Hurricanes

Just as earthquakes and volcanic

eruptions have a single cause

(geologic activity within the Earth),

so do natural disasters caused by extreme

weather. Hurricanes, tornadoes, floods and

sometimes wildfires are all caused by extreme

weather conditions such as very low air pressure, high temperature, high humidity (water vapor in the air), and strong winds.

Learning Objectives

To understand how hurricanes cause natural disasters, you should be able to:

Describe how hurricanes form.
Explain three conditions necessary for hurricane formation.
Compare and contrast a hurricane eye and a hurricane eyewall.
Describe how a hurricane creates health and safety risks.

Vocabulary

air mass—a large body of air in which the temperature and moisture content stay about the same.

air pressure—the weight of all the air from the Earth’s surface to the top of the atmosphere.

climate—the weather conditions in an area over a long period of time.

eye—the center of the hurricane.

eyewall—huge storm clouds outside the eye of a hurricane.

hurricane—a tropical storm that forms over oceans.

Saffir-Simpson Hurricane Scale—a scale that measures hurricane strength based on wind speed and damage.

storm surge— an abnormal rise in seawater level during a storm caused by the storm’s winds pushing water onshore.

weather—short term atmospheric conditions such as temperature (hot or cold), precipitation (rain, snow, sleet, hail), pressure, or cloudiness.

Earth’s Atmosphere

Weather occurs in Earth’s atmosphere. The atmosphere is constantly in motion, driven mostly by differences in pressure and temperature between air masses. Air masses are large bodies of air in which temperature and moisture content stay about the same. Air masses are so large that only several may cover the United States at any one time.

Air pressure is the weight of all the air from the Earth’s surface to the top of the atmosphere. A column of air above each square centimeter (1 cm by 1 cm) of the Earth’s surface weighs 1 kilogram, or 2.2 pounds.

An astronaut in a spacecraft orbiting Earth took this picture of the Earth’s atmosphere with the Moon far distant in space. The orange part is the troposphere or lowest 12 km of the atmosphere, where humans and other life live, weather occurs, and airplanes fly. Image: NASA.

Air pressure changes for a variety of reasons. One is elevation. As you drive up a mountain there is less air above you than when you are lower, so its weight and pressure are less. Air pressure also changes when atmospheric temperatures change.

The greatest source of heat to the atmosphere is the Sun, so for about 12 hours every day the part of the Earth facing the Sun is warmed, air expands and air pressure decreases. At night, the air is cooler and its pressure increases. And, as we all know, temperature also changes according to the season. During the summer, the land and air are warm, and during winter, they are cold.

Hurricanes

Hurricanes are huge storms that

form over warm ocean water. They

move thousands of miles across

oceans and if they reach land they

eventually weaken and die. The warm,

moist air of hurricanes transports

billions of tons of water from oceans

onto land.

Image: Hurricane Florence, 2018, seen from the space station. Photo: NASA

The solar heating near the equator causes hurricanes to form over oceans in the tropics. Heat from the sun is greatest near the equator, where the sun passes overhead every day. At all other locations, the sun is lower in the sky. Less sunlight hits the Earth surface, making it cooler, and few hurricanes form.

Three conditions are necessary to form a hurricane:

● Warm water—at least 26.5

degrees Celsius (79.9 degrees

Fahrenheit)

● Deep enough warm water—to a

depth of at least 50 meters

● Low wind shear—low differences

in the speed of winds around a

forming column of air. Wind shear can tear apart the forming column and a hurricane does not form.

Image: The paths of all Atlantic Ocean hurricanes since 1851. Wikipedia

The column of air becomes larger as more moisture gathers. The center of the storm called the eye, is relatively calm. The eyewall, the huge surrounding storm clouds of the hurricane, contains the harshest weather and winds.

Trade winds from Africa push the huge storm clouds westward, across the ocean. All along the ocean path the hurricane continues to absorb heat from the ocean and grows larger and more dangerous.

As the hurricane hits land, ocean water surges across the shore, heavy rains flood low areas, and strong winds blow down trees and buildings. As the hurricane moves away from an ocean its wind speeds fall. When the wind becomes slower than 74 mph, it is no longer a hurricane, even though it still can cause intense rains as it keeps being pushed inland by winds.

Image: View of a hurricane eyewall from inside the eye. Photo: NOAA

When the winds blow down trees and buildings and become slower than 74 mph, it is no longer a hurricane, even though it still can cause intense rains as it keeps being pushed inland by winds.

Hurricanes have lower air pressure than any other kind of storm, and the lower the pressure, the more dangerous the storm. Hurricanes are classified on a 1 to 5 scale proposed by two scientists – Herbert Saffir and Robert Simpson. Their scale depends on how fast the winds blow and how much damage a hurricane causes.

Saffir-Simpson Hurricane Scale

Category 1 and 2 hurricanes are strong. They blow down trees, knock over trailers, tear roofs off buildings, and knock out electrical power to thousands to millions of customers. But it is Category 3 to 5 storms where more severe damage occurs, destroying homes, tossing around cars, and bringing in deep and rapidly flowing flood waters.

Another threat from hurricanes is storm surge flooding. A storm surge occurs when ocean water gets pushed toward the shore by the force of a big storm. The size of the surge and the flooding depends on the force of the storm, the angle of the approach, and the shape of the coastline.

Hurricane Ike hit the Houston area of Texas in 2008 with 110 mph winds, making it nearly a Category 3 storm. The photo shows a coastal area with total destruction, where nearly every house and building was destroyed by wind and surging water.

As a hurricane approaches, many people prepare by covering their home’s windows with plywood, packing their car, and evacuating further inland where flooding is less likely and winds will be weaker. Some people ignore mandatory orders to evacuate, and stay inside and get to the highest floors of their homes to avoid drowning. If flood waters are very high, people punch holes through attic ceilings to climb onto their roofs for safety and to be visible for helicopter rescue.

As you can imagine, hurricanes can cause many injuries. Hurricane-force winds damage buildings and homes and debris flying through the air can cause serious injuries if people are outside in the winds. Many other injuries, such as broken bones, sprains, and abrasions occur in the cleanup phase when the hurricane has passed by.

Check Your Understanding

1. Explain three conditions necessary for hurricane formation.

2. Compart and contrast a hurricane eye and a hurricane eyewall.

3. How are hurricane strengths measured?

4. Describe how a hurricane creates health and safety risks.

image of palm trees blowing in hurricane winds under gray sky

image of Earth's atmosphere showing different layers with moon in the background

Image of swirling hurricane winds from space

Image of tracks of hurricane paths in the Atlantic Ocean

Tall cloud formation of a hurricane eye from inside the eye

Image of damage to the coast of Florida by Hurricane Ike