Few things in nature can compare to the destructive force of a hurricane. Called the greatest storm on Earth, a hurricane is capable of annihilating coastal areas with sustained winds of 155 miles per hour or higher, intense areas of rainfall, and a storm surge. In fact, during its life cycle a hurricane can expend as much energy as 10,000 nuclear bombs!
The term hurricane is derived from Huracan, a god of evil recognized by the Tainos, an ancient aboriginal tribe from Central America. In other parts of the world, hurricanes are known by different names. In the western Pacific and China Sea area, hurricanes are known as typhoons, from the Cantonese tai-fung, meaning great wind. In Bangladesh, Pakistan, India, and Australia, they are known as cyclones, and finally, in the Philippines, they are known as bagyo.
The scientific name for a hurricane, regardless of its location, is tropical cyclone. In general, a cyclone is a large system of spinning air that rotates around a point of low pressure. Only tropical cyclones, which have warm air at their center, become the powerful storms that are called hurricanes.
Hurricane Formation and Decay
Hurricanes form over tropical waters (between 8 and 20 degrees latitude) in areas of high humidity, light winds, and warm sea surface temperatures [typically 26.5 degrees Celsius (80 Fahrenheit) or greater]. These conditions usually prevail in the summer and early fall months of the tropical North Atlantic and North Pacific Oceans, and for this reason, hurricane “season” in the northern hemisphere runs from June through November.
The first sign of hurricane genesis (development) is the appearance of a cluster of thunderstorms over the tropical oceans, called a tropical disturbance. Tropical disturbances generally form in one of three ways, all of which involve the convergence of surface winds. When winds come together (converge), the force of the collision forces air to rise, initiating thunderstorms.
One trigger for convergence is the meeting of the Northern and Southern Hemisphere easterly trade winds near the equator. The meeting of these wind belts triggers numerous, daily thunderstorms in a region called the Intertropical Convergence Zone (ITCZ). Occasionally, a cluster of thunderstorms will break away from the ITCZ and organize into a more unified storm system.
Another mechanism that can lead to the formation of a hurricane is the convergence of air along the boundary between masses of warm and cold air. Along the boundary, denser cold air can help lift warm and moist air to form thunderstorms. Occasionally such boundaries, called mid-latitude frontal boundaries, drift over the Gulf of Mexico or the Atlantic Ocean off the East Coast of the United States, where developing storms can organize into hurricanes in one of two ways. Either thunderstorms organize into a large system that forms a new area of low pressure, or a pre-existing, weak, non-tropical cyclone will form along the front and will develop into a hurricane.
Cyclones that form along mid-latitude frontal boundaries are often called mid-latitude or extratropical cyclones, and they typically have cold air at upper levels over the cyclone center. In contrast, hurricanes (tropical cyclones) have warm air over their centers. To change into a tropical cyclone, the cold air over an extratropical cyclone must change to warm air. This change can happen if thunderstorms occur near the cyclone center. The thunderstorms form along the frontal boundary as warm air rises over the colder air mass. As the air rises, it cools, and water vapor condenses into clouds. The heating released by condensation then helps to warm the air, and eventually the extratropical cyclone transitions into a tropical cyclone.
The last and most common mechanism that triggers the development of a cyclone is the African easterly wave, an area of disturbed weather that travels from east to west across the tropical Atlantic. Essentially, an easterly wave forms because of a “kink” in the jet of air that flows west out of Africa. The jet is created by the strong temperature difference between the Sahara Desert and the Gulf of Guinea. The warm air over the Sahara rises and, several kilometers above the surface, turns southward toward the cooler air over the Gulf. The rotation of the Earth turns this air current westward to form the African Easterly Jet, which then continues out over the Atlantic Ocean. Occasionally, a “kink” will develop in the jet and move from east to west, hence the name easterly wave. Converging winds on the east side of the easterly wave trigger the development of thunderstorms, and some of these large thunderstorm systems go on to become hurricanes. Most Atlantic hurricanes can be traced to easterly waves that form over Western Africa.
