Previously Asked Questions

Tropical Weather

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Hurricane/Tropical FAQ from NOAA

2005 Hurricane Season: A record breaker

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How does a hurricane form?
El Paso, TX


From late spring to early fall, weather conditions come together to form swirling tropical cyclones over the Atlantic and Pacific oceans.  This is when all the ingredients for tropical cyclones come together, including warm ocean waters, a warm moist atmosphere that can produce lots of thunderstorms, plenty of spin in the atmosphere, and not much turning of winds as you go higher up. It all begins with a thunderstorm forming over seawater that is at least 80 degrees Fahrenheit.  More storms develop and merge together to form a tropical wave. The thunderstorms give off heat, and that warms the atmosphere.  Air rises and so the air pressure falls even more.  That falling air pressure causes the winds to increase.  The Earth's rotation causes the increasing winds to spiral counter-clockwise.  As the winds increase and turn, a closed circulation may form and that’s when we get a tropical depression.  When steady winds reach 39 miles an hour, the cyclone is called a tropical storm and it gets a name.  If winds reach a speed of 74 miles an hour inside the tropical cyclone, we call it a hurricane. 

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How do you track hurricanes?

Several tools are used to track the path of a hurricane.  One important tool is a weather satellite, which takes photographs of hurricanes and other storms from space.  With those photographs, meteorologists can see where the tropical cyclone is currently, plot its location on a map, and get an idea of the direction it is headed.  To get an exact fix on the location of the cyclone’s center, reconnaissance aircraft actually fly into the storms.  In several locations throughout the cyclone, a meteorologist aboard the plane takes measurements of wind speed, wind direction, and barometric pressure, among other things.  From these measurements, the meteorologist can determine the latitude and longitude of the hurricane’s center and plot the location on a map.  Plotting these locations over a period of hours and/or days shows us the hurricane’s track.

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I just saw a "Storm Stories" of a cyclone in Australia. What is the difference between a hurricane, cyclone and typhoon?
Frankie Hall
, MO

Dear Frankie, 

When it comes to tropical weather, the difference between a hurricane, typhoon and cyclone is basically in location. They all fall under the generic name of "tropical cyclone,” and they all develop from areas of low pressure and thunderstorms over warm oceans. In the North Atlantic Ocean, the Northeast Pacific Ocean east of the International Dateline (western hemisphere), or the south Pacific Ocean east of Longitude 160 east, we call them hurricanes. Hurricanes affect not only the United States, but also Mexico, Central America and the Caribbean islands.  In the Northwest Pacific Ocean west of the International Dateline (Eastern Hemisphere) they are called Typhoons. Typhoons affect areas such as Japan, China, Taiwan and the Philippines.  In the Indian Ocean, and southern Pacific Ocean, the same storms are simply called “tropical cyclones” or "cyclones" for short, and sometimes called “severe tropical cyclones.” These tropical cyclones often affect northern Australia during their tropical season, which is during fall and winter to early spring in the Northern Hemisphere.

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How and why were the various wind speed numbers selected for Tropical Storms (39 mph), hurricanes (74 mph) and their various categories?
Fred Bennett
Earth Science
Arlington Junior High School
Jacksonville, FL

In 1926, the International World Meteorological Committee (which later became the World Meteorological Organization) adopted a universal scale of wind speed values, with a few revisions in later years.  There is nothing particularly magical about these numbers, and the reason they’re not more “even” (such as 40 miles an hour and 75 miles an hour etc.) is that they’re the result of conversions to miles per hour from meters per second and knots.  But when we see a tropical cyclone with maximum sustained wind speeds of 39 mph (34 knots), we also usually see the thunderstorms in the cyclone concentrated near the center, and we also see rainfall in the outer part of the cyclone organizing into distinct bands, and we call it a Tropical Storm.  When sustained winds reach around 74 mph (64 knots), we begin to see an eyewall and spiral rain bands forming in the tropical cyclone, and it officially becomes a hurricane.   

In the late 60s civil engineer Herbert Saffir was doing a study on low-cost housing in hurricane-prone areas when he realized there was no scale for showing the likely damage hurricanes of varying strength.  He came up with a one-to-five scale based on wind speed and gave it to the National Hurricane Center.  Bob Simpson, who was the director of the NHC at the time, added to the scale the effects of storm surge and flooding, and the scale became known as the Saffir-Simpson scale.  According to the scale, the various categories of hurricanes are: Category 1 (74-95 mph), Category 2 (96-110 mph), Category 3 (111-130 mph), Category 4 (131-155 mph) and Category 5 (156+ mph). Again, there’s nothing special about these specific numbers, as, for example, a strong Category 2 hurricane with 110 mph winds can bring nearly the same wind damage as a weak Category 3 with 111 mph winds.  Even so, the scale gives us some idea of what wind damage to expect from a particular hurricane.

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I thought that the word “cyclone” came from the word “cyclonic,” meaning counter-clockwise. So how can a storm in the southern hemisphere which rotates clockwise be called a tropical cyclone?

As you have said, the cyclonic flow, or general direction of wind flow around an area of low pressure in the Northern Hemisphere, is counter clockwise.  However, the cyclonic flow around a low in the Southern Hemisphere is clockwise.  “Cyclonic” only refers to the inward-flowing air around a low pressure center due to Earth’s rotation, and that can be either clockwise or counter clockwise, depending on which hemisphere you’re talking about.  So in the Southern Hemisphere, tropical cyclones, which, as you say, rotate clockwise, are appropriately named.  Of course we have tropical cyclones in the Northern Hemisphere as well.  In the Atlantic and Eastern Pacific we call them hurricanes, and in the Western Pacific they’re known as typhoons, and they rotate counter clockwise.

