BACKGROUND: Scientists have detected La Niña this year, and this could have effects on spring and summer weather in the United States, not to mention the upcoming hurricane season.
GIRLS AND BOYS: La Niña is the opposite of El Niño, which is a condition of warming in the Pacific warming. La Niña is a slight cooling of the Pacific Ocean, and typically creates more rainfall across Indonesia and northern Australia and the Amazon basin. The last La Niña lasted from 2000 to 2001.
ABOUT HURRICANES: A hurricane is a type of tropical cyclone, a low-pressure system that usually forms in the tropics and is accompanied by a counterclockwise circulation of winds near the earth's surface. Storms are considered hurricanes when their wind travels faster than 74 mph. Every hurricane arises from the combination of warm water and moist warm air. Tropical thunderstorms drift out over warm ocean waters and encounter converging winds from near the equator. Warm, moist air from the ocean surface rises rapidly, encounters cooler air, and condensed into water vapor to form storm clouds, releasing heat in the process. This heat causes the condensation process to continue, so that more and more warm moist air is drawn into the developing storm, creating a wind pattern that spirals around the relatively calm center, or eye, of the storm, much like water swirling down a drain. The winds keep circling and accelerating to form a classic cyclone pattern.
RATING HURRICANES: Hurricanes are categorized according to the strength of their winds according to the Saffir-Simpson Hurricane scale. They are rated from lowest wind speeds (Category 1) to highest (Category 5). But even lower category storms can cause a great deal of damage, mostly from storm surges – when water is pushed towards the show by strong winds and combines with normal tides to create hurricane storm tides – and the resulting flooding. The worst devastation from hurricane Katrina, for example, occurred when flooding caused the New Orleans levees to fail.
The American Meteorological Society contributed to the information contained in the TV portion of this report.