NOAA text modified and supplemented by BYU-Idaho faculty

Hurricane Katrina, August 2005 (NOAA) Based on a slideshow by Robbie Berg, National Hurricane Center Learning Objectives

Your goals in studying this chapter are to:

• Understand the types of tropical cyclones. • Understand how hurricanes form. • Understand where and when hurricanes form. • Understand safety measures to take before, during, and after a hurricane. • Understand watches and warnings. • Understand the factors that help or hinder hurricane development. • Understand the hurricane categories. • Understand the hazards caused by hurricanes.

Galveston, Texas before and after Hurricane Ike. Why Learn About Hurricane Hazards?

Hurricane Ike, 2008 (NOAA)

History teaches that a lack of hurricane awareness and preparation are common threads among all major hurricane disasters. By knowing your vulnerability and what actions you should take, you can reduce the effects of a hurricane disaster.

Hurricane hazards come in many forms, including storm surge, heavy rainfall, inland flooding, high winds, tornadoes, and rip currents. The National Weather Service is responsible for protecting life and property through issuance of timely watches and warnings, but it is essential that your family be ready before a storm approaches. Furthermore, mariners should be aware of special safety precautions when confronted with a hurricane.

Download the Tropical Cyclone Preparedness Guide (PDF) or follow the links for more information. But remember, this is only a guide. The first and most important thing anyone should do when facing a hurricane threat is to use common sense. Definitions A tropical cyclone is a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has a closed low-level circulation. Tropical cyclones rotate counterclockwise in the Northern Hemisphere. They are classified as follows:

Tropical Depression: A tropical cyclone with maximum sustained winds of 38 mph (33 knots) or less. Tropical Storm: A tropical cyclone with maximum sustained winds of 39 to 73 mph (34 to 63 knots). Hurricane: A tropical cyclone with maximum sustained winds of 74 mph (64 knots) or higher. In the western North Pacific, hurricanes are called typhoons; similar storms in the Indian Ocean and South Pacific Ocean are called cyclones. Major Hurricane: A tropical cyclone with maximum sustained winds of 111 mph (96 knots) or higher, corresponding to a Category 3, 4 or 5 on the Saffir-Simpson Hurricane Wind Scale.

Tropical cyclones forming between 5 and 30 degrees North latitude typically move toward the west. Sometimes the winds in the middle and upper levels of the atmosphere change and steer the cyclone toward the north and northwest. When tropical cyclones reach latitudes near 30 degrees North, they often move northeast.

The Atlantic hurricane season runs from June 1st to November 30th, and the Eastern Pacific hurricane season runs from May 15th to November 30th. The Atlantic basin includes the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

Resources http://www.nhc.noaa.gov/ National Hurricane Center http://www.ready.gov/hurricanes http://www.nws.noaa.gov/om/hurricane/inde x.shtml National Weather Service, hurricane preparedness and information (NOAA) How Do Hurricanes Form?

Cumulonimbus clouds (NASA)

Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean rises upward from near the surface. Because this air moves up and away from the surface, there is less air left near the surface. Another way to say the same thing is that the warm air rises, causing an area of lower air pressure below.

Air from surrounding areas with higher air pressure rushes into the low pressure area. Then that "new" air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface. Storms that form north of the equator spin counterclockwise. Storms south of the equator spin clockwise. This difference is because of Earth's rotation on its axis.

As the storm system rotates faster and faster, an eye forms in the center. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye. Diagrams showing the structure of a hurricane. Low pressure draws in air from all sides, and the converging air mass begins to spin so it can efficiently rise up in the center, like an upside- down bathtub drain. A hurricane transfers heat from the warm ocean waters to the high atmosphere, further accelerating winds and circulation. (NASA diagrams) Rain bands are composed of rows of cumulonimbus clouds – big thunderstorms. These are usually the first part of the cyclone to arrive on land.

(NASA)

Cyclones Occur in Tropical and Subtropical Oceans Around the Globe

“hurricanes”

“typhoons” “hurricanes” “cyclones”

“typhoons”

Tropical Versus Extratropical Cyclones An extra-tropical cyclone is a storm system that primarily gets its energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones (also known as mid-latitude or baroclinic storms) are low pressure systems with associated cold fronts, warm fronts, and occluded fronts. Tropical cyclones, in contrast, typically have little to no temperature differences across the storm at the surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics

(tropical cyclone) (extratropical cyclone)