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What is a subtropical storm?
Birmingham, AL

A subtropical storm is a low pressure area that has characteristics of both a tropical cyclone and a regular mid-latitude cyclone (the kind that normally brings precipitation across the U.S.)  Subtropical storms usually form between about 20 to 35 degrees north latitude over water.  The winds are strongest about 10-15 thousand feet high, and weaker near the surface, where winds are in the 39-73 mile an hour range, which is less than hurricane strength. 

Subtropical storms are usually slow-moving and can produce abundant rain.  They also can last a long time, from one to two weeks. They are fairly rare, but can sometimes be found in the north Indian Ocean, in south Pacific and near Hawaii, and in the Atlantic Ocean.  You’ll see them most often in the fall or early winter, but they can occur in any month of the year. 

Before 2002 Atlantic subtropical storms were not given names, even though the National Hurricane Center issued warnings and forecasts for them.  Now, a subtropical storm will get a name from the tropical cyclone list the same way a tropical storm would.

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This hurricane season (2005) seems to be most active than I can ever remember for named storms.  What year have we had the most named storms in one hurricane season?
, GA


As of October 22, 2005 there is a new record for tropical cyclones in the Atlantic Ocean, with 22 named storms. With Tropical Storm Alpha, and now Beta, we have, for the first time, exhausted all 21 names in the alphabet and have had to use letters from the Greek alphabet. In 1933 there were 21 tropical cyclones strong enough to be called hurricanes or tropical storms.  Hurricanes were not named back then, but if they had been named under the current system, we would have gone all the way through the alphabet. (We skip the letters Q, U, X, Y, and Z because not enough names begin with those letters.) 

In 1995 we got all the way to Hurricane Tanya, the 19th named storm of that season.  Now, ten years later, here we are with 23 storms in the Atlantic, breaking the 1933 record.

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If the military set off a nuclear explosion in the path of a hurricane, would it be strong enough to deflect the storm?


Every time we have major hurricanes that threaten the United States, I get this question, or one similar to it.  The idea you suggest has been considered, but unfortunately even an atomic or hydrogen bomb is not powerful enough to weaken the energy of a hurricane. The National Hurricane Center says that a hurricane releases heat energy at a rate of 50 trillion to 200 trillion watts. (Trillion in this case is a number followed by 12 zeros) This is the equivalent of a 10-megaton nuclear bomb exploding about every 20 minutes.  

Hurricanes are "heat engines" that depend on the temperature contrast between warmth at the ocean surface and cold air aloft.  Knowing that, one wonders whether the heat from any kind of bomb would actually add to the storm's natural heat supply, making the storm stronger. In any case, it has been said that trying to heat the upper atmosphere with bombs in an effort to lessen the heat contrast would be as fruitless as trying to heat the city of Minneapolis in January by opening the windows of a house.  

The best thing we can do right now is to learn to anticipate and prepare for nature’s power, since we have not found any ways to significantly change it.  The Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory has answers to other questions about hurricane modification here.


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Awhile back I recall hearing about a way to seed storm clouds with a substance that would dissipate hurricanes. With all the technology we have today, why can't scientists do something to lessen the impact of these terrible storms?


You’re right. About forty years ago the U.S. Government supported research into hurricane modification.  The theory was that seeding the thunderstorm clouds just outside the storm’s eyewall with silver iodide would form a new ring of clouds that would compete with the storm’s natural circulation and thus weaken it.  For cloud seeding to work, there must be a lot of “supercooled” water in the clouds; that is, water droplets that have remained liquid at temperatures below freezing.  The problem with seeding hurricanes is that most hurricanes are warmer than other storm systems, so they don’t have enough supercooled water for cloud seeding to work.  The project was eventually abandoned.

There have been other suggestions over the years, including burning soot on the ocean waters near a hurricane.  The idea was that the soot would absorb the sun’s heat and help form thunderstorms outside the core of the hurricane and, similar to the previous idea, weaken the storm’s center.  I also once heard of a similar idea to deposit oil over the seawater ahead of a tropical cyclone to prevent evaporation from occurring and maybe cut off the storm’s moisture supply.  Unfortunately the possible environmental catastrophes from these experiments could be worse than the storms’ impacts, and the effectiveness of them remains in doubt. 

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Do Atlantic hurricanes ever cross over Central America or Mexico and end up in the Pacific?
East Meredith, NY

Yes, Atlantic hurricanes can move across Central America into the Pacific, and vice-versa.  This happened in July 1996 when Atlantic Hurricane Cesar moved across Central America and was renamed Douglas when it moved into the Northeast Pacific Ocean. In 1989, Cosme in the Pacific became Allison in the Atlantic. The rule was that if the tropical storm or hurricane moved into a different basin, then it was renamed to whatever name was next on the list for the area.  But that is no longer the case.  Now, the National Hurricane Center says that if the system remains a tropical cyclone as it moves across Central America, then it will keep the original name. However, if the tropical cyclone dissipates and if there is not an identifiable circulation remaining as it moves over Central America, and if it reforms when it gets into another ocean basin, the Hurricane Center will give the storm a new name.

