooled water available in hurricane convection - the 
buoyancy is fairly small and the updrafts correspondingly small compared 
to the type one would observe in mid-latitude continental super or 
multicells. The few times that they did seed and saw a reduction in 
intensity was undoubtedly due to what is now called "concentric eyewall 
cycles".
Concentric eyewall cycles naturally occur in intense tropical cyclones 
(wind > 50 m/s [100 kt, 115 mph]). As tropical cyclones reach this 
threshold of intensity, they usually - but not always - have an eyewall 
and radius of maximum winds that contracts to a very small size, around 
10 to 25 km [5 to 15 mi]. At this point, some of the outer rainbands may 
organize into an outer ring of thunderstorms that slowly moves inward and 
robs the inner eyewall of its needed moisture and momentum. During this 
phase, the tropical cyclone is weakening (i.e. the maximum winds die off 
a bit and the central pressure goes up). Eventually the outer eyewall 
replaces the inner one completely and the storm can be the same intensity 
as it was previously or, in some cases, even stronger. A concentric 
eyewall cycle occurred in Hurricane Andrew (1992) before landfall near 
Miami: a strong intensity was reached, an outer eyewall formed, this 
contracted in concert with a pronounced weakening of the storm, and as 
the outer eyewall completely replaced the original one the hurricane 
reintensified.
Thus nature accomplishes what NOAA had hoped to do artificially. No 
wonder that the first few experiments were thought to be successes. To 
learn about the STORMFURY project read Willoughby et al. (1985). To learn 
more about concentric eyewall cycles, read Willoughby et al. (1982) and 
Willoughby (1990).
Subject: C5b) Why don't we try to destroy tropical cyclones by placing a 
substance on the ocean surface ?
Contributed by Chris Landsea
There has been some experimental work in trying to develop a liquid that 
when placed over the ocean surface would prevent evaporation from 
occurring. If this worked in the tropical cyclone environment, it would 
probably have a limiting effect on the intensity of the storm as it needs 
huge amounts of oceanic evaporation to continue to maintain its intensity 
(Simpson and Simpson 1966). However, finding a substance that would be 
able to stay together in the rough seas of a tropical cyclone proved to 
be the downfall of this idea.
There was also suggested about 20 years ago (Gray et al. 1976) that the 
use of carbon black (or soot) might be a good way to modify tropical 
cyclones. The idea was that one could burn a large quantity of a heavy 
petroleum to produce vast numbers of carbon black particles that would be 
released on the edges of the tropical cyclone in the boundary layer. 
These carbon black aerosols would produce a tremendous heat source simply 
by absorbing the solar radiation and transferring the heat directly to 
the atmosphere. This would provide for the initiation of thunderstorm 
activity outside of the tropical cyclone core and, similarly to 
STORMFURY, weaken the eyewall convection. This suggestion has never been 
carried out in real-life.
Subject: C5c) Why don't we try to destroy tropical cyclones by nuking them ?
Contributed by Chris Landsea
During each hurricane season, there always appear suggestions that one 
should simply use nuclear weapons to try and destroy the storms. Apart 
from the fact that this might not even alter the storm, this approach 
neglects the problem that the released radioactive fallout would fairly 
quickly move with the tradewinds to affect land areas and cause 
devastating environmental problems. Needless to say, this is not a good 
idea.
Now for a more rigorous scientific explanation of why this would not be 
an effective hurricane modification technique. The main difficulty with 
using explosives to modify hurricanes is the amount of energy required. A 
fully developed hurricane can release heat energy at a rate of 5 to 
20x1013 watts and converts less than 10% of the heat into the mechanical 
energy of the wind. The heat release is equivalent to a 10-megaton 
nuclear bomb exploding every 20 minutes. According to the 1993 World 
Almanac, the entire human race used energy at a rate of 1013 watts in 
1990, a rate less than 20% of the power of a hurricane.
If we think about mechanical energy, the energy at humanity's disposal is 
closer to the storm's, but the task of focusing even half of the energy 
on a spot in the middle of a remote ocean would still be formidable. 
Brute force interference with hurricanes doesn't seem promising.
In addition, an explosive, even a nuclear explosive, produces a shock 
wave, or pulse of high pressure, that propagates away from the site of 
the explosion somewhat faster than the speed of sound. Such an event 
doesn't raise the barometric pressure after the shock has passed because 
barometric pressure in the atmosphere reflects the weight of the air 
above the ground. For normal atmospheric pressure, there are about ten 
metric tons (1000 kilograms per ton) of air bearing down on each square 
meter of surface. In the strongest hurricanes there are nine. To change a 
Category 5 hurricane into a Category 2 hurricane you would have to add 
about a half ton of air for each square meter inside the eye, or a total 
of a bit more than half a billion (500,000,000) tons for a 20 km radius 
eye. It's difficult to envision a practical way of moving that much air 
around.