Given favorable conditions, the tropical disturbance can become better organized into a more unified storm system. As the storm organizes, surface air pressures fall in the area around the storm and winds begin to spin in a cyclonic circulation (counter-clockwise in the Northern Hemisphere). Surface pressures fall when water vapor condenses in rising air and releases energy, or latent heat, into areas within the tropical disturbance. The heat boosts the air (increases the buoyancy), so it continues rising. To compensate for the rising air, surrounding air sinks. As this air sinks towards the surface, it is compressed by the weight of the air above it and warms. The pressure rises at the top of the layer of warming air, pushing air at the top of the layer outward. Because there is now less air in the layer, the weight of the entire layer is less, and the pressure at the ocean surface drops. The drop in pressure draws in more air at the surface, and this air converges near the center of the storm to form more clouds.
Like an ice skater whose body spins faster as his arms are drawn inward, air near the surface speeds up as it spirals in towards the center of the low pressure area. The increasing winds that spin around the center of the storm draw heat and moisture from the warm ocean surface, providing more fuel for the rising motions that produce the clouds and increase the temperatures.
A chain reaction (or feedback mechanism) is now in progress, as the rising temperatures in the center of the storm cause surface pressures to drop even more. The lower the surface pressure, the more rapidly air flows into the storm at the surface, increasing the winds and causing more thunderstorms. More thunderstorms release more heat, forcing air at higher altitudes outward. The air pressure at the surface drops even more, triggering stronger winds, and so on.
The storm takes the distinctive, spiraling hurricane shape because of the Coriolis Force, generated by the rotation of the Earth. This is the same force that causes the south-blowing African jet to bend westward over the Atlantic, spawning easterly waves. In the Northern Hemisphere, the Earth’s rotation causes moving air to veer to the right. As air rushes towards the low-pressure center of the storm at the Earth’s surface, it curves right. If the storm is far enough from the equator (generally at least 8 degrees of latitude), the deflection or curvature is great enough that the air starts spinning counterclockwise around the center of the storm.
Once sustained wind speeds reach 37 kilometers (23 miles) per hour, the tropical disturbance is called a tropical depression. As winds increase to 63 kilometers (39 miles) per hour, the cyclone is called a tropical storm and receives a name, a tradition started with the use of World War II vintage code names such as Able, Baker, Charlie, etc. For a number of years beginning in 1953, female names were used exclusively until the late 1970s, when storm names began to be alternated between male and female names. Finally, when wind speeds reach 119 kilometers (74 miles) per hour, the storm is classified as a hurricane.
Even when the conditions are ripe for hurricane formation at the surface, the storm may not form if the atmospheric conditions five to ten kilometers above the surface are not favorable. For example, around the area of 20 degrees latitude, the air aloft is often sinking, due to the presence of the sub-tropical high—a semi-permanent high pressure system in the subtropical regions. The high pressure pushes air towards the surface. The sinking air warms and creates a temperature inversion, an extremely stable air layer in which temperature increases with altitude, the opposite of the usual temperature profile in the lower atmosphere. Called the trade wind inversion, this warm layer is very stable, which makes it difficult for air currents to rise and form thunderstorms and (eventually) hurricanes. In addition, strong upper-level winds tend to rip apart developing thunderstorms by dispersing the latent heat and preventing the warming temperatures that lead to lower air pressure at the surface.
At the surface, hurricanes can diminish rather quickly given the right conditions. These conditions include the storm moving over cooler water that can’t supply warm, moist tropical air; the storm moving over land, again cutting off the source of warm, moist air; and finally, the storm moving into an area where strong winds high in the atmosphere disperse latent heat, reducing the warm temperatures aloft and raising the surface pressure.
During hurricane development, certain characteristics become more prominent as the storm strengthens. At the center of the hurricane is the eye, a cloud-free area of sinking air and light winds that is usually from 10 to 65 kilometers in diameter. As air rises in the thunderstorms surrounding the eye, some of it is forced towards the center, where it converges and sinks. As this air sinks, it compresses and warms to create an environment (mostly) free of clouds and precipitation. The eye is the calmest part of the storm because the strong surface winds converging towards the center never actually reach the exact center of the storm, but instead form a cylinder of relatively calm air.
