Q:    What’s the difference between a hurricane and a typhoon?

A:    They are two different names for the same kind of storm – a tropical cyclone, which is a low-pressure area in which the central core is warmer than the surrounding atmosphere. They’re called hurricanes in the Atlantic Ocean and the Eastern Pacific Ocean, and typhoons in the Western Pacific. In addition, these storms are known as cyclones in the Indian Ocean and around the Coral Sea off northeastern Australia.

Q:    Does the size of a hurricane’s eye have a relationship
to its strength?

A:    When forecasters are tracking a tropical cyclone, the eye typically becomes more well-defined and smaller as the system intensifies. However, depending on the overall size of the storm, there is a limit to how small the eye can become. Sometimes the eyewall becomes ragged or disappears completely, with a corresponding weakening of peak winds, only to be replaced by a new eyewall and restrengthening of the hurricane. This is called an eyewall replacement cycle.
While most hurricanes are about 300 miles in diameter, the overall size of the hurricane is also not necessarily indicative of its strength. Hurricane Andrew in 1992, a relatively small hurricane, packed quite a punch as it made landfall in southeast Florida as a Category 5 hurricane. Another good example of a small but potent storm was Hurricane Charley in 2004.

Q:    Where do most hurricanes originate – in the
Atlantic off the African coast, in the Gulf of Mexico
or in the Caribbean?

A:    Most hurricanes begin in the Atlantic as a result of tropical waves that move westward off the African coast. Some form in the vicinity of the Cape Verde islands and are known as “Cape Verde” hurricanes. The Atlantic is the favoured location for tropical cyclones that occur during the peak of hurricane season from mid-August through mid-October. During the early and late stages of hurricane season, which runs from June 1 through November 30, storms are typically less numerous and are more likely to originate in the Caribbean or Gulf of Mexico.

Q:    A hurricane “picks up” saltwater over the ocean,
but rains fresh water. Where does the salt go?

A:    The process by which a hurricane “picks up” water over the ocean is called evaporation. That is, heating by the sun makes saltwater warm enough that molecules of water leave the liquid state and become water vapor in the atmosphere. The salts that are diluted in the water are left behind, making the remaining ocean water even saltier. The water vapour condenses to form clouds within the hurricane and eventually precipitates out of the storm as fresh rainwater.

Q:    Why does wind shear tend to strengthen thunderstorms but weaken hurricanes?

A:    Wind shear, changes in wind speed and direction with elevation, gives a thunderstorm its spin in the form of rotating updrafts called mesocyclones. Wind shear also helps to keep a thunderstorm’s updraft separated from its downdraft, increasing the storm’s strength and duration. A hurricane, on the other hand, is a large collection of thunderstorms and occurs on a much larger scale than do individual thunderstorms. A hurricane gets its spin from the rotation of the Earth rather than from wind shear. Hurricanes rapidly intensify when the thunderstorms form tightly around the centre of circulation. Like pushing over a spinning figure skater, wind shear tends to disrupt the balance of the storm.

Q:    Do hurricanes hit Central America?

A:    Yes, Central America can be affected by hurricanes. Storms that form in the north Atlantic and eastern Pacific basins often hit or come close to the region. One of the worst storms was in October 1998, when catastrophic Hurricane Mitch killed more than 9,000 people, mostly in Honduras and Nicaragua.
South America, on the other hand, is rarely affected by tropical systems. Cooler water, high wind shear, and the lack of an Intertropical Convergence Zone – a constant area of low pressure and thunderstorm activity along the Earth’s equator – in the south Atlantic and south eastern Pacific basins limit any tropical activity that may affect South America.

Q:    What is a tropical wave?

A:    Tropical waves, also known as easterly waves, are elongated low-pressure areas that cause cloudiness and thunderstorms. They form when warm air drifts off of Africa into the Atlantic Ocean, and they are usually pushed westward by prevailing winds. Tropical waves can often develop into tropical cyclones:    60 per cent of all Atlantic tropical cyclones originate from tropical waves, and about 85 per cent of Atlantic hurricanes of Category 3 or greater come from tropical waves.

About 60 tropical waves form off of the African coast each year. Sometimes, tropical waves can have tropical-storm force winds (39-73 mph) without being considered tropical storms, since tropical storms must have closed circulation.