Attacking weak tropical waves or depressions before they have a chance to 
grow into hurricanes isn't promising either. About 80 of these 
disturbances form every year in the Atlantic basin, but only about 5 
become hurricanes in a typical year. There is no way to tell in advance 
which ones will develop. If the energy released in a tropical disturbance 
were only 10% of that released in a hurricane, it's still a lot of power, 
so that the hurricane police would need to dim the whole world's lights 
many times a year.
Subject: C5d) Why don't we try to destroy tropical cyclones by adding a 
water absorbing substance ?
Contributed by Hugh Willoughby (FIU)
"Dyn-O-Gel" is a special powder (produced by Dyn-O-Mat) that absorbs 
large amounts of moisture and then becomes a gooey gel. It has been 
proposed to drop large amounts of the substance into the clouds of a 
hurricane to dissipate some of the clouds thus helping to weaken or 
destroy the hurricane.
At HRD we tried the one possible way that "Dyn-O-Gel" could weaken a 
hurricane in the MM5 numerical model. We saw an effect but it was small 
(~1 m/s). The argument was that the glop would make raindrops lumpy (i. 
e., non-aerodynamic) they would fall slower and increase condensate 
loading, thus weakening the eyewall updraft. If, by contrast, one 
increases the fall speed of the hydrometeors, the storm strengthens 
(again by only ~1 m/s). In the numerical experiments "decrease" meant 
reduce the fall velocity to half the real value, and "increase" meant 
double the real value. The foregoing effect is larger than anything one 
could hope to produce in the real atmosphere.
The observation that the experiment that "Dyn-O-Gel" conducted actually 
"dissipated" clouds is problematic. Did they watch any unmodified clouds?  
Isolated Florida cumuli have short lifetimes, and these are just the 
ones an experimenter would logically pick.
Accepting for the sake of argument that they actually did have an effect, 
the descriptions seem more consistent with an increase in hydrometeor 
fall speed and accelerated collision coalescence, which the numerical 
model results argue would strengthen the hurricane, but not much. If this 
speculation proves to be correct, "Dyn-O-Gel" might be useful for 
rainmaking during a dry spell, unlike glaciogenic seeding which (in the 
tropics at least) tends to make rainy days even more rainy--if it does 
anything at all.
One of the biggest problems is, however, that it would take a LOT of the 
stuff to even hope to have an impact. 2 cm of rain falling over 1 square 
kilometer of surface deposits 20,000 metric tons of water. At the 
2000-to-one ratio that the "Dyn-O-Gel" folks advertise, each square km 
would require 10 tons of goop. If we take the eye to be 20 km in diameter 
surrounded by a 20km thick eyewall, that's 3,769.91 square kilometers, 
requiring 37,699.1 tons of "Dyn-O-Gel". A C-5A heavy-lift transport 
airplane can carry a 100 ton payload. So that treating the eyewall would 
require 377 sorties. A typical average reflectivity in the eyewall is 
about 40 dB(Z), which works out to 1.3 cm/hr rain rate. Thus to keep the 
eyewall doped up, you'd need to deliver this much "Dyn-O-Gel" every 
hour-and-a-half or so. If you crank the reflectivity up to 43 dB(Z) you 
need to do it every hour. (If the eyewall is only 10 km thick, you can 
get by with 157 sorties every hour-and-a-half at the lower reflectivity.)
Subject: C5e) Why don't we try to destroy tropical cyclones by cooling 
the surface waters with icebergs or deep ocean water ?
Contributed by Neal Dorst
Since hurricanes draw their energy from warm ocean water, some proposals 
have been put forward to tow icebergs from the arctic zones to the 
tropics to cool the sea surface temperatures. Others have suggested 
pumping cold bottom water in pipes to the surface, or releasing bags of 
cold freshwater from near the bottom to do this.
Consider the scale of what we are talking about. The critical region in 
the hurricane for energy transfer would be under or near the eyewall 
region. If the eyewall was thirty miles (48 kilometer) in diameter, that 
means an area of nearly 2000 square miles (4550 square kilometers). Now 
if the hurricane is moving at 10 miles an hour (16 km/hr) it will sweep 
over 7200 square miles (18,650 square kilometers) of ocean. That's a lot 
of icebergs for just 24 hours of the cyclone's life.
Now add in the uncertainty in the track, which is currently 100 miles 
(160 km) at 24 hours and you have to increase your cool patch by 24,000 
sq mi (38,000 sq km). For the iceberg towing method you would have to 
increase your lead time even more (and hence the uncertainty and area 
cooled) or risk your fleet of tugboats getting caught by the storm.
For the bag/pipe method you would have to preposition your system across 
all possible approaches for hurricanes. Just for the US mainland from 
Cape Hatteras to Brownsville would mean covering 528,000 sq mi (850,000 
sq km) of ocean floor with devices.
Lastly, consider the creatures of the sea. If you suddenly cool the 
surface layer of the ocean (and even turn it temporarily fresh), you 
would alter the ecology of that area and probably kill most of the sea 
life contained therein. A hurricane would be devastating enough on them 
without our adding to the mayhem.
Last updated August 13, 2004
Subject: C5f) Why don't you harness the energy of tropical cyclones?