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What happens if there are more hurricanes than names in a given year?   Haven they ever run out of names for hurricanes in the Atlantic?
Knoxville, TN

There are 21 names on the tropical cyclone list for the Atlantic basin.  The letters Q, U, X, Y and Z are not used.  If there are more than 21 named storms in the Atlantic in one year, we would start using letters from the Greek alphabet, which we did in 2005.  So the 22nd named storm is Alpha, the 23rd would be Beta, then Gamma, Delta, Epsilon, and so on.  Since there are 24 letters in the Greek alphabet, it's not likely we'd even get close to using up that list or going beyond what would be "Tropical Storm Omega!

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Why don’t you use the letters Q, U, X, Y and Z for hurricane names in the Atlantic Ocean?
, TX


The committee of the World Meteorological Organization that maintains the list of hurricane names feels that there are not enough common names that start with those letters to use them.  Hurricane names are picked from common names of people in the areas affected by the storms, so in the Atlantic basin, for example, American, Caribbean, Mexican and Central American names will appear on the lists of tropical cyclones. 

In the Eastern Pacific Ocean east of longitude 140 W, the letters Q and U are not used, but X, Y and Z are, so you may hear of Hurricane Xina or Xavier, York or Yolanda or Zelda or Zeke there. 

For more on the various names of tropical cyclones used around there world, go to this page from the National Oceanic and Atmospheric Administration:

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Hi Nick,
Is it possible for rain to contain sea water? The rain we got from Hurricane Katrina (2005) seemed to smell salty to me.
, OH

Rain water is always fresh water, even if its original source was from the ocean. That's because when water evaporates, it leaves all solids, including salt, behind. When water vapor condenses back to water droplets to form clouds, the condensation does occur onto what are called “condensation nuclei” which are microscopic particles of dust or salt, but the particles are too small to provide any significant salt content in the rainwater. Even the clouds from a hurricane that form from evaporated seawater contain only fresh water.

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Has there ever been a hurricane to hit Seattle, Washington?
, KY

Hurricanes are tropical cyclones with warm air near the center of low pressure. They form over the warm waters of tropical oceans. This kind of storm cannot form in the cold Pacific waters off the coast of Washington State.  However, the Pacific Northwest has seen storms with hurricane-force winds.  The worst of them struck on October 12, 1962. Known as the Columbus Day Storm, it is actually the strongest non-hurricane wind storm to strike the continental U.S. in the twentieth century.  Though winds in Seattle were measured just below hurricane strength, winds in Portland gusted to 116 miles an hour, and winds on the coast peaked at 138.  46 people died and at least 100 homes were destroyed.  The storm was fueled by a dying typhoon as it crossed the Pacific and merged with an upper-level trough off the Pacific Coast.  As it moved from the south toward the Oregon coast, air pressure rapidly fell ahead of the storm center and rose quickly behind the center.  The short distance between very low pressure and very high pressure resulted in the strong winds.  Other big wind storms have hit the Northwest since then, but none has been as strong as the Columbus Day Storm.

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What are the chances of New York getting a hurricane in the form of a category 3 or higher?
Highland Mills, NY

The chances of a major hurricane striking the coast of New York are fairly remote, though not unprecedented. It has happened before.  The infamous “Long Island Express” in 1938 was a category three hurricane.  If such a strong hurricane hit the same area, which is much more developed and densely-populated today, the damage would be disastrous, and evacuations would take dangerously long.

So anyone who lives near the shore in the United States (even people on Long Island and Cape Cod, MA) need to recognize the danger of a hurricanes. They need to have a plan of action; they need to have emergency supplies in case they decide to stay home. They need to have copies of important records somewhere else beside their home. They need to have prescriptions, extra cash, financial records and anything else that they may not be able to live without. They need to have a place to go inland and stay for awhile if need be. (Motels fill up fast!) If they have pets, they need to have a plan for them. They need to know how to put plywood over windows and have the tools to do it.  These preparations are necessary even for a category one or two hurricane, and the New York tri-state area has seen plenty of those over the years.


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How are hurricanes first formed?


From late spring to early fall, weather conditions come together to form swirling tropical cyclones over the Atlantic and Pacific oceans.  This is when all the ingredients for tropical cyclones come together, including warm ocean waters, a warm moist atmosphere that is most conducive to thunderstorms, a lot of large-scale spin in the atmosphere, and not much vertical shear (turning of winds with height). It all begins with a thunderstorm forming over seawater that is at least 80 degrees Fahrenheit.  More storms develop and merge together to form a tropical wave. The thunderstorms give off heat that warms the atmosphere.  Air rises and the barometric pressure falls even more.  The falling air pressure causes the winds to increase, the Earth's rotation causes the increasing winds to spiral counter-clockwise.  Once the wind inside the wave forms a closed circulation, a tropical depression forms.  When steady winds reach 39 miles an hour, the cyclone is called a tropical storm and it gets a name.  If winds reach a speed of 74 miles an hour inside the tropical cyclone, we call it a hurricane.   

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Why do hurricanes turn counter-clockwise and not clockwise?
Whitehall, NY

Hi Wesley,

Actually, the direction of a tropical cyclone’s spin depends on whether it forms in the top half of Earth or the bottom half.  All tropical cyclones in the Northern Hemisphere rotate in a counter-clockwise direction, and clockwise in the Southern Hemisphere. 

What’s the difference? Earth’s rotation is responsible for what is called the Coriolis force, which tends to pull winds to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere.  So when a low pressure center forms north of the equator, winds are pulled to the right as they blow into the center of the low and the wind flow is counter-clockwise.  South of the equator, winds toward the low’s center are deflected to the left, so the rotation is clockwise.

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When a hurricane has winds of 100 mph, where do these winds come from? How are they made?

The winds come from the vast difference in air pressure between the center of the tropical cyclone and the edges of it.  As you may know, wind flows generally from areas of high pressure to areas of low pressure.  Take a bicycle pump, for example.  When you put pressure on the air inside the pump, the air under the high pressure flows through the pump’s tube to where the air pressure is lower.  So as a tropical cyclone strengthens and the air pressure in the center drops, air flows in from all sides of the cyclone in a counter-clockwise direction in the Northern Hemisphere and a clockwise direction in the Southern Hemisphere.  This is due to the Coriolis Force set up by Earth’s rotation. (See previous question) The bigger the difference in air pressure between the center of the hurricane and the outskirts of it, the stronger the winds will be.  That’s why when we see the center pressure drop, the hurricane grows stronger. 

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I was just wondering why hurricanes seem to move east to west and most US weather systems move west to east.
Jacksonville, FL 

You are correct. Most tropical systems, at least initially, basically move in an east to west direction.  To get the best answer for you about why this is true, I asked The Weather Channel's tropical weather expert Steve Lyons for his input.  Here’s his answer:

“Non-tropical weather systems (low pressure areas, gales and storms) generally move from west to east while tropical cyclones (tropical depressions, tropical storms and hurricanes) generally move from east to west. The answer to this is simple, they each move with the "steering" current they are embedded in. The steering current basically pushes them along. In non-tropical areas steering winds blow from west to east and hence so to do weather systems. In tropical areas steering winds blow from east to west and hence so to do tropical weather systems. The boundary that separates steering from west to east from steering from east to west is the subtropical ridge of high pressure, typically located near 30 degrees north latitude (farther south in winter and farther north in summer). South of this ridge of high pressure we find "trade winds" (blowing from east to west), north of this high pressure ridge we find "westerlies" (blowing from west to east).

Frequently tropical cyclones will move generally toward the west but also move north of about 30 degrees north latitude. When this happens we see the cyclone "re-curve" and begin moving toward the east in the same direction as non-tropical weather systems do in those locations. After all, a steering current will move tropical and non-tropical weather systems about the same, if they are about the same depth.

Unfortunately the world is not so simple and we have changing winds and steering currents in the east-west direction as well as in the north-south direction. It is for this reason we see rather wild and highly varied tracks to tropical cyclones. Steering currents are constantly changing in speed and direction due to continuously varying atmospheric weather patterns.”

Thanks for your observation, Max.  And thank you, Steve, for your expertise.

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When a big hurricane passes through the Gulf of Mexico, is enough heat energy sucked out of the ocean to significantly lower the temperature of the water?
, NM

You are correct; a hurricane will almost always cool the ocean waters over which it passes. It causes "upwelling," which is pulling cooler water up from lower depths toward the surface of the ocean. How much a tropical cyclone cools the ocean depends on the size of the hurricane, how fast it moves, and the vertical lapse rate of temperature in the ocean (how fast the temperature changes from surface to lower depths, which changes from one location to the next and even from one day to the next.) These variables in different combinations would produce highly variable results depending on the specific situation.

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I was told that if a man were to stand outside in a hurricane that the wind would be so powerful it would rip the shirt and tie clean off his body.  Could this really happen?

I forwarded your question to Dr. Steve Lyons, our hurricane expert at The Weather Channel. Here is his response:

”Having gone into a wind tunnel in sustained winds from toe to head of 160 mph, I can answer this question without a doubt of being wrong! Hurricane force winds of 74 mph are strong, but will NOT rip off your shirt or tie.  In fact, if no debris is flying through the air, it is sort of fun!  You can lean way into the wind without falling down, but it will not blow you away.  However, it is difficult to walk forward without losing your balance.
Once winds get to about 110 mph, standing without holding onto something is nearly impossible.  The wind will blow you back and/or down and it is nearly impossible to keep your eyes open...the noise is deafening. But at least your clothes are not blown off. Once winds get to about 130 mph, you need to be tied into the wind tunnel or you will blow away, the wind is far too strong to stand in, and the noise is so loud that even when you shout as loud as you can into the wind you cannot hear anything except the wind’s roar. It becomes difficult to breathe out, as air only wants to rush into your mouth and lungs.

When winds get to 160 mph it is pure survival even being tied to the floor of a wind tunnel. It is very difficult to breathe.  The force of the wind is so strong that it broke one of my ribs as I was pushed back so hard against my "climbing harness". One must wear ear plugs or you risk damaging your hearing because the wind's roar is so loud. These winds still will not tear the shirt off your back, but if there was blowing sand, you would be sandblasted to the extent that it would "blast" your clothing off your body.  Also, your face would become so permanently defaced that you would be unrecognizable (both were observed in victims of the CAT 5 1935 hurricane in the Florida Keys.) It was impossible to keep my lips closed, my mouth was shut tight, but my lips were spread wide open and I was not able to close them.”

Wow!  Adam, I think you'll agree this gives you a pretty good picture of the force of hurricane winds.   Thanks for your question.

