Hurricanes, known broadly as tropical cyclones, are rotating systems of clouds and thunderstorms that form over tropical or subtropical waters. One of nature’s most powerful storms, hurricanes can bring strong winds, storm surge flooding, heavy rainfall that can lead to inland flooding, tornadoes, and rip currents.
For a storm to be categorized as a tropical cyclone, it must be a warm core (or center) low pressure system without any "front" attached, develop over tropical or near tropical waters, and have a closed circular wind circulation around a well-defined center.
Tropical cyclones are powered by warm tropical ocean waters and a warm, moist atmosphere. Hurricanes are typically steered by easterly winds, generally south of 25° north latitude and by low- to mid-level westerly winds north of 25° north latitude. There are several favorable environmental conditions that must be in place before a tropical cyclone can form. They are:
- Warm ocean waters (at least 80°F/27°C) throughout a depth of about 150 ft. (46 m).
- An atmosphere that cools fast enough with height such that it is potentially unstable to moist convection.
- Relatively moist air near the mid-level of the troposphere (16,000 ft/4,900 m).
- Generally a minimum distance of at least 300 miles (480 km) from the equator.
- A pre-existing near-surface disturbance.
- Low values (less than about 20 mph/32 kph) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the change in wind speed and direction at different height.
Hurricanes, tropical storms, and tropical depressions pose a variety of threats to people. Most importantly, long-lasting damage can occur from wind, heavy rainfall, and storm surge (the number oneoffsite link cause of loss of life during hurricanes). Tornados and rip currents are additional threats that can result from hurricanes. The best time to prepare for a hurricane is before hurricane season begins. The Atlantic hurricane season runs from June 1 to November 30, and the Eastern Pacific hurricane season runs from May 15 to November 30.
Although hurricanes are well known for their strong and destructive winds, a hurricane’s storm surge is by far the greatest threat to life and property along the immediate coast. Storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more. In addition, wind driven waves are superimposed on the storm tide. This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides. Because much of the United States' densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level, the danger from storm surges is tremendous.
As the climate changes and the strength and paths of hurricanes also change, it is increasingly important for students to understand hurricanes. This collection provides educators and students with resources to explore how hurricanes form, their potential effects to humans and ecosystems, ways to prepare for hurricanes, and even a citizen science project to classify hurricanes from satellite imagery.
Updated May 2019