CH Section 13.1: ