SciTech

Hurricanes are powerful, yet destructive forces of nature

How Things Work: Hurricanes (credit: Diane Lee/) How Things Work: Hurricanes (credit: Diane Lee/)

There had not been a Category 5 hurricane in the Atlantic since 2007 until Hurricane Matthew struck Caribbean islands and the southeastern United States earlier this month, causing serious damage to life and property. The high winds, intense rain, and deadly flooding associated with the hurricane were demonstrations of our planet’s uncontrollable power. How exactly is a storm so powerful formed? What makes a hurricane capable of so much damage? It may be surprising to see that the reason behind all of this isn’t so different from the reason behind the weather we experience in Pittsburgh.

Hurricane Matthew, like all other hurricanes that affect the eastern coast of the United States, is an Atlantic hurricane. Atlantic hurricanes form just off the western coast of Africa, in the Atlantic Basin, which is one of the seven tropical cyclone basins. The Inter-Tropical Zone located near the equator is the region where the northeast and southeast trade winds collide. Between June 1 and Nov. 30, the winds of this zone shift a continual line of storms down to the equator all around the globe. Just as low-pressure systems follow the jet stream across the United States and are the reason behind the dreary cloudy days and rain of Pittsburgh, hurricane storm systems start as low-pressure systems slowly moving across this line.

Depending on the ocean’s conditions, certain storms begin to gain strength. Deep warm water, large amounts of moisture in the air, and limited vertical wind shear can lead to this intensification.Wind shear is the difference in wind velocity between two points. Low wind shear means that the winds don’t “turn” or stay stable, which allows the hurricane to strengthen. As conditions become favorable for storm growth, the low-pressure systems begin to develop distinct characteristics. As increased wind speed and counter-clockwise rotation become more pronounced, a tropical depression becomes a tropical storm. The tropical storm then becomes a Category 1 hurricane, increasing from Category 1 to Category 5 to match the logarithmic increase in storm strength.
The clear “eye” of the hurricane forms from a vacuum in the center of the hurricane. The walls of this calm center have the fastest speeds; this concept could be thought of as related to conservation of momentum.

Large rain bands form, often extending hundreds of miles outside the hurricane eye. While the hurricane forms, the low-pressure system continues to move in translation. The global winds, along with rotation from the earth’s pressure systems, combine to direct the hurricane, with high pressures rotating clockwise and low pressures rotating counter-clockwise. In the case of Hurricane Matthew, as it began moving north along the southern United States coastline, a cold front, which refers to a line of colder wind, pushed eastward against the storm, keeping it along the coast instead of above land.

Each part of a hurricane contributes its own dangerous element. The wide and extensive rain bands deposit large amounts of rain for long periods of time. For Hurricane Matthew, this was a serious issue in South Carolina where more than ten inches of rain fell during the storm and deadly flooding resulted.The counterclockwise rotation of the bands creates winds of differing directions in each quadrant of the hurricane.

In the northeast quarter of the storm, the nature of the bands can often lead to tornadoes when they reach land. When the direction of the winds lines against the coast, the continual high winds actually push water from the ocean farther into land. This storm surge was a very serious complication of Hurricane Sandy in 2012, increasing water levels and flooding large parts of New York City.

Categories are assigned to hurricanes based on their wind-speeds. Category 5 hurricane winds are sustained at more than 157 miles per hour. This intensity in speed causes catastrophic damage to trees, buildings, and homes. The winds move fastest closest to the eye-wall, thus the Category 4 Hurricane Matthew’s shift away from the coast of Florida may have saved the state from a grimmer outcome. The eye-wall did eventually reach land in South Carolina, but the hurricane had weakened to a Category 1 storm by that time.

When hurricanes transition from the sea to the land, they lose the moist air and encounter rugged terrain. This significantly weakens the storms; but before this, they are able to cause incredible damage.However, due to increased weather-sensing technology, hurricanes can now be tracked in real time.

This provides valuable information that can be used to predict when people need to be evacuated from their homes and how much potential damage will result from the hurricane. For Hurricane Matthew, thousands of people in the United States were urged to evacuate, including people in Florida, Georga, and South Caroline, where risks for flooding were most extreme.The primary technology for monitoring hurricanes continues to be weather satellites and planes with special weather-sensing equipment. The data is then fed into supercomputers containing programs that can project a model trajectory of the hurricane.

Though deadly, hurricanes remain marvels of nature. The energy that pulls a roof off of a house on the coast is the same energy that is being removed from the heat of the ocean. These storms balance some of the ocean’s heat and sometimes the result is catastrophic. But looking beyond the damage caused, these storms are a powerful, almost respectable feature of nature.