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Is there a "purpose" for hurricanes?
New Port Richey, FL

What a thoughtful and thought-provoking question!  We tend to concentrate on the destruction of hurricanes and tropical storms, implying that they are always a “bad” thing.    

 The answer is, yes. There is an excess of heat energy in the tropical oceans around the globe.  When a hurricane forms, it feeds off the warm moisture from the tropical waters and often travels toward the poles .  The storm essentially carries some of the heat energy away from the area of excess energy, and transports it to areas with a deficit of heat energy.  Thus tropical cyclones help to keep our planet's atmospheric energy supply in balance.

Something else to consider is that much of the United States’ east coast, as well as other areas affected by hurricanes, rely on tropical rainfall to keep the soil and groundwater supplies adequate for agriculture, human consumption, and recreation.  When tropical cyclones do not bring that rain for a long time, these areas can sometimes experience  drought. 

Now don’t get me wrong.  I’m not saying that destructive hurricanes such as Katrina. Rita and Wilma are good for our way of life.  But tropical cyclones are one of nature’s ways of keeping our weather and climate in balance, and we should treat them with awe and respect. 


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Dear Mr. Walker,
Are there hurricanes or typhoons in the southern hemisphere?

Yes there are, though they usually go by a different name.  They can form in the Indian Ocean, and southern Pacific Ocean where they are simply called “tropical cyclones” or sometimes “severe tropical cyclones.”  In the southern Atlantic Ocean, there is usually not enough spin or convergence in the atmosphere and the upper level winds are too strong for such storms to form, though a hurricane did form in the southern Atlantic in March, 2004 and made landfall in Brazil.  This was the first time on record a hurricane formed in this area.

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One of our students had a great question about how fast a hurricane moves over the ground. They all know that a hurricane weakens when it moves over land due to the lack of warm water convection feeding the storm. But all of the sources we have checked online have mentioned that the land surface causes an increase of friction due to the uneven surface. The students and I both have a problem with increased friction and increased ground speed occurring at the same time. They have looked at several past hurricane tracks and speeds to discover that the storms indeed do speed up in their motion once they make landfall.  Please let me know the correct reasoning for this phenomenon.
Thank you,

What a great question! You are correct. Tropical cyclones often pick up speed after making landfall, despite the friction with land.  This is not always the case, however.  Some tropical cyclones slow down over land, especially at lower latitudes such as in Texas and Mexico.  In 1994 Tropical Storm Alberto moved over Georgia and Alabama and slowed to a snail’s pace.  As it crawled over the southeastern United States, heavy rain led to disastrous flooding.   

One thing to remember: friction with land is associated with wind speed and not the speed of motion of the entire weather system.  Tropical cyclones can pick up speed after landfall, but the winds within the system will usually weaken.  A good example of this is Hurricane Isabel.  After making landfall on the North Carolina coast, Isabel encountered faster steering winds as it became assimilated into a mid-latitude weather system.  The system picked up considerable speed over land while the winds within the system generally slowed.   

A big factor in whether or not the tropical cyclone moves faster or slower after landfall is its latitude. Atlantic storms typically move with the steering currents in the tropics from east to west.  If they turn northward, they are influenced more by the mid-latitude westerly winds. The transition from being steered by the “trade winds” to being steered by the ”westerlies” is what causes many hurricanes and tropical storms to head toward the U.S. coast only to turn away from land and back out to sea before they make landfall. These strong west winds in the upper levels often (but not always) help speed up the tropical cyclone. This increase in speed would occur whether it made landfall or not, simply because the storm is moving into a higher latitude.  Again, this is a general statement, and steering wind currents can vary in strength and direction both in the tropics as well as in the mid-latitudes. 

Thanks again for such a thoughtful question.  My thanks to meteorologists Buzz Bernard, Stu Ostro and Dr. Greg Forbes for their help.

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Has there ever been a case where two hurricanes come together and merged into one larger hurricane?
San Sebastian Sixth graders
Miami, Florida

No, it's actually impossible for two or more hurricanes to merge together. But if two hurricanes or tropical storms do come close together, they can interact with one another. Their close proximity may weaken one or both of the storms.  Or they may begin circling around each other.  This is called the Fujiwhara effect, named after a Japanese meteorologist who first explained the phenomenon.  In this case, the tropical cyclones begin to move around a center point between them.  It's sort of like two people on opposite sides of a tetherball pole each holding onto a ball and walking around the pole.  The resulting "dance" is an amazing thing to see on an animated weather satellite photograph! Go here to see a photo.

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Has a hurricane ever hit the west coast of the United States?
San Diego, CA

No hurricanes have hit California, but one tropical storm has.   In late September 1939, a tropical storm moved onshore near Long Beach with 50 mph winds.  It caused two million dollars in damage and brought 5.66 inches of rain to Los Angeles and 11.6 inches at Mount Wilson.

Although it hasn’t happened yet, it is possible for hurricane to strike the U.S. west coast.  But there are two factors that make it fairly unlikely.  As you may know, hurricanes tend to move toward the west after they form.  In the Atlantic, this brings them toward the East Coast of the U.S.  In the Pacific, this movement brings them away from our shores.  Another reason is that hurricanes thrive on warm ocean water.   Along the East Coast of the United States, the Gulf Stream provides plenty of water at temperatures of above 80 degrees during the summer and early fall.   But on the West Coast, ocean temperatures rarely get above the low 70s.  That’s just too cool to allow a hurricane to maintain its strength.

