This Month in Science – Hurricane Season

New Hanover County has a significant history of hurricanes making landfall. Hurricanes Hazel (Category 3, 1954), Floyd (Category 2, 1999), and Florence (Category 1, 2018) were some of the most damaging storms to hit the area, not only due to wind, but also rainfall and flooding. September is peak hurricane season for the Atlantic Ocean because ocean surface temperatures are at their warmest, above 82°F.

Hurricane formation requires sea surface temperatures of at least 80°F. Hurricanes begin as tropical depressions that form in areas of high humidity, low pressure, and low wind shear. A hurricane typically starts in an area where a cluster of thunderstorms is present. Thunderstorms are sites of low pressure that provide the water vapor necessary to generate a tropical depression that has hurricane potential.

Water vapor is the fuel that initiates and maintains hurricane rotation. Warm water evaporates from the surface of the ocean, transferring heat from the ocean to the atmosphere. As the warm, moist air evaporates, it rises up through the center of the hurricane and then spirals out at the top of the storm. This rising air creates an area of low pressure below it. Cool, dry air from higher pressure areas rushes into the low pressure area. As it approaches the ocean surface, the cool, dry air warms and then rises through the center. This cycle continues with the circulation of warm air from the bottom to the top, fueling the hurricane’s “heat engine”. With enough energy transfer through evaporation of warm, moist air, the storm begins to rotate, eventually forming a center and an eyewall. Hurricanes lose strength over land because they no longer have access to the warm water that keeps it spinning.

Hurricanes rotate because of the Coriolis effect—a force generated by the rotation of the Earth. Hurricanes tend to form about 300 miles away from the equator, where the Coriolis effect is strong and water is warm. Without the Coriolis effect, evaporation of warm air would just cause thunderstorms. The direction of rotation depends on the location of the storm; hurricanes spin counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. The spin creates a calm center with almost no wind, called the eye. In contrast, the eye wall surrounding the eye produces the strongest winds. The outer rain bands are storm clouds that bring rain and, in some cases, tornadoes.

Hurricanes can be extremely damaging to both life and property. In order to communicate about severity of storms, meteorologists developed the Saffir-Simpson wind scale in the late 1960s and early 1970s. The scale categorizes hurricanes by their maximum sustained wind-speed, from the least severe category one storm (74-95 mph) up to a catastrophic category five (greater than 156 mph). While these categories are useful for predicting damage due to hurricane force winds, they do not take into account the risk of flooding for an area in the path of a storm.

Meteorologists use computer simulations and models to predict how active a particular season will be and the path a hurricane will take once it forms. While these models are useful, they don’t provide information about what is happening within an active storm. This is where Hurricane Hunters and weather satellites come in. Hurricane Hunters are pilots that fly into storms to gather data about wind speeds, barometric pressure, and rainfall. The pilots release devices called dropsondes directly into storms to gather these data. As they fall, they send two to four measurements per second by radio to a nearby aircraft. Weather satellites are also crucial for tracking hurricanes. They take pictures and measure weather patterns with radar and infrared. The data collected provide real-time information that helps meteorologists update forecasts and emergency responders plan for incoming storms.

As we enter peak season, make sure you are prepared. For more emergency preparedness resources, check out New Hanover County’s ReadyNHC site for severe weather and the National Hurricane Center.


Previous Columns

July: Plastic Free July
June: All About Alligators
May: Rain Gardens
April: Bald Cypress Trees and The Climate Record
March: Ocean Waves
February: The Scientific Method
January 2021: Static Electricity
December: Surviving the Winter Season
November: Marbled Salamanders
October: Controlling Wildfires
September: The Equinox and Changing Seasons
August: Perseid Meteor Shower
July: Plastic Free July
June: Sargasso Sea
May: Getting to Mars
April: Earth Day 50th Anniversary
March: Ghost Trees
February: Fire
January 2020: Biodiversity of the Cape Fear



Hurricane Florence makes landfall in the Carolinas, 2018.
Credit: NOAA

The anatomy and dynamics of a hurricane.
Credit: Kelvinsong

The Saffir-Simpson Scale of Hurricane Categorization.
Credit: Smithsonian Ocean
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