Hurricane formation and its progression always begins with a tropical disturbance. Tropical disturbances can be tropical waves, but not always. The disturbance then develops into a tropical depression, which has a closed circulation. The system then forms into a tropical storm, before finally developing into a hurricane.

Q:    Where do most Atlantic Ocean hurricanes originate?

A:    The origins of many Atlantic hurricanes can be traced to tropical waves that develop over sub-Saharan Africa and move west into the eastern Atlantic. During the mid-August to mid-October peak of the hurricane season, which runs from 1 June 1 to 30 November, tropical storms and hurricanes tend to develop in the eastern Atlantic, often near the Cape Verde Islands off the coast of Senegal. Early and late-season tropical storms and hurricanes are more likely to develop in the western Caribbean or even in the Gulf of Mexico.

Q:    Why do hurricanes strengthen over warm water and
weaken over cooler water?

A:    Hurricanes do not always strengthen over warmer water or weaken over cooler water. The atmospheric environment in which the hurricane is embedded has a profound effect on whether a hurricane weakens or strengthens. This is what makes forecasting hurricane intensity change so difficult.

However, sea water temperatures can cause changes in intensity. Cooler water puts less moisture and energy into the atmosphere, typically reducing its ability to make thunderstorms. Conversely, warmer water puts more moisture and energy into the atmosphere, making it more amenable to thunderstorm development.
Since thunderstorms are one important conduit for quickly transferring ocean energy into the atmosphere, their absence prevents strengthening and/or causes weakening.

Hence, cold water is unfavourable for hurricane development. In general, the ocean water temperature necessary to form a hurricane is around 80°F. But once a hurricane forms, it can persist in water below 80°F, sometimes for many days.

Q:    I know typhoons and hurricanes are the same type of storm, but it seems like most typhoons are usually stronger than hurricanes. Is this true?

A:    Hurricanes form in limited areas of the Atlantic and northeast Pacific, but typhoons in the northwest Pacific draw on the world’s largest pool of deep, warm seawater. The journal Science reported in September that, from 1990 to 2004, 41 per cent of northwest Pacific typhoons and southwest Pacific tropical cyclones reached Category 4 or 5 strength, while only 25 per cent of Atlantic hurricanes did.

Q:    How is a hurricane’s path predicted?

A:    It should be recognised that, even for the experts at the National Hurricane Center, predicting a hurricane’s path often comes with great uncertainty.

The forecast of a hurricane’s path depends largely upon the accuracy of the predicted winds from computer forecast models. The speed and direction of these steering winds typically vary with altitude. Weak tropical cyclones tend to be steered more by lower-level winds, while upper-level winds usually influence the paths of stronger hurricanes.

So not only does an accurate path forecast depend on the accuracy of the computer models, but it is also tied into the forecast intensity of the storm as well.

Q:    What is meant by the “Atlantic basin?”

A:    It’s a geographic term used to describe where Atlantic tropical storms and hurricanes form. The Atlantic basin includes a part of the North Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea. There are seven separate basins in oceans around the world, each of which uses a different list of names chosen by the countries affected in that particular basin.

Q:    Has a tropical storm or hurricane ever travelled from the Atlantic basin into the Pacific basin?

A:    This has indeed occurred, most recently in 1996 when Atlantic Hurricane Cesar moved across Nicaragua and El Salvador and emerged in the northeast Pacific as Hurricane Douglas. Since 1949, storms have traveled from the Atlantic to the Pacific seven times. Two storms have moved from the Pacific into the Atlantic.

There is no record of a storm ever crossing from the Atlantic to the Pacific and back again into the Atlantic.

Q:    Why don’t Pacific hurricanes hit the West Coast?

A:    Winds in the tropics generally blow from east to west. This means that a west-northwest motion of a hurricane in the Atlantic basin will carry it toward the US mainland. As they approach the US the storms are maintained – and sometimes strengthened – by the warm water of the Gulf Stream off the Eastern Seaboard or warm water in the Gulf of Mexico.

Storms that form in the eastern Pacific Ocean tend to move away from the Mexican coastline, as well as the US mainland. The few that stray to the north occasionally make landfall in Mexico. Any further north and they run into the cooler ocean water off the California coast.