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It was said that Charley (August, 2004)  was a Category four storm with sustained 145 mph winds. Yet, now, in the aftermath, it is being said that the highest gust only touched 127, far below what it was supposed to have been. Does this mean the storm wasn't really as powerful as first thought?

New Jersey

Not necessarily. Since surface observations of a tropical cyclone at sea are impossible, the winds are estimated. But it's usually a pretty good guess. Hurricane Hunters drop instruments from airplanes above the storm to measure mid and upper level winds and barometric pressure, and formulas are used to derive the surface wind speed from those readings. Specific barometric pressure ranges correspond to specific wind speed ranges. Also, observations from satellite can be used to estimate the wind, since the storms exhibit certain characteristics with particular wind speeds.

Clocked wind speeds on land are less than the wind speeds of a tropical cyclone at sea for several reasons. First of all, the wind speed posted for a tropical cyclone at sea represents the forecasted maximum sustained wind over an open ocean ten meters (33 ft) above the surface. In other words, this is the highest wind you would find in some part of the center of the tropical storm or hurricane over water, not over land. Winds below 33 feet, where most surface measurements are taken, will not be as strong. Also, as a hurricane or tropical storm makes landfall, the center of maximum wind rarely passes over an anemometer, and even if it did, the wind-measuring instrument would probably be destroyed by the wind, so you wouldn’t get a maximum reading anyway. Portable wind instruments aren't substantial enough to carry into the eye, which would be a difficult and foolhardy endeavor. Besides, the hurricane loses strength as soon as it hits land, so only anemometers located on the beaches would give a maximum reading. In that case, wind gusts at the coast would be close to the maximum sustained winds of the cyclone while it was at sea.

One other element to consider is that the wind speeds of a hurricane are often revised upward or downward after-the-fact for the record books. Once meteorologists get a look at the damage, they are able to get a pretty good idea of what sort of wind speeds actually occurred. This is the same way they determine the F strength of tornadoes--by surveying the damage. This is why Andrew was eventually raised to a Category 5 hurricane--up from a Category 4--which it was classified as for years.

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Hey Nick,
We are thinking of going to Virginia Beach during the middle of September and are concerned about the hurricane season. When are hurricanes most likely to occur there? We're also curious about Gulf Shores, Alabama.


Dresden, ME

September is considered the height of the hurricane season, with September 10th  being the most active in the Atlantic Ocean historically.  But before you cancel your plans, consider this:  Hurricanes hit the southern Virginia coast, on average, every seven to fifteen years, so the odds are still in your favor, though the odds are much better in June and July.  The odds on the Alabama coast are only slightly worse, with a hurricane affecting that area every seven to ten years.  Thousands of people visit the Atlantic and Gulf Coast beaches in September and never experience a hurricane or tropical storm.  But if you want the best odds during the summer months, the earlier you go, the better.  Just bear in mind that even then, it is not impossible to have a land-falling hurricane or tropical storm.

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Nick, I am planning a cruise to the Caribbean in August 2005.  I hear that hurricane season in the Caribbean is from June until September.  Should I be concerned about weather spoiling my trip?

I don’t think you should worry about this.  While it’s true that there have been, and there will certainly again be tropical cyclones in the Caribbean during the month of August in any year, the chances of a cruise ship directly encountering one is pretty remote.   First of all, the odds of a tropical storm being in the Caribbean the exact week of your trip is fairly low.  Also, today’s modern cruise ships have state-of-the-art weather observing equipment that tells ship captains well in advance what sort of weather they will encounter at sea.  Cruise ships will steer clear of any hint of foul weather, and are fast enough to outrun almost any storm coming their way.  If a tropical cyclone were to be in your ship’s path, you might end up visiting a different destination than you had planned, or your ship might encounter heavier than normal waves.  Seasick patches would come in handy in this case. 

The only scenario I can think of in which you would face a serious problem would be if for some reason, your ship lost power and couldn’t move away from an approaching storm.  That’s highly unlikely, but of course, no one can guarantee a trouble-free vacation.  If August is your best month to go, I’d say, with envy in my voice, go and have a great time! 

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What is the difference between a hurricane and a typhoon?
Williamsburg, VA

Hi Alex,
The difference is basically in location. Both hurricanes and typhoons are tropical cyclones that develop from areas of low pressure and thunderstorms over warm oceans. In the North Atlantic Ocean, the Northeast Pacific Ocean east of the International Dateline (western hemisphere), or the south Pacific Ocean east of Longitude 160 east, we call them hurricanes. In the Northwest Pacific Ocean west of the International Dateline (eastern hemisphere) they are called Typhoons. Hurricanes affect areas such as the United States, Mexico, Central America and the Caribbean islands. Typhoons affect areas such as Japan, China, Taiwan and the Philippines. But they are exactly the same type of storm.

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If you were standing on a California beach and the waves were coming in toward you, then if you were standing on a beach in China would the waves be going away from you and heading for California? Or is there a place in the middle of the ocean where the waves split heading toward all the coasts? What makes the waves head towards the land?
Des Plains, IL 

I consulted our hurricane expert Steve Lyons on this, and here is his answer: 

"All ocean surface waves (except Tsunamis) are caused by the wind.   Ocean waves move in the same direction the wind is blowing. But waves have a very long lifetime, and usually only dissipate when they hit a coast. So as waves move away from the wind area that generates them they are called "swells."  Swells can move thousands of miles away from the area where the wind generated them.  For example, swells that routinely affect Southern California in summer come from the south.  When we track them we find they have been formed by strong winter storms in the Southern Hemisphere.  That means that they have traveled more than 3500 miles to the California coast, a trip that takes about one week.  

