Hurricanes, tropical cyclones, or typhoons (depending on where you live) are immensely powerful weather systems. They act like massive heat pumps, extracting energy from the warm sea surface to form a rotating (clockwise in the southern hemisphere and counterclockwise in the northern hemisphere) mass of incredibly strong winds, extreme rainfall, and thunderstorms, creating massive waves and catastrophic storm surges. Hurricanes form when a minor atmospheric disturbance, such as a tropical wave, intensifies under the following conditions:
1. Sea surface temperatures above 27 °C: High sea surface temperatures provide the necessary energy to intensify a hurricane. To sustain a hurricane, high temperatures need to extend below the surface of the ocean. The large waves generated by a hurricane cause mixing in upper layers of the ocean. If the high temperatures do not extend into these layers, then the wave action will rapidly reduce the heat energy available to the storm and prevent further intensification.
2. Low wind shear: Wind shear is used to describe winds that change direction with height. Hurricanes require vertical motion, and too much wind shear will disturb the vertical motion of a system, preventing development and intensification. The strongest wind shear occurs near the jet streams, and so the position of the subtropical jet is a good indicator of the likelihood for hurricane development.
3. Sufficient distance from the equator: The Coriolis force is too weak at the equator to generate the rotation needed for hurricane formation. As a general rule, hurricanes do not form within 300 nm of the equator, nor do they cross the equator.
Before a tropical disturbance becomes a hurricane, it is first classified as a tropical depression (maximum one-minute sustained wind speed less than 33 kts). followed by a tropical storm (maximum one-minute sustained winds less than 64 kts). Once the maximum one-minute sustained wind speeds are greater than 64 kts, the system is called a hurricane. A major hurricane is one where the one-minute sustained wind speed exceed 96 kts. Hurricane strength is can be further categorized from 1 to 5 using the Saffir-Simpson hurricane wind scale.
The good news about hurricanes is that they are among the most accurately forecast weather systems on the planet. Hurricane track and intensity can now be forecast with a high degree of confidence many days in advance, providing notice for communities to prepare for a hurricane landfall, and for sailors to move away from its track.
Before the start of each hurricane season, it is now common for forecasters to provide a seasonal hurricane forecast. Seasonal hurricane forecasts consider factors such as sea surface temperature anomalies and the El Nino Southern Oscillation to predict the expected severity of the hurricane season. While these forecasts do not make predictions based on hurricane tracks, they attempt to predict the expected number of tropical storms, hurricanes, and major hurricanes. Warm sea surface temperature anomalies frequently contribute to more intense hurricane seasons, while the phase of the El Nino Southern Oscillation provides an indication of the expected position of the jet stream, and its influence on wind shear.
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