ObjectivesExplain why some thunderstorms are more severe than others.

Recognize the dangers of severe weather, including lightning, hail, and high winds.

Describe how tornadoes form.

Severe WeatherSection 13.2

Review Vocabulary

air mass: large body of air that takes on the characteristics of the area over which it forms

All thunderstorms produce wind, rain, and lightning, which can have dangerous and damaging effects under certain circumstances.

Severe WeatherSection 13.2

New Vocabulary

Severe WeatherSection 13.2

supercell

downburst

tornado

Fujita tornado intensity scale

Weather Cells

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

Severe WeatherSection 13.2

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.

Weather Cells

Severe WeatherSection 13.2

Supercells- rotating updrafts.

Weather Cells

Severe WeatherSection 13.2

An anvil-shaped cumulonimbus cloud is characteristic of many severe thunderstorms.

Supercells

Strong Winds

Severe WeatherSection 13.2

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.

Severe WeatherSection 13.2

Hail

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

Severe WeatherSection 13.2

Hail

For hail to form,

water droplets rise to the heights 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 which tosses the hail up and down repeatedly.

Severe WeatherSection 13.2

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.

Severe WeatherSection 13.2

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.

Tornadoes

Development of tornadoes

Severe WeatherSection 13.2

Please click the image above to view the video.

Severe WeatherSection 13.2

Tornadoes

The Fujita tornado intensity scale, which ranks tornadoes according to

1.their path of destruction,

2.wind speed,

3. and duration,.

The scale ranges from F0 –F5

Tornado classification

Severe WeatherSection 13.2

Tornadoes

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.

Tornado distribution

Severe WeatherSection 13.2

Tornadoes

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.

Tornado distribution

Severe WeatherSection 13.2

Tornadoes

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.

Tornado safety

CH

All thunderstorms produce wind, rain, and lightning, which can have dangerous and damaging effects under certain circumstances.

Intense rotating updrafts are associated with supercells.

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

Section 13.2 Severe Weather

Study Guide Key Concepts

CH

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.

Section 13.2 Severe Weather

Study Guide Key Concepts

CH

The strongest thunderstorms develop under highly stable atmospheric conditions.

a. true

b. false

13.2 Section Questions

The Nature of Storms

CH

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

The Nature of Storms

CH

How do tornadoes form?

13.2 Section Questions

The Nature of Storms

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.

CH

Standardized Test Practice

The Nature of Storms

Why do hailstones consist of concentric layers?

CH

Standardized Test Practice

The Nature of Storms

Possible answer: The layers form as hailstones are tossed in a thunderstorm by strong updrafts. A new layer of ice is added each time the hailstone moves into a level of supercooled water droplets. These water droplets exist at temperatures that are well below the normal freezing point of water and so change to ice as soon as they come into contact with the hailstone.