2006-07-07 / News

Hurricanes: Mother Nature's most powerful storms

By Abby Anthony

Most people would agree that hot and humid weather leaves us feeling zapped of energy, lethargic, and lazy. Ironically, heat and humidity are the energy sources that fuel hurricanes.

Hurricanes are essential features of nature because they transfer surplus energy from the equator to the cooler regions around the north and south poles.

A hurricane is a large rotating system of clouds, strong winds, and thunderstorms centered on an area of low air pressure. The meteorological term for a hurricane is a tropical cyclone, but various terms are used to describe these storms in different regions. In the Northwest Pacific, for example, tropical cyclones are called typhoons.

Hurricanes form under two required conditions. First, the ocean water must be at least 80 degrees Fahrenheit to a depth of 150 feet. Second, the earth must be spinning at a sufficient speed for hurricanes to form. The earth rotates faster near the equator, so hurricanes form within 300 miles of the equator.

The warm sea heats the air above it; a current of warm, moist air rises up into the atmosphere, quickly creating an area of low pressure.

The low-pressure area acts like a vacuum, and air from the surrounding high-pressure areas rushes in to fill the void. The rotation of the Earth causes the system to spin, a phenomenon called the Coriolis Effect, giving it cyclonic characteristics and affecting its trajectory. In the Northern hemisphere the cyclone spins in a counterclockwise direction and in the Southern hemisphere it spins clockwise.

Warm ocean water evaporates from the tropical seas and creates very moist air. The moist air rises and condenses as the atmosphere becomes cooler, creating large cumulus clouds and rain. When the water vapor in the clouds condenses into water droplets, it releases the latent heat that originally caused the water to evaporate. The release of this latent heat energy warms the surrounding air, causing the air pressure to drop. Again, air in high-pressure areas rushes in to fill the low-pressure areas and more warm moist air is drawn off the sea.

This positive feedback loop can be thought of as a "heat engine" and the warm moist water is the engine's fuel.

Researchers at the Atlantic Oceanographic and Meteorological Laboratory division of the National Oceanic and Atmospheric Administration have estimated that a hurricane can release 6.0 x 104 watts per day through cloud and rain formation.

This amount of energy is equivalent to 200 times the worldwide electrical generating capacity!

Even though hurricanes are large systems generating an enormous amount of energy, their movements over the Earth's surface are compared to that of leaves carried along in a stream. The Earth's strong prevailing winds move and steer hurricanes. In the North Atlantic, tropical systems are generally steered westward, carried by the east-to-west winds on the south side of the Bermuda High, a prevailing highpressure system in the North Atlantic. The Earth's rotation also affects the storm's track. The Coriolis Effect causes cyclonic systems to turn towards the poles; storms in the North Atlantic are pushed westward by the prevailing winds in the beginning, and then pulled north by the Coriolis Effect. Usually, the storms then encounter the west-to-east prevailing winds traveling across North America and the storm is blown east and out to sea.

A tropical cyclone becomes a hurricane when its wind speed reaches 74 miles per hour. According to the Atlantic Oceanographic and Meteorological Laboratory. In 1805 Commander Francis Beaufort devised a wind scale to standardize the weather reports in his ships' logs.

The wind scale was divided into 14 forces and each force was given a common name (light breeze, gentle breeze, moderate breeze, etc.), as was a description of what would happen to a ship in those wind conditions.

Since a hurricane was the most powerful storm a ship was likely to encounter in the North Atlantic, it became the top of the scale. The International Meteorological

Committee assigned wind speeds to each wind speeds to each

force level in 1926.

Seventyfour mph is not a magical number, but scientists have observed that at this

speed, tropical cyclones begin to take on the structure of mature hurricanes, including an eye.

Whether global warming will cause more frequent and intense hurricanes has become a common question. Most climatologists agree that no single storm, or even a season of

storms, can be attributed to any single cause, including global warming. According to the Hurricane Research Division of the U.S. National Oceanic and Atmospheric Administration, "the current state of climate science does not support so close a linkage" between storm intensity, frequency, and greenhouse gas emissions. Nonetheless, scientists are not ignoring climate change, and are looking closely at the relationship between storms and greenhouse gases in the atmosphere.

NOAA does report that since 1995 there has been an increase in the frequency and intensity of hurricanes, but the changes in the last decade indicate that they are the result of the multi-decadal signal, a historical climate pattern in which several decades of high hurricane activity are followed by several quiet decades; the 1940s, '50s, and '60's saw considerable hurricane activity, while the 1970s, '80s, and early '90s were relatively uneventful.

Nonetheless, NOAA predicts that the 2006 hurricane season will be very busy. Warmer water, weaker easterly winds, and favorable wind patterns in the midlevel of the atmosphere are factors that will favor the development of more frequent and intense storms in the Atlantic.

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