Contributed by Neal Dorst
If someone can figure out a way to harness that energy, the more power to 
them. They could earn millions of dollars and the gratitude of everyone 
on the shore. Every dyne of energy harvested would be one less dyne 
blowing over trees.
The biggest technical impediment is that a hurricane's energy is low 
grade. It's abundant, but it's spread over a tremendous area. For energy 
to be high grade it should be concentrated, making it easy to gather and 
use. You would need a field of wind turbines covering dozens of square 
miles in order for it to be profitable. And it would have to be mobile, 
so you could intercept landfalling storms, or chase those that change 
direction. Of course, you have to expend energy to move them around, so 
you run the risk of losing money on the operation. The same is true of 
wave turbines plus you would need to find a way of anchoring them 
securely without compromising mobility.
It would be a daunting technical task, plus you have to worry about your 
turbines being robust enough to sustain damage from windblown debris and 
be able to transmit the energy gathered quickly. So after you draw up 
your engineering specs, you'd better have an investor or two, because it 
will cost you a great deal of money to build so many of these reinforced, 
mobile turbine units even before you collect you first erg.
Created May 24, 2007
Subject: C5g) Why don't we try to destroy tropical cyclones by (fill in 
the blank) ?
Contributed by Chris Landsea
There have been numerous techniques that we have considered over the 
years to modify hurricanes: seeding clouds with dry ice or Silver Iodide, 
cooling the ocean with cryogenic material or icebergs, changing the 
radiational balance in the hurricane environment by absorption of 
sunlight with carbon black, exploding the hurricane apart with hydrogen 
bombs, and blowing the storm away from land with giant fans, etc. (Some 
of these have been addressed in detail in this section of FAQ's.) As 
carefully reasoned as some of these suggestions are, they all share the 
same shortcoming: They fail to appreciate the size and power of tropical 
cyclones. For example, when Hurricane Andrew struck South Florida in 
1992, the eye and eyewall devastated a swath 20 miles wide. The heat 
energy released around the eye was 5,000 times the combined heat and 
electrical power generation of the Turkey Point nuclear power plant over 
which the eye passed. The kinetic energy of the wind at any instant was 
equivalent to that released by a nuclear warhead. Perhaps if the time 
comes when men and women can travel at nearly the speed of light to the 
stars, we will then have enough energy for brute-force intervention in 
hurricane dynamics.
Human beings are used to dealing with chemically complex biological 
systems or artificial mechanical systems that embody a small amount (by 
geophysical standards) of high-grade energy. Because hurricanes are 
chemically simple --air and water vapor -- introduction of catalysts is 
unpromising. The energy involved in atmospheric dynamics is primarily 
low-grade heat energy, but the amount of it is immense in terms of human 
experience.
Attacking weak tropical waves or depressions before they have a chance to 
grow into hurricanes isn't promising either. About 80 of these 
disturbances form every year in the Atlantic basin, but only about 5 
become hurricanes in a typical year. There is no way to tell in advance 
which ones will develop. If the energy released in a tropical disturbance 
were only 10% of that released in a hurricane, it's still a lot of power, 
so that the hurricane police would need to dim the whole world's lights 
many times a year.
Perhaps some day, somebody will come up with a way to weaken hurricanes 
artificially. It is a beguiling notion. Wouldn't it be wonderful if we 
could do it ?
Perhaps the best solution is not to try to alter or destroy the tropical 
cyclones, but just learn to co-exist better with them. Since we know that 
coastal regions are vulnerable to the storms, building codes that can 
have houses stand up to the force of the tropical cyclones need to be 
enforced. The people that choose to live in these locations should be 
willing to shoulder a fair portion of the costs in terms of property 
insurance - not exorbitant rates, but ones which truly reflect the risk 
of living in a vulnerable region. In addition, efforts to educate the 
public on effective preparedness needs to continue. Helping poorer 
nations in their mitigation efforts can also result in saving countless 
lives. Finally, we need to continue in our efforts to better understand 
and observe hurricanes in order to more accurately predict their 
development, intensification and track.
Subject: C6) During a hurricane are you supposed to have the windows and 
doors on the storm side closed and the windows and doors on the lee side open?
Contributed by Chris Landsea
No! All of the doors and windows should be closed (and shuttered) 
throughout the duration of the hurricane. The pressure differences 
between inside your house and outside in the storm do not build up enough 
to cause any damaging explosions. (No house is built airtight.)
The winds in a hurricane are highly turbulent and an open window or door 
- even if in the lee side of the house - can be an open target to flying 
debris. All exterior windows should be boarded up with either wooden or 
metal shutters.
Subject: C7) Should I tape my windows when a hurricane threatens? 
Contributed by Chris Landsea
No, it is a waste of effort, time, and tape. It offers little strength to 
the glass and NO protection against flying debris. After the storm passes 
you will spend many a hot summer afternoon trying to scrape the old, 
baked-on tape off your windows (assuming they weren't shattered). Once a 
Hurricane Warning has been issued you would be better off spending your 
time putting up shutters over doors and windows.