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(Photo courtesy NOAA)

What is a hurricane?

Hurricanes are large rotating tropical storms with winds in excess of 119 kilometers per hour (74 mph). They usually form between June 1 and November 30 in the Atlantic Ocean but can develop in other oceans as well. They are known as typhoons in the western Pacific and cyclones in the Indian Ocean.

The anatomy of a hurricane is simple. Its center is a cloud-free, relatively calm area called the eye. The eye is surrounded by the much more active eye wall, a ring of thunderstorms where the hurricane’s winds are the strongest and rains are the heaviest. Spiral bands of clouds, rain, and more thunderstorms extend out from the eye wall like a pinwheel on top of a rotating funnel. These rain bands, as they’re known, can stretch for hundreds of miles and sometimes contain tornadoes.

Scientists classify the strength of a hurricane using a system developed in the 1970s called the Saffir-Simpson Scale. It consists of five categories, based on wind strength: 1 is the weakest and 5 is strongest, with winds exceeding 251 kilometers per hour (156 mph).

How do hurricanes form?

The exact combination of conditions needed for hurricanes to form is still poorly understood, but one key factor is well documented: warm ocean water.  Warm water induces evaporation, causing more water vapor to rise from the ocean surface into the atmosphere. Hurricanes begin over the ocean as tropical storms triggered by a disturbance in the atmosphere. Once triggered, the Earth’s rotation causes the warm, moist air at the ocean surface to rise in a spiral pattern. Below this rising air mass, an area of low pressure forms. As moist air rises, it releases heat, cools down, and condenses into windy bands of clouds and thunderstorms. The low-pressure base acts like a vacuum that sucks more warm, moist air into the spiral.

For a storm to gain enough energy to develop into a hurricane, the temperature of surface waters needs to rise above 26⁰ C (79⁰ F).  The warmer the water, the more energy to fuel the hurricane, and the stronger it becomes. Energy released as the air rises and condenses sustains the hurricane as it moves over the ocean. Once a hurricane makes landfall its energy decreases and the hurricane weakens.

how hurricanes form

Most Atlantic hurricanes begin to form over Africa, where hot, dry desert air meets cool, wet air. In the seam between these high- and low-pressure air systems, a powerful westward stream known as the African Easterly Jet forms. Atmospheric disturbances that break off from the swerving jet can trigger hurricanes. (Illustration by Natalie Renier, Woods Hole Oceanographic Institution)

How do scientists study hurricanes?

Scientists use a variety of data, tools, and approaches to study hurricanes. They include satellite imagery of the surface temperature of the ocean, radar to locate precipitation and estimate its motion and type (rain, snow, etc.), computer models that rely on current and past weather patterns to make predictions, floats that collect measurements of ocean water temperature, and a fleet of specialized aircraft known as “hurricane hunters.”

Hurricane hunters are piloted by members of the U.S. Air Force 53rd Weather Reconnaissance Squadron and the National Oceanic and Atmospheric Administration (NOAA). The airplanes are specifically designed to fly into hurricanes to measure their maximum wind speeds and get a firsthand view of these powerful storms.

How well can we forecast hurricanes?

Like any weather phenomenon, hurricanes can be difficult to forecast. Right now, scientists can do a much better job of predicting the track of a hurricane than of estimating how strong the hurricane will be when it reaches the coast.

NOAA’s National Hurricane Center (NHC) has been responsible for hurricane prediction since the early 1950s. Once a tropical storm forms over the ocean, NHC tracks it closely, and together with partner organizations, they issue regularly updated forecasts about whether it will develop into a hurricane and where and when it will make landfall. The predicted path of a hurricane is called the “cone of certainty.” It is developed using computer models that process historical hurricane data with data on current atmospheric conditions. These forecasts extend out for about five days and are updated to reflect the dynamic nature of the storms. On average, models have accurately predicted the path of hurricanes about two-thirds of the time.

hurricane track

This montage of satellite images shows Hurricane Irma and its shifting intensities as it tracked across the Atlantic Ocean and Caribbean Sea in August 2017. (Cooperative Institute for Meteorological Satellite Studies Space Science and Engineering Center/University of Wisconsin-Madison)

Why are hurricanes important?

Hurricanes are powerful natural energy systems that can unleash torrents of destructive winds and rain that can have significant impacts on lives, communities, and entire regions. They can knock out power lines and foul water systems; flood and destroy homes, businesses, and infrastructure; turn everyday objects into deadly missiles; and claim lives.

They are also very costly: In 2017, a year that included the major hurricanes Harvey, Irma, Jose, and Maria, damages in the U.S. exceeded $300 billion. 2017 broke the previous cost record of $215 billion set in 2005, when hurricanes Katrina, Rita, and Wilma hit.

