Chapter 13 The Nature of Storms

SES5. Students will investigate the interaction of insolation and Earth systems to produce weather and climate. e. Describe the hazards associated with extreme weather events and climate change (e.g., hurricanes, tornadoes, El Niño/La Niña, global warming).

Chapter 13 – The Nature of Storms

•13.2 Severe Weather

•13.3 Tropical Storms

13.2 Severe Weather

13.2 Severe Weather

• Objectives– Explain why some thunderstorms are more

severe than others.– Recognize the dangers of severe weather,

including lightning, hail, and high winds.– Describe how tornadoes form.

13.2 Severe Weather

• Main Idea– All thunderstorms produce wind, rain, and

lightning, which can have dangerous and damaging effects under certain circumstances.

• Review Vocabulary– air mass: large body of air that takes on the

characteristics of the area over which it forms

Weather Cells• All thunderstorms are not created

equal.

• The increasing instability of the air intensifies the strength of a storm’s updrafts and downdrafts, which makes the storm severe.

Weather Cells• Supercells

– Severe thunderstorms can develop into self-sustaining, extremely powerful storms called supercells.

– These furious storms can last for several hours and can have updrafts as strong as 240 km/h (150 mph).

Weather Cells

• Supercells – An anvil-shaped cumulonimbus cloud is

characteristic of many severe thunderstorms.

Strong Winds

• Violent downdrafts that are concentrated in a local area are called downbursts.

• Based on the size of the area they affect, downbursts are classified as either macrobursts or microbursts.– Macroburts: path of destruction up to 5 km

wide, wind speeds >200 km/h, last up to 30 mins.

– Microbursts: path of destruction <3 km, winds >250 km/h, last <10 mins.

Hail

• Hail is precipitation in the form of balls or lumps of ice. It forms because of two characteristics common to thunderstorms.

• For hail to form, water droplets rise to the heights of a cumulonimbus cloud where the temperature is below freezing, encounter ice pellets, and freeze on contact with the pellets, which causes the ice pellets to grow larger.

• The second characteristic that allows hail to form is an abundance of strong updrafts and downdrafts moving side by side within a cloud.

Hail

Tornadoes

• A tornado is a violent, whirling column of air in contact with the ground.

• Before a tornado reaches the ground, it is called a funnel cloud.

Tornadoes

• Development of tornadoes– A tornado forms

when wind speed and direction change suddenly with height, a phenomenon associated with wind shear.

– Although tornadoes rarely exceed 200 m in diameter and usually last only a few minutes, they can be extremely destructive.

TornadoesTornado classificationThe Fujita tornado intensity scale, which ranks tornadoes according to their path of destruction, wind speed, and duration, is used to classify tornadoes.

Fujita Tornado Intensity Scale

Tornadoes• Tornado distribution

– Most tornadoes—especially violent ones—form in the spring during the late afternoon and evening, when the temperature contrasts between polar air and tropical air are the greatest. This type of large temperature contrast occurs most frequently in the central United States.

– Many of the more than 700 tornadoes that touch down in the United States each year occur in a region called “Tornado Alley,” which extends from northern Texas through Oklahoma, Kansas, and Missouri.

Tornadoes• Tornado safety

– If you are caught in a tornado, take shelter in the southwest corner of a basement, a small downstairs room or closet, or a tornado shelter.

13.2 Section Summary

• Intense rotating updrafts are associated with supercells.

• Downbursts are strong winds that result in damage associated with thunderstorms.

• Hail is precipitation in the form of balls or lumps of ice that accompany severe storms.

• The worst storm damage comes from a vortex of high winds that moves along the ground as a tornado.

13.2 Section Questions

The strongest thunderstorms develop under highly stable atmospheric conditions.

a. true

b. false

13.2 Section Questions

a. rain

b. snow

c. hail

d. sleet

Which type of precipitation requires strong updrafts and downdrafts to exist side by side in a cloud?

13.2 Section Questions

How do tornadoes form?

Answer: The rotation of a tornado begins as a result of wind shear, wind at different levels of the atmosphere blowing in different directions or at different speeds. The horizontal rotation is then tilted to a vertical position by thunderstorm updrafts. A tornado forms if the rotating column extends to the ground.

