The Science of Hurricanes

Hurricane season in the Atlantic brings with it a plethora of dangerous and devastating weather. But how does all of this get started?

All hurricanes are essentially fueled by two simple things: heat and water. Hurricanes generally begin above the equator, where the waters are significantly warmer. The air just above the ocean’s surface takes in heat and moisture and, as the hot air rises, it leaves the air below it with a lower pressure. This cyclical process repeats and produces swirls in the air, according to NASA. This creates a thunderstorm that has the potential to turn into a hurricane.

Warm water in the Atlantic is really the first key element hurricanes need to get started. Another important key factor is wind shear. Wind shear, according to Federal Aviation Administration, is a change in wind speed and/or direction over a short distance. Wind shears can be either horizontal or vertical and are most commonly caused by strong differences in temperature. These contribute to the cyclone of warm and cool air that starts a hurricane.

These two ingredients for hurricanes don’t just happen on their own, though. Gerry Bell, the lead seasonal hurricane forecaster for NOAA’s Climate Prediction Center, says that “there are two dominant climate patterns that really control the wind and pressure across the Atlantic.

The El Niño/La Niña cycle is the first of these climate patterns. El Niño is a large-scale ocean-atmosphere climate interaction that causes periodic warming in the ocean surface temperatures. These effects are seen in the winter season, with wetter-than-average conditions expected. La Niña is essentially the opposite of El Niño, in which it represents below-average ocean surface temperatures, but slightly warmer temperatures in the Southeast region.

The second climate pattern that impacts hurricanes is the Atlantic Multidecadal Oscillation (AMO). This is a climate trend that has a time span of 25 to 40 years and is generally associated with warmer waters in the Atlantic. When the AMO is in a warm phase, there tends to be stronger hurricane patterns. Currently, we have been in a warm phase AMO, conducive to hurricanes, since 1995.

Hurricanes start out as tropical cyclones. Once a tropical cyclone’s winds have a sustained speed of 39 to 73 mph, it is then considered a tropical storm and it gets a name from the list put out by the World Meteorological Organization. If the sustained winds reach speeds of 74 to 95 mph, the storm becomes a Category 1 hurricane.

CategorySustained Wind SpeedPotential Damage
174-94 mphMinimal – some roof leakage, gutter damage, snapped tree branches
296-110 mphModerate – major roof and siding damage, uprooted trees causing road blockage, power loss for days to weeks
3111-129 mphExtensive – gable and decking damage, many uprooted trees, extended power outages
4130-156 mphExtreme – roof and exterior walls destroyed, snapped trees, power outages for weeks to months
5157 mph or higherCatastrophic – high fraction of framed houses destroyed, power outages for weeks to months, large areas uninhabitable for weeks to months

 

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