Bordering the eye of a mature hurricane is the eye wall, a ring of tall thunderstorms that produce heavy rains and very strong winds. The most destructive section of the storm is in the eye wall on the side where the wind blows in the same direction as the storm’s forward motion. For example, in a hurricane that is moving due west, the most intense winds would be found on the northern side of the storm, since the hurricane’s winds are added to the storm’s forward motion.
Surrounding the eye wall are curved bands of clouds that trail away in a spiral fashion, suitably called spiraling rain bands. The rain bands are capable of producing heavy bursts of rain and wind, perhaps one-half or two-thirds the strength of those associated with the eye wall.
As a hurricane moves closer to land, coastal communities begin to feel the effects of heavy rain, strong winds, and tornadoes. However, its most destructive weapon is the accompanying storm surge, a rise in the ocean levels of up to 10 meters (about 33 feet). When a hurricane approaches the coast, an 80-to-160-kilometer-wide dome of ocean water sweeps over the coastline. Storm surges have demolished marinas, piers, boardwalks, houses, and other shoreline structures, while eroding beaches and washing out coastal roads and railroads. Strong onshore winds pushing the ocean surface ahead of the storm on the right side of the storm track (left side in the Southern Hemisphere) is the primary cause of the storm surge. This wall of water is greatest when the arrival of the storm coincides with the occurrence of an astronomical high tide.
The Saffir-Simpson Scale
In the early 1970s, a classification system was designed to quantify the level of damage and flooding expected from a hurricane. This system was conceived by Herbert Saffir, a consulting engineer, and Robert Simpson, then the director of the National Hurricane Center. Using a mix of structural engineering and meteorology, they constructed the Saffir-Simpson Hurricane Intensity Scale, or simply, the Saffir-Simpson Scale. Consisting of 5 categories (1 being the weakest and 5 being the strongest), the scale corresponds to a hurricane’s central pressure, maximum sustained winds, and storm surge. Sustained wind speeds are the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region. Categories 3, 4, and 5 are considered major (intense) hurricanes, capable of inflicting great damage and loss of life.
The number of hurricanes occurring annually on a global basis varies widely from ocean to ocean. Globally, about 80 tropical cyclones occur annually, one-third of which achieve hurricane status. The most active area is the western Pacific Ocean, which contains a wide expanse of warm ocean water. In contrast, the Atlantic Ocean averages about ten storms annually, of which six reach hurricane status. Compared to the Pacific Ocean, the Atlantic is a much smaller area, and therefore supports a smaller expanse of warm ocean water to fuel storms. The Pacific waters also tend to be warmer, and the layer of warm surface waters tends to be deeper than in the Atlantic. The frequency and intensity of hurricanes varies significantly from year to year, and scientists haven’t yet figured out all the reasons for the variability.
Hurricanes and El Niño
Scientists continue to investigate the interactions between hurricane frequency and El Niño. El Niño is a phenomenon where ocean surface temperatures become warmer than normal in the equatorial East Pacific Ocean. In general, El Niño events are characterized by an increase in hurricane activity in the eastern Pacific and a decrease in activity in the Atlantic, Gulf of Mexico, and the Caribbean Sea. During El Niño years, the wind patterns are aligned in such a way that there is an increase in vertical wind shear (upper level winds) over the Caribbean and Atlantic. The increased wind shear helps to prevent tropical disturbances from developing into hurricanes. Oppositely, in the eastern Pacific, El Niño alters wind patterns in a way that reduces wind shear, contributing to more storms.
Hurricanes and Global Warming
Since warm ocean waters and warm, moist air fuel storms, theory predicts that global warming should increase the number and intensity of tropical cyclones. As the oceans soak up extra heat from the atmosphere, ocean surface temperatures rise, increasing the extent of warm water that can support a hurricane. Not only should this mean that more hurricanes can form, but increased ocean surface temperatures could also increase a storm’s maximum potential intensity, the strongest a storm can get in ideal conditions.