Q:    I thought that weather systems move from west to east; however, why do hurricanes move to the west?

A:    Not all weather systems move from west to east. With the exception of Florida, the continental USA is generally located within the mid-latitudes, which stretch from 30°N to 60°N. Mid-latitude weather systems in the Northern Hemisphere do indeed typically move from west to east. However, in the subtropics, where hurricanes typically form, most weather moves from east to west.

Q:    What determines the different categories of a hurricane: wind speed or barometric pressure?

A:    The Saffir-Simpson scale rates storms according to their intensity, as measured by the sustained wind speed of the storm. While wind speed is the determining factor in the scale, central air pressure and storm surge are also used to gauge the damage potential of the hurricane.

Q:    How are hurricane intensity and storm surge related?

A:    Stronger, more intense hurricanes tend to produce a higher storm surge, which is the mound of water that builds up as a hurricane moves over water. Forecasting a hurricane’s path is just as important as forecast intensity to determine the potential for storm surge. As the mound of water approaches the shore, the topography of the ocean floor and coast, along with the timing of local tides, will ultimately determine the size of the storm surge.

Q:    How does the jet stream affect the paths of hurricanes and has the jet stream moved north, thus causing or allowing such bad tropical weather the past few years?

A:    The jet stream, particularly the subtropical jet stream, does impact the development of tropical systems.
 
A strong subtropical jet tends to produce shear that weakens or destroys tropical systems. A weaker jet allows for development and active hurricane seasons.

The ingredients of warm ocean waters and relatively weak atmospheric wind shear have helped to increase tropical activity in recent years.

El Nino and La Nina tend to impact the location and strength of the jet stream, and therefore, impact hurricane season activity.

Q:    If hurricanes draw their water from the sea, why isn’t the rain in hurricanes salt water?

A:    When over the ocean, hurricanes draw much of their moisture and energy from the condensation of water vapour that evaporated from warm ocean water. Having undergone the evaporation process and, thereby, leaving its salt content in the ocean, the water is fresh. However, the hurricane does cause some sea water to get into the lower atmosphere due to the strong winds taking the tops of waves and blowing them apart. Thus, the rain from hurricanes falls as drinkable water, with only slight salt content.

Q:    Does the reported wind speed for a hurricane also include its forward wind speed?

A:    A hurricane’s sustained winds and wind gusts include the winds from the storm’s rotation as well as the forward speed of the storm itself. For typical westward moving tropical systems spinning counterclockwise in the Northern Hemisphere, the forward speed adds to winds on the north side of the storm, while the forward speed subtracts from winds on the south side.

One excellent example of this addition of a storm’s rotational and forward speeds is “The Long Island Express,”

the great hurricane of 1938. The 70 mph forward speed of the storm, the fastest known forward speed ever recorded, produced hurricane wind speeds that exceeded 180 mph in eastern Long Island and New England.

Q:    What’s the difference between a tropical cyclone and a subtropical cyclone?

A:    A cyclone is defined as an area of low atmospheric pressure that has a closed circulation. Cyclones (or more commonly called low- pressure areas) are typically divided into tropical and extratropical cyclones, the difference being the source of energy for each type of system. Tropical cyclones form in warm, humid air masses found over warm ocean waters during the summer months. Extratropical cyclones, on the other hand, are those familiar mid-latitude storms that occur in all seasons and can form over land or water. Temperature contrasts are their major source of energy and they have fronts, or boundaries between warm and cold air.

Now that we have the basic differences between tropical and extratropical, we can tackle subtropical cyclones, non-frontal low pressure systems that have characteristics of both tropical and extratropical cyclones. Subtropical cyclones usually form out of extratropical storms over subtropical oceans, north of 23.5°N or south of 23.5°S latitude.

While the subtropical cyclone is initiated by a temperature contrast between air masses, thunderstorms near its center generate latent heat given off by the condensation of water vapor. This warms the storm’s central core.

With a warm core established, meteorologists studying hybrid storms observed eye-type features, more typical of hurricanes, forming in many intensifying storms. They also determined the Northern Hemisphere version of these evolving systems exhibited a tight temperature contrast on their northern side, a feature exclusive to extratropical storms, not tropical storms. With these storms exhibiting characteristics of both tropical and extratropical cyclones, the term subtropical encapsulates the “hybrid” nature of these storms.
Q:    Could Venezuela ever receive a direct hit from a hurricane?