So at any given time waves are being generated locally by the local wind and swells are moving across oceans from distant wind generation areas. Those swells move along what are called "great-circle" tracks.  Those are the shortest distance between two points on a sphere, or in this case, Earth.  As swells move away from the wind generation area that made them, they "decay," or get smaller and smaller as they spread out.  In Southern California when you see swells this summer from the south they could be from a hurricane or tropical storm off the coast of Mexico or from a winter storm in the Southern Hemisphere (remember when it is summer in the northern hemisphere it is winter in the southern hemisphere).  When you see swells of 8-10 feet in California that have traveled 3500 miles or so, you can be sure that they were very large (30-40 feet) waves in the area where winds generated them. 

At any moment, waves and swells at a location (such as China or California) are the result of any wind that is pointing toward that location and generates swells.  So waves and swells in China may be generated from completely different winds or storms than those in California.

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What do the numbers 29.58 in/1002 mb on your tropical maps tell us?  What does that information mean?
Ms. J. Lenox's 4th grade class
McDonough, GA

When we show you the air pressure numbers inside a tropical depression, tropical storm or hurricane, we give the pressure readings in both inches of mercury and in millibars.  Inches of mercury are the familiar standard of air pressure measurement in the United States.  The scale refers to how high the mercury rises in a barometer due to air pressure.  Millibars, also called hectopascals. are the standard method of air pressure measurement for meteorologists.  Unlike an inch of mercury, a millibar is the unit of pressure in the metric system, like pounds per square inch would be the unit used to express air pressure in the U.S.  Since they refer to a direct measurement of pressure, millibars can be used in metric-based mathematics to calculate weather conditions.  One millibar is equal to .02953 inches of mercury.  To give you some perspective, the average pressure at sea level is 29.92 inches of mercury of 1013.25 millibars.  On the other hand, the air pressure in a category three hurricane is 27.91 or 28.47 inches on average, or 945 to 946 millibars.  Air pressure decreases with altitude, so the measured air pressure over land is usually less than what the value would be at sea level.  One important thing to remember is that when we refer to the air pressure on surface maps over land, the raw observations at various locations and elevations are adjusted to what the air pressure measurements would be at those locations if the air extended all the way to sea level.

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I have a couple of questions about the recent tropical cyclones in the South Pacific. Are they rare this time of year? Is there a difference between a hurricane/typhoon and a tropical cyclone?

Long Beach, NY

From March 6-15, 2005, tropical cyclone Ingrid ravaged the northern coast of Australia with a total of four landfalls over the various peninsulas that extend out from the Australian coastline. It was the worst in terms of damage of eleven tropical cyclones in that area this season, many of which did not make landfall. 

Tropical season comes in summer and early autumn.  So the prime time for Atlantic hurricanes is June to November, whereas most tropical cyclones south of the Equator form from November to April.  This is the time when all the ingredients for tropical cyclones come together, including warm ocean waters, a warm moist atmosphere that is most conducive to thunderstorms, a lot of large-scale spin in the atmosphere, and not much vertical shear (turning of winds with height). 

Hurricanes/Typhoons/Tropical Cyclones (with winds of 74mph or higher) are just different names (in various parts of the world) for the same weather feature. Tropical cyclones (the generic name) are called “hurricanes” in the North Atlantic and the Northeast Pacific Ocean east of the international dateline (180W).  They are called “typhoons” in the Northwest Pacific west of the dateline.  The generic term “tropical cyclone” is used to describe tropical circulations in the Southwest Pacific and Indian oceans, with an intensity attached to each.

Thanks to Hurricane Expert Dr. Steve Lyons for his input.

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What are the characteristics of a Category 5 Hurricane?
Houston, TX 

To be classified as a Category 5, which is the strongest rating on the Saffir-Simpson scale, a hurricane must have sustained winds exceeding 155 miles an hour. Winds that strong will blow trees and signs down and do extensive damage to the roofs, window and doors of buildings, and can blow roofs completely off of many homes.  Small buildings and mobile homes may even be destroyed or blown away.   A category five hurricane may also cause the ocean to rise more than 19 feet above normal levels, flooding homes within five hundred yards of the shoreline.  When such a hurricane approaches the coast, residents within five to ten miles of the shore should evacuate well before the storm hits, since low-lying escape routes may be blocked by rising water several hours before the hurricane’s center arrives.  Only three category 5 hurricanes have struck the United States in recorded history: 1935’s “Labor Day Hurricane” that struck the Florida Keys, Hurricane Camille which in 1969 struck Mississippi and Louisiana, and Hurricane Andrew that hit southern Florida and Louisiana in 1992.

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Why do hurricanes form near the equator?
Columbia, SC

Hurricanes need the warm humid air above tropical oceans in order to develop.  That’s why they form over ocean waters close to the equator.  And that’s why they form only during the summer and early fall, when those waters are about 80 degrees F or above.  But you won’t normally see hurricanes form right at the equator. That’s because at zero degrees latitude there isn’t enough turning of winds (Coriolis Force) in the atmosphere to give tropical cyclones the “spin” they need to get started. You have to get about three degrees or almost 200 miles away from the equator before you get into an area where hurricanes and tropical storms can easily form.  In 2001, Typhoon Vamei in the South China Sea formed about one hundred miles north of the Equator, but this was an extremely rare exception. 