Models based on scientists’ current understanding of hurricanes suggest that if ocean temperatures increased by 2-2.5 degrees, the average intensity of hurricanes would increase by 6 to 10 percent. Since 1970, the average ocean temperature has warmed about half a degree, which means that theoretically, storms could be one to three percent stronger. Such an increase translates to a few knots in wind speed, too small a change to accurately measure. Hurricane wind speeds have historically been measured in increments of five knots, so any increase in intensity that has already occurred as a result of global warming would, in theory, be too small to detect yet.
However, in 2005 and 2006, several studies suggested that global warming may be impacting hurricanes more than theory predicts. In an analysis of the historical record, there appeared to be an increase in the number of intense (Category 4 and 5) storms in recent years. Another analysis charted sea surface temperatures and the number of tropical cyclones. It revealed that as sea surface temperatures went up, the number of cyclones went up. Was the increase in sea surface temperatures responsible for the increased number of storms or did some outside factor drive both?
The studies triggered many questions. Both theory and the studies suggested that there should be a link between global warming and hurricanes, but the studies showed a much greater increase in storm frequency and intensity than theory predicted. What caused the discrepancy? Is humanity’s current understanding of hurricanes flawed? Can the theory be adjusted to explain why hurricanes would have a stronger reaction to warming than previously predicted?
One theory put forth to explain the recent increase in storm intensity and frequency in the Atlantic basin is the multi-decadal oscillation. Storms in the Atlantic may go through a natural cycle of 20-30 years of increased activity followed by a quieter period. The record seems to show such a cycle, with more intense hurricanes in the 1950s and 1960s followed by two decades of relative quiet, and then increased intensity from the mid-1990s to the present. Some scientists argue that this natural cycle may actually be a product of global warming and atmospheric aerosols. In the 1970s and 1980s, aerosol pollution may have “shaded” the Earth, keeping temperatures cooler than they had been in previous decades. This cooling would have suppressed hurricane formation. In the 1990s, global warming may have increased enough to overcome aerosol cooling and allowed hurricane intensity and frequency to climb again.
Other scientists argued that the flaw isn’t necessarily in the theory, but in the historical records. Satellite data used to estimate hurricane intensity only goes back to the 1970s for the Atlantic basin, and other basins have a shorter record. A thirty-year record may not be long enough to coax out real trends. Further, satellite technology and the methods used to estimate a storm’s intensity have improved, so a storm that may have been classified a Category 1 or 2 in the 1970s through the mid-1980s would be classified as a much stronger storm today. The change in intensity-predicting methods could skew the record to show fewer intense storms in the 1970s and 1980s than there are today.
From the 1940s to the 1970s, hurricane intensity estimates were based on aircraft and ship data. This means that fewer storms were recorded than probably actually occurred. The intensity records may also be skewed because the early flights did not go directly over the eye of the hurricane, but measured winds in safer flying areas farther from the center of the storm. From those measurements, wind speeds at the center of the storm and thus the storm’s intensity were estimated. As a result, many storms may have been stronger than they were estimated to have been.
Before the 1940s, intensity estimates were made based on surviving ship’s records. It is likely that any ship at the center of a Category 4 or 5 storm didn’t survive, so the record probably contains fewer big storms than actually occurred. From changes in the methods used to estimate hurricane intensity to spotty ship records, the historical record may well be skewed towards weaker storms, argue many scientists. If all these factors were accounted for, the trend toward greater hurricane frequency and intensity could disappear.
Regardless of their position, scientists need a longer and more accurate data record to fully understand the connection between global warming and other factors that may influence hurricane intensity and frequency. A longer, more accurate record will help improve theory and models, and it will amplify or flatten the currently observed trends.
NASA Missions to Study Hurricanes
The ability to detect and track severe storms has been dramatically enhanced by the advent of weather satellites. Satellites have also helped scientists understand the mechanisms that drive hurricane formation and development. In its mission to study the Earth, NASA has developed and launched several innovative satellites that are providing unprecedented information on hurricanes.