Importantly, the strength of a storm does not necessarily correlate directly with its potential devastation. Some “weaker” storms—as defined by their wind strength—can wreak havoc on coastal communities by pushing waves of ocean water into the coast—a phenomenon known as storm surge. The likelihood of damage and fatalities has increased as a growing number of people move to coastal cities.

How do hurricanes affect the environment?

Hurricanes affect ecosystems in a variety of ways. Some species of plants and animals fare better than others. Winds can uproot trees, and storm surges can carry salt water up inland rivers, harming or killing plants and animals that cannot tolerate salt. High tides can easily wipe out sensitive sea turtle and bird nests along shorelines. Violent wave action kills many fish. Coastal waters that typically nourish seagrass beds—home to crabs and fish—can grow clouded and toxic with sediments and pollutants. The drop in air pressure resulting from a hurricane often disorients manatees and dolphins. On the other hand, sharks can detect the drop and safely head for deeper waters. Whereas some birds detect the pressure shift and escape in advance of storms or safely weather them on the ground, others can be thrown far off course or get trapped in the eye of a hurricane.

Conversely, some animals actually benefit from hurricanes. These include scavengers such as raccoons that take advantage of new food sources after storms; certain frogs and toads that breed in heavy rainfall; and plants that use wind to spread their seeds.

Are hurricanes getting worse because of climate change?

Since the 1970s the number of Category 4 and 5 storms has roughly doubled, and studies indicate that hurricanes seem to gain strength more quickly than they did 25 years ago. Recent hurricanes in the North Atlantic Ocean have broken records for intensity and duration. But the mechanisms driving hurricanes are complicated, and more research is needed to fully understand them.

A few trends are clear. For example, scientists know that climate change is warming the surface of the oceans, especially in the North Atlantic, and warm water fuels hurricanes. Increased evaporation and water vapor from a warming ocean means hurricanes produce more rain. Sea level rise will exacerbate the effects of coastal storm surges triggered by hurricanes. What remains less well understood is whether climate change will increase the temperature of deeper ocean waters, which would sustain the warmth of surface waters and fuel more—and more powerful—hurricanes.

Climate change is also altering global air circulation patterns, which can affect hurricanes. As the Arctic region warms, the temperature difference between the poles decreases. This weakens the Jet Stream, the west-to-east winds that circumnavigate temperate regions of the Earth and usually propel hurricanes out to sea. Climate change also creates the conditions for meandering, eddy-like offshoot winds—the atmospheric equivalent of whirlpools alongside a boat’s wake. Those meandering eddies act like bumpers in a ping-pong game that keep a ball in play. Consequently, some storms such as 2012’s hurricane Sandy, which typically would have headed out to sea after making landfall, tend to linger on land and cause more damage.

What are scientists doing to improve hurricane forecasting?

Scientists continue to improve their ability to forecast hurricanes. The sooner they can access accurate information about a storm’s location and intensity, the better the chances to minimize the storm’s impacts. Historically, scientists have been better able to forecast a storm’s trajectory than its intensity because they have lacked a key data point: real-time ocean water temperatures. If waters just beneath the ocean surface are cold, they will mix with warm surface water—reducing its temperature and sapping a storm’s energy. If subsurface water is warm, it will help sustain warm surface water temperatures and maintain or intensify a hurricane’s strength.

Until now, scientists mostly have had access only to satellite imagery, which can only show the temperature of the first few inches of surface water. More recently, WHOI scientists have developed new, specialized, 50-pound instruments called ALAMO floats (Air-Launched Autonomous Micro-Observers) to help address this problem. ALAMO floats are robotic cylinders that are launched from airplanes into the ocean in front of hurricanes. The floats can change their buoyancy, allowing them to sink to 1,000 meters and collect measurements of temperature and salinity as they slowly rise back up to the surface.

Starting in the 2014 season, ALAMO floats were tossed off the back of hurricane-hunting airplanes during a storm to collect data and transmit it via satellite in real time. ALAMO floats can make hundreds of underwater roundtrips before, during, and after a storm, collecting data that could dramatically improve hurricane forecasting.

What can we learn from past hurricanes?

Scientists study evidence left by hurricanes from several hundred years ago to improve our understanding of future hurricanes. How does a scientist study a hurricane that hit in the 1400s? When a hurricane approaches a coastline, it tends to whip up sandy seafloor sediments and organic debris and fling them into nearby coastal areas where they get buried over time. Scientists can unearth this ancient submerged evidence with long, metal, straw-like tubes that extract cores from the bottom of coastal ponds or lagoons, salt marshes, or blue holes in the open ocean. They analyze the cores to help determine the frequency and severity of long-ago storms, understand the factors that generated past hurricanes, and help predict future hurricane patterns.

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