Question: What Is A Hurricane?

Question: What Are The Different Hurricane Development Phases?

Question: What Is An Easterly Wave?

Question: What Is Storm Surge?

Question: What Is Wind Shear?

Question: What Is The Saffir-Simpson Scale?

Question: Why are hurricanes predicted to be more frequent?

Question: Is there a link between hurricanes and climate change?

Question: What Is A Hurricane?

Answer: An intense storm that develops in the tropics with sustained wind speeds of 74 mph or greater. In the Atlantic Ocean and the Eastern Pacific Ocean these storms are called "hurricanes." In the Western Pacific, the same type of storm is called a "typhoon," and in the Indian Ocean and the Southwest Pacific they are called "cyclones." These storms have large, closed circulations that develop over warm tropical and sub-tropical water, and rotate counterclockwise in the northern hemisphere or clockwise in the southern hemisphere.

INTERACTIVE: Hurricanes 101

Scientists have observed that tropical cyclones tend to develop when two critical conditions are reached: (1) ocean temperature of at least 80°F (26.5°C) and (2) penetration of this heat through a layer extending from the surface to 150 ft (50m) deep. To cool off the over-heated tropical waters, the thermal engines of the Earth respond by transporting this excess heat to cooler regions. This temperature adjustment is made possible by the interplay between the ocean waters and overlaying atmosphere through which tropical cyclones are spawned. In the Atlantic Ocean, these mechanisms take place off western coastal Africa.

Question: What Are The Different Hurricane Development Phases?

Answer: As a storm builds and wanes, its strength level is referred to in terms that are shorthand for its wind speeds. These phases are:

Tropical Disturbance: An organized tropical system without a closed circulation.

Tropical Depression: A tropical system with a distinct closed circulation, but peak sustained wind speeds less than 39 mph.

Tropical Storm: A large, organized tropical system with a distinct closed circulation and sustained wind speeds of 39 mph or greater. When the winds first exceed 39 mph, the tropical storm is given a name. An international committee at The World Meteorological Organization (WMO) in Geneva, Switzerland creates separate lists of names for tropical storms that form in 12 different ocean regions throughout the world. The names tend to come from culturally common names in each region, and progress down the alphabet.

Hurricane: At this stage the cyclone has achieved maturity with wind speeds at or greater than 74 mph (33 m/s). Meteorologists will use the Saffir-Simpson scale to explain the speed and potential impact of winds in a particular hurricane; category 1 is the least intense while category 5 is the most potent.

Question: What Is An Easterly Wave?

Answer: An easterly wave is a tropical wave that spurs a disturbance in the normal air flow, resulting in a trough or low-pressure system.

Between May – October, an inverted temperature gradient (inversion) is generated over western and central North Africa. In the lower troposphere, temperatures generally decrease with altitude, however, during an inversion the opposite results and temperatures rise with height.

When waves, generated on the lee-side (opposite of the prevailing winds) of mountains, mix with moist humid air from western coastal Africa they generate upper-air instability that can promote cyclonic development. According to NOAA, there appears to be some link between easterly waves and the intensity of tropical cyclones but not necessarily an association with the actual number of such storms.

Question: What Is Storm Surge?

Answer: Storm surge is the onshore flow of a massive volume of water that comes as a tropical cyclone makes landfall. It is measured by subtracting the astronomical tide from the actual tide. A "Sea, Lake, and Overland Surges from Hurricanes" (SLOSH) model was created to estimate the amount of inland water deluge can be expected, but it does not take into account the amount of rain water the storm may add to this. Because of this SLOSH provides a useful baseline.

Question: What Is Wind Shear?

Answer: Wind shear is a variation in wind speed and/or direction at varying heights. Wind shear can nullify a tropical cyclone by weakening the potential for vertical development. For example, easterly winds are the predominant winds for Atlantic hurricanes. If a westerly wind moves in at any level, it has the potential to destroy the vertical development of a cyclone.

Question: What Is The Saffir-Simpson Scale?

Answer: The Saffir-Simpson scale was developed by Herbert Saffir, an engineer, and Dr. Bob Simpson, a meteorologist and former head the National Hurricane Center at NOAA. In 1971, Saffir, while helping to write building code for Dade County, FL, created a scale of potential wind damage that can be expected from a range of wind speeds. In 1972, Simpson correlated the wind data with potential flood damage. This correlation resulted in the Saffir-Simpson scale with categories 1 – 5. Category 1 having the least expected damage and category 5 having the most. Wind speed is the determining factor for assigning a cyclone to a particular category.

Over the life a cyclone, it may change category at any point in time:

Category 1: winds 74 – 95 mph (19-153 km/hr), with surges 4-5 feet above normal. expect: minor damage to trees/shrubbery and poorly constructed buildings.

Category 2: winds 96-110 mph (154-177 km/hr), with surges 6-8 feet above normal. expect: medium damage to roofs, windows; considerable damage to trees/shrubbery, piers, mobile homes, and poorly constructed buildings.

Category 3: winds 111-130 mph (178-209 km/hr), with surges 9-12 feet above normal. expect: blockage of low-lying evacuation routes, destruction of smaller coastal structures and severe battering of larger buildings, low level escape routes are flooded 3 –5 hours before hurricane.

Category 4: winds 131-155 mph (210-249 km/hr), with surges 13-18 feet above normal. expect: extensive damage to doors, windows and building walls, collapsed trees/shrubbery, complete destruction of mobile homes, massive flooding, particularly of areas lower than 10 feet above sea level, considerable damage due to floating or flying debris, massive evacuation for inland areas reaching in as far as 6 miles (10 km).

Category 5: winds greater than 155 mph (greater than 249 km/hr), with surges greater than 18 feet above normal. expect: severe and extensive window, building damage, massive flooding for areas lower than 15 feet above sea level and within 500 yards of the coast, massive evacuation for low-level inland areas 5-10 miles (8-16 km) from the coast.

Question: Why are hurricanes predicted to be more frequent?

Answer: You may have heard that hurricanes are likely to grow in frequency and intensity during the next 20-40 years since the mid-1990s. What is this all about?

The Atlantic Multi-decadal Oscillation (AMO). The AMO is a climate scale phenomena that results in a seesaw-like effect in North Atlantic Ocean temperatures over several decades. At present, the AMO is in a warming phase, which means that tropical storms will have a large energy reservoir from which to develop into hurricanes. Scientists at the National Oceanic and Atmospheric Administration (NOAA) estimate that tropical storms are twice more likely to develop into hurricanes during this warming phase than during a cool period.

Paleoclimate evidence suggests that the AMO is most probably a natural part of the climate system. Large fluctuations in the AMO have the potential to mask or inflate the impact of global warming. Scientists are working hard to understand what controls the AMO and to more fully understand its full nature.

Question: Is there a link between hurricanes and climate change?

Answer:Yes, but the issue is whether human- or natural-induced climate change is at the root. Scientists who advocate the human-induced element say that there is link a between greenhouse concentrations and hurricane frequency and intensity.

Those who argue that argue natural-induced climate change is responsible sight a close link with hurricane intensity with fluctuations in the Atlantic Ocean temperatures (see Atlantic Multi-decadal Oscillation (AMO) (more on AMO: link to A6)), which have been in a warm phase starting in 1995. In support of this view, 1995 was the start of a very intense Atlantic hurricane season that is projected to last the next 20-40 years. This time span is the expected duration of the AMO warm-phase.

Who is right? Probably both. It is possible that human-induced changes are enhancing natural climate variability. The models are unable to discern the difference.