NASA’s Quick Scatterometer (QuikSCAT) spacecraft was launched from Vandenberg Air Force Base in California on June 19, 1999. QuikScat carries the SeaWinds scatterometer, a specialized microwave radar that measures near-surface wind speed and direction under all weather and cloud conditions over the Earth’s oceans.
Data from the SeaWinds scatterometer augments traditional satellite images of clouds by providing direct measurements of surface winds. Scientists can compare the winds with the observed cloud patterns in an effort to better determine a hurricane’s location, direction of motion, structure, and strength. Specifically, these wind data are helping meteorologists to more accurately identify the extent of gale-force winds associated with a storm, while supplying inputs to numerical models that provide advanced warning of high waves and flooding.
QuikSCAT’s measurements of near-surface wind direction also help scientists answer questions about hurricane formation and evolution. For example, QuikSCAT wind measurements reveal when circulation first forms at the surface. In the earliest stages of tropical cyclone formation, winds start to circle around a low-pressure region at a mid-level in the atmosphere. When that circulation reaches the bottom of the storm, immediately above the ocean, the circling winds kick up the water vapor from the ocean needed to fuel the storm into a full-blown hurricane. As the only instrument that currently measures wind direction at the surface, QuikSCAT can reveal when a storm’s winds have reached the surface, allowing scientists to study this crucial stage of hurricane evolution.
The Tropical Rainfall Measuring Mission (TRMM), a joint project of NASA and the Japanese Space Agency, is the first space mission dedicated to studying tropical and subtropical rainfall. The satellite launched on November 27, 1997, from the Japanese Space Center in Tanegashima, Japan. TRMM carries a suite of advanced instruments that includes the world’s first spaceborne Precipitation Radar, the TRMM Microwave Imager, the Visible and Infrared Scanner (VIRS), the Clouds and the Earth’s Radiant Energy System (CERES), and a Lightning Imaging Sensor (LIS).
Scientists are using the Precipitation Radar and the Microwave Imager to peer inside the tropical thunderstorms associated with hurricanes to understand how precipitation is organized in hurricanes and how that organization relates to storm intensity and environmental effects. This information is adding to the knowledge needed to improve computer-based weather models. With these data, meteorologists may be more able to precisely predict the path and intensity of hurricanes.
For example, the Precipitation Radar detects both the horizontal and vertical structure of rain in a storm, giving scientists an unprecedented three-dimensional view of hurricanes. This view has revealed that hurricanes often produce deep, towering clouds of heavy rain just before intensifying. The presence of tall towers in TRMM data can help forecasters improve their predictions of storm intensification.
In addition to providing clues about storm intensity, TRMM reveals the storm’s structure. Since the heaviest rain clusters around the center of a cyclone’s circulation, the precipitation patterns show where the center of a storm is located well before an eye can be seen in photo-like satellite images of clouds. Knowing exactly where a storm’s center is located helps forecasters improve their predictions of where the storm will go.
On a broader scale, TRMM data are being used to answer the question of how the latent heat released during condensation of water vapor into raindrops affects global weather patterns.
Together, QuikSCAT and TRMM are providing scientists with the opportunity to observe a hurricane’s wind and rain before it makes landfall. The coincident measurements of surface wind and rain reveal the interplay between precipitation processes and surface energy exchanges within the storm. These variables are important in understanding the structure of the hurricane and predicting its path.