A:    While it is technically possible, no hurricane in recorded history has ever directly hit Venezuela. Most storms only wind up grazing the country’s northern coast. Just like Hurricane Ivan in September 2004, the effects of which killed three Venezuelans, tropical storms have skirted the coast many times.

An unnamed tropical storm in 1933 was known to have killed about 25 people in Venezuela. In 1988, Tropical Storm Joan skirted the coast of Venezuela and Colombia as did Tropical Storm Bret in 1993. In 1996, Hurricane Cesar formed just off of Venezuela, and in 2000, Tropical Storm Joyce flirted with the Venezuelan coast.

There are several reasons that Venezuela is not a common target for Atlantic hurricanes. First of all, because northernmost South America (Venezuela and Colombia) is only at a latitude of about 12° north, the Coriolis force is still relatively weak and lacks the sufficient spin for tropical system development.

Storms that do form in these low latitudes are often steered toward Central America or the Gulf of Mexico by ocean currents and trade winds. Additionally, land masses inhibit the formation of tropical depressions and typically prevent them from intensifying into hurricanes.

Q:    How does an African tropical wave turn into a tropical storm or hurricane?

A:    Once the wave moves over the ocean, showers and thunderstorms often intensify. When this occurs, the surface air pressure drops and the wave becomes a strong low-pressure area. Sometimes the wave is so strong that it exits Africa as a closed low-pressure area, and then intensifies. Given enough time, and with persistent showers and thunderstorms, the wave transforms into a well-organized tropical depression, then a tropical storm and finally a hurricane.

Q:    Could it ever snow during a hurricane, say if a late-season hurricane meets up with true arctic air? Has it ever happened?

A:    Hurricanes are driven by warm, moist air. When cold air intrudes, hurricanes quickly become extratropical lows, with a different structure built on temperature contrasts. Over a foot of snow fell in parts of New England on October 10, 1804, on the edge of a storm believed to have once been a hurricane.

Weather histories have reported other snowfalls related to systems believed to have had tropical origins, much like the “snow hurricane” of 1804. However, it is difficult to verify if actual tropical storms or hurricanes were involved. Nor’easters
were sometimes confused with hurricanes before the 20th Century, because of their very high winds they bring and the mild air that often sweeps inland near the coastline in places such as Boston.

Q:    What causes African tropical waves (that later turn into hurricanes) to form?

A:    Tropical waves are thought to be formed by instability in a low-level jet stream of east winds over tropical West Africa (known as the African easterly jet stream). Undulations in these east winds result in waves in the easterly flow and are called “easterly” or “tropical” waves. They are tied to the monsoon trough of low pressure that is a regular summer climate feature over this region of Africa.

Q:    Do hurricanes in the Atlantic basin relate to those in the eastern Pacific basin?

A:    Hurricanes that form in the Atlantic Ocean don’t have any direct relation or cause for hurricanes forming in the eastern Pacific Ocean. However, during an El Niño
year, when the eastern Pacific water is unusually warm, there tend to be more eastern Pacific hurricanes and fewer Atlantic hurricanes.

Q:    What is the worst side to be on when there is a hurricane? I have heard there is one side that is worse than the other.

A:    The right side of a hurricane usually has the fastest winds, while the left side often has the heaviest rain. (Most people describe hurricane severity by wind speed not rain amounts, even though the number one cause of death in a hurricane is fresh water flooding.)

A hurricane is a whirling mass of thunderstorms that spins counterclockwise. As a hurricane moves, the wind on its right side blows in the direction of the storm’s motion. That means a hurricane packing 100 mph winds and moving at 20 mph has a combined wind of 120 mph on that side of the storm. It also means that the forward motion of 20 mph takes away from the opposing wind on the left side of the storm:    100 mph–20 mph = 80 mph. In this example, that’s a 40 mph difference in wind from one side of the storm to the other. The forward motion has no effect on the wind in the front or rear part of the hurricane.

While storm surge generally occurs in the centre of a hurricane, higher waves and the onshore wind in the right side of the storm can worsen the surge there. Also, tornadoes are most likely in the right-front part of a hurricane, which makes the right side that much worse.