13.3 – Tropical Storms

• Real-World Link: If you try mixing cake batter in a shallow bowl, you might find that a low speed works well, but a high speed creates a big mess. Tropical cyclones form from processes similar to other storm systems, but their high winds can bring devastation to locations in their path.

13.3 – Tropical Storms

• Main Idea– Normally peaceful, tropical oceans are capable

of producing one of Earth’s most violent weather systems—the tropical cyclone.

• Review Vocabulary– Coriolis effect: caused by Earth’s rotation,

moving particles, such as air, are deflected to the right north of the equator, and to the left south of the equator

13.3 – Tropical Storms

• Objectives– Identify the conditions required for tropical

cyclones to form.– Describe the life cycle of a tropical cyclone.– Recognize the dangers of hurricanes.

Overview of Tropical Cyclones

During summer and fall, the tropics experience conditions ideal for the

formation of large, rotating, low-pressure tropical storms called tropical

cyclones.

Overview of Tropical Cyclones

Favorable conditions for cyclone formation exist in all tropical oceans except the South Atlantic Ocean and the Pacific Ocean off the west coast of South America.

Cyclone location

Overview of Tropical Cyclones

Tropical cyclones require two basic conditions to form: an abundant supply of warm ocean water and some sort of mechanism to lift warm air and keep it rising.

Cyclone formation

Overview of Tropical Cyclones

The first indication of a building tropical cyclone is a moving tropical disturbance. When a disturbance over a tropical ocean acquires a cyclonic circulation around a center of low pressure, it has reached the developmental stage and is known as a tropical depression.

Cyclone formation

Overview of Tropical Cyclones

When wind speeds around the low-pressure center of a tropical depression exceed 65 km/h, the system is called a tropical storm.

If air pressure continues to fall and winds around the center reach at least 120 km/h, the storm is officially classified as a cyclone.

Cyclone formation

Overview of Tropical Cyclones

Once winds reach at least 120 km/h, another phenomenon occurs—the development of a calm center of the storm called the eye.

The eye of the cyclone is a span of 30 to 60 km of calm weather and blue sky.

Cyclone formation

Overview of Tropical CyclonesCyclone formation

The strongest winds in a hurricane are usually concentrated in the eyewall—a tall band of strong winds and dense clouds that surrounds the eye.

Overview of Tropical CyclonesCyclone formation

A hurricane will last until it can no longer produce enough energy to sustain itself. This usually happens when the storm has moved either over land or over colder water.

Overview of Tropical CyclonesTropical cyclone movement

Like all large-scale storms, tropical cyclones move according to the wind currents that steer them.

Hurricane Hazards

The Saffir-Simpson hurricane scale classifies hurricanes according to wind speed, potential for flooding due to the effect on the height of sea level, and potential for property damage.

Hurricane Hazards

Damage

Hurricanes can cause extensive damage, particularly along coastal areas, which tend to be where human populations are the most dense.

Hurricane Hazards

Winds

Much of the damage caused by hurricanes is associated with violent winds.

A storm surge occurs when hurricane-force winds drive a mound of ocean water toward coastal areas where it washes over the land.

Hurricane Hazards

Storm surge

The National Hurricane Center issues a hurricane warning at least 24 hours before a hurricane is predicted to strike.

Awareness, combined with proper safety precautions, has greatly reduced death tolls associated with hurricanes in recent years.

Hurricane HazardsHurricane advisories and safety

13.3 – Summary

• Cyclones rotate counterclockwise in the northern hemisphere.

• Cyclones are also known as hurricanes & typhoons.• Cyclones go through the same stages of formation &

dissipation as other storms.• Cyclones are moved by various wind systems after they

form. • The most dangerous part of a tropical cyclone is the

storm surge.• Hurricane alerts are given at least 24 hours before the

hurricane arrives.

13.3 – Review Questions

At what latitudes do tropical cyclones usually form?

a. between 0 and 5

b. between 5 and 30

c. between 30 and 50

d. between 50 and 70

13.3 – Review Questions

What weather condition usually exists within the eye of a hurricane?

a. high wind

b. calm wind

c. tornadoes

d. lightning

13.3 – Review QuestionsWhat is the source of a hurricane’s energy?

Answer: A hurricane’s energy comes from the warm water over which it develops. As ocean water evaporates, some heat is taken from the ocean. The water vapor then rises high into the atmosphere. The heat that was taken from the ocean is released to the atmosphere as the water vapor condenses.