Carrying a suite of six instruments for observing Earth’s oceans, atmosphere, land, ice and snow, and vegetation, Aqua was launched on May 4, 2002, from Vandenberg Air Force Base, California. Four of the instruments collect information valuable to hurricane research. The Advanced Microwave Scanning Radiometer for EOS (AMSR-E) measures sea surface temperatures by recording microwave energy emitted from the Earth. Warm ocean waters fuel hurricanes, so measurements of sea surface temperatures are crucial to accurately predicting how much a storm might intensify. Because microwave energy passes through clouds, the sensor can record sea surface temperatures both in the hurricane’s path and directly beneath the storm, something previous satellites, which used infrared light to detect temperature, could not provide. These measurements reveal the cold water wake created when hurricanes churn up a shallow layer of warm water allowing cooler, deep water to come to the surface. This interaction between the storm and the ocean’s surface is visible immediately in ASMR-E microwave data, but can only be seen after the storm passes in infrared-based sea surface temperature measurements. AMSR-E also provides information on the precipitation structure of hurricanes similar to the TRMM Microwave Imager. By combining precipitation information from the TRMM Microwave Imager, AMSR-E, and other satellites, scientists are able to monitor changes in the precipitation structure of storms and to estimate the total rainfall occurring along the path of the storms.
Three additional instruments on Aqua provide information about atmospheric conditions. The Atmospheric Infrared Sounder and the Advanced Microwave Sounding Unit work together to give temperature and humidity profiles of the atmosphere. Data from the Moderate Resolution Imaging Spectroradiometer can be used to measure atmospheric water vapor, aerosols, and clouds. Taken together, this information about the atmosphere is helping scientists understand what influences hurricane formation and development.
CloudSAT and CALIPSO
Launched together from Vandenberg Air Force Base in California, on April 28, 2006, CloudSat and CALIPSO fly in close formation to provide nearly simultaneous, three-dimensional measurements of cloud structure. CloudSat carries the first satellite-based, millimeter-wavelength cloud radar—a radar that is more than 1000 times more sensitive than existing ground-based weather radars. The radar sends out radio waves and records their return to determine the location of tiny particles of water and ice that make up clouds. CALIPSO, for Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations, similarly uses laser light pulses to measure the location of ice, water, and aerosol particles. By plotting the location of particles, scientists get an unprecedented two-dimensional view of a cloud’s structure.
One of the questions that NASA scientists are trying to answer with data from the Aqua, Cloudsat, and CALIPSO satellites is what impact warm, dry, dusty air blowing out of the Sahara Desert might have on hurricane formation in the Atlantic. Does the dry air suppress hurricane formation, or does dust provide seeds for clouds, prompting storm formation? Together, the three satellites reveal humidity, aerosols (dust), temperature, and cloud structure within the layer of Saharan air, which allows scientists to map out these characteristics of the air mass in relation to where and when hurricanes form. By understanding where Saharan air is in relation to hurricanes, scientists can then observe what impact the air might have on storms.
Supplementing satellite observations, NASA has conducted a number of field campaigns, which use aircraft and ground-based instruments to get a more detailed view of hurricanes. Field studies generally look at focused questions such as why do towering clouds lead to intensification, or how does the wind structure at various points in the storm change as a cluster of thunderstorms transitions into a full-blown hurricane. In field studies, scientists fly aircraft carrying an array of radar and other instruments over or within a storm. The instruments measure rain structure, temperature, humidity, and winds directly under the plane, giving a very detailed view of a slice of the storm. Satellites (with the exception of CloudSat and CALIPSO), by contrast, show a wider area in less detail.
While satellite and field observations are essential to understanding hurricanes, it is impossible to observe all of a storm in complete detail throughout its lifetime. Weather satellites that have a constant view of storms provide distant, low-resolution pictures. Satellites that capture the storm in greater detail and aircraft see the storm only when they fly over it. To fill in the gap, scientists use observations as input to computer models, mathematical descriptions of the atmosphere or storm. Some models focus on a very small scale, looking at the processes within individual hurricanes, while others focus on a global scale, looking at global ocean and atmosphere conditions in relation to storm formation.
Not only do the models allow scientists to look at what might be happening in a storm moment by moment, but they also allow scientists to experiment. In a model, you can change the amount of dust in the Saharan air layer, for example, or the circulation of air inside a storm, and see what effect the change has on the storm. By combining models with field and satellite data, NASA scientists are working to more fully understand the structure and mechanics of hurricanes and the large-scale climate patterns that influence them.