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Do hurricane winds spin in or out?
Sound Beach, NY  

The simple answer to your question is “yes.”   A hurricane reaches all the way from the surface of the ocean to thousands of feet into the atmosphere.  At its lower levels near the ocean’s surface, air rushes inward in a counterclockwise direction in the Northern Hemisphere.  That spinning air rises upward, building the giant thunderstorms that make up the tropical cyclone and lowering the air pressure at the surface.  At the top of the hurricane in the upper atmosphere is an area of high pressure where the air is flowing outward.  The rising air flows away from the cyclone, which helps force more air upward from the surface to replace that rising air.  The hurricane’s energy is fed by that continuous flow of warm humid air into the cyclone, and the outward flow above.  In fact, if air moves into the cyclone at the lower levels slower than it moves out at high levels, the hurricane will strengthen.  That’s because the air is forced upwards faster, so the air pressure gets even lower.

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Is it possible to have hurricanes and tornadoes in the same area?
Minneapolis, MN

Yes, tornadoes often occur within a hurricane from a couple of days before landfall to as many as three days afterward, though most tornadoes that are produced by a tropical cyclone occur on the day of landfall.  Although the structure of a hurricane is much different than that of the usual tornado-producing thunderstorm, there is enough vertical shear (turning of wind direction as you go higher in the atmosphere) to produce supercell storms within the hurricane, which can bring tornadoes.  One of the biggest tornado outbreaks ever reported in the United States was from Hurricane Beulah.  It spawned 115 tornadoes in southeast Texas after it made landfall in September 1967.  1992’s Hurricane Andrew spawned 62 tornadoes in southern Florida.

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Who names the hurricanes and how do they come up with them?
Wichita, KS 

The National Hurricane Center started naming tropical cyclones in the Atlantic Ocean in 1953.  At first, the list featured only women’s names, but in 1979 the list was changed to have alternating men’s and women’s names.  The list is now kept and updated by a committee of the World Meteorological Organization, affiliated with the United Nations.  Six lists are rotated from year to year, so 2005's list of names will reappear in 2011.  The names are picked from common names of people in the areas affected by the storms, so American, Caribbean, Mexican and Central American names will appear on the lists.

Go here for a list of hurricane and tropical cyclone names.

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If the wind comes into a hurricane from all directions and rises to form the eye, why is the "right" side of the storm the most powerful?
Brattleboro, VT 

Facing the direction the tropical cyclone is moving, the right side has the reputation for being the strongest.  As The Weather Channel's Tropical Expert Steve Lyons explains, this is because in the Northern Hemisphere, the winds in a hurricane flow counterclockwise around its center. As the storm nears the shoreline, these winds blow in the direction of the coast instead of away from it, pushing seawater toward shore.  This results in higher surge and higher waves in that quadrant of the tropical cyclone.  Also, the tornado threat can be greater in this quadrant.  Incidentally, winds circulate clockwise around tropical cyclones in the Southern Hemisphere, so in their case, the left side will usually cause more damage. 


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Do hurricanes that form in the Caribbean ever turn south and make landfall on the coast of northern South American countries like Venezuela?
Lisle, Illinois                 

A hurricane that formed in the Caribbean is not likely to turn south toward South America.  However, Northern Hemisphere tropical cyclones can affect the northern countries of South America if they originate far south enough.  A recent example was Hurricane Ivan in 2004.  It set a record as the lowest latitude major hurricane.
As Ivan traveled through the southern Caribbean, hurricane watches and tropical storm warnings were issued for the coasts of Columbia and Venezuela.  In fact, one fatality in Venezuela was attributed to heavy rain from Ivan. Most of the tropical cyclones that have impacted this area have been tropical storms. For example, one in 1933 killed about 25 people in Venezuela.  Again in 1988, Tropical Storm Joan impacted the coast of Venezuela and Colombia, and Tropical Storm Bret moved over the same area in 1993.  

Tropical cyclones that enter the Caribbean either turn northward or continue westward due to the steering currents of trade winds from the east, then a clockwise flow around a semi-permanent area of high pressure to the north.  This would have a tendency to turn them northward away from South America. 

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How can wind speed be converted into force? Is there are chart to show at what speed the wind must blow to equal pounds of force applied to a surface? For example, if the wind from a hurricane reaches speeds above 70 mph, what is the force produced by the wind?

I put this question to The Weather Channel's severe weather expert, Dr. Greg Forbes, and here is his answer: 

“The force of wind is proportional to the square of the wind speed.  For example, a 50 mph wind has 4 times the force of a 25 mph wind.  A 70 mph wind would have about twice the force of a 50 mph wind.  But there’s more to it than that, and computing the exact value of the force is more difficult.  The force is also proportional to the density of the air, which can be computed if you know the air pressure and temperature.  But there’s another variable in the equation, and that is the drag coefficient of the object that the wind is blowing against.  Engineers determine various drag coefficients by putting objects in a wind tunnel.  They measure the force on the objects and working backwards from the known speed and air density to determine the drag coefficient.”   

So Mark, it looks as if converting wind speed to exact pounds per square inch is possible, but can be a little complicated. Thanks for your question!

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