A hurricane is a powerful, rotating tropical weather system characterized by a low-pressure center, strong winds, and heavy rainfall. These systems form over warm ocean waters, drawing energy from the heat and moisture of the sea surface. They often span hundreds of miles, with potential impacts on coastal and inland areas including destructive winds and extensive flooding.
Tropical Disturbance
The initial stage of a hurricane begins as a tropical disturbance, an unorganized cluster of thunderstorms over tropical waters. At this point, there is little to no discernible rotation or a defined low-pressure circulation. These disturbances are areas of organized convection, with consistent rising air and thunderstorm activity. Many tropical disturbances form annually, but only a small fraction develop into more organized weather systems.
Tropical Depression
A tropical disturbance can strengthen into a tropical depression when it develops a closed circulation and its sustained wind speeds reach up to 38 miles per hour (61 kilometers per hour). This indicates an increase in the system’s organization, with a more defined low-pressure center taking shape. The closed circulation allows the system to draw in and rotate air more effectively.
Tropical Storm
Further intensification leads to a tropical storm, occurring when sustained winds increase to a range of 39 to 73 miles per hour (63 to 118 kilometers per hour). At this stage, the system is assigned a name from a predetermined list, making it easier to track and identify. Tropical storms exhibit more defined spiral bands of thunderstorms as their circulation becomes increasingly organized and powerful.
Hurricane Classification
A tropical storm becomes a hurricane when its sustained wind speeds reach a minimum threshold of 74 miles per hour (119 kilometers per hour). The system is then classified using the Saffir-Simpson Hurricane Wind Scale (SSHWS), which rates hurricanes from Category 1 to Category 5 based on their maximum sustained wind speeds.
- Category 1: Winds 74-95 mph (119-153 km/h), causing some damage to roofs and power outages.
- Category 2: Winds 96-110 mph (154-177 km/h), leading to extensive damage and near-total power loss.
- Category 3: Winds 111-129 mph (178-208 km/h), causing devastating damage, including structural damage to homes and widespread utility outages.
- Category 4: Winds 130-156 mph (209-251 km/h), resulting in catastrophic damage to well-built homes and prolonged power outages.
- Category 5: Winds 157 mph (252 km/h) or higher, causing catastrophic damage where a high percentage of homes are destroyed, making affected areas uninhabitable for extended periods.
Dissipation
Eventually, a hurricane will weaken and dissipate as it encounters conditions that remove its energy source. Moving over cooler ocean waters, typically below 80 degrees Fahrenheit (26.5 degrees Celsius), is a common reason for weakening, as hurricanes require warm water to sustain their strength. Making landfall also causes rapid weakening because the storm is cut off from the warm, moist air supplied by the ocean. The increased friction over land and lack of moisture contribute to the storm’s breakdown. Strong wind shear, a change in wind speed or direction with height, can also tear the storm apart and disrupt its organized structure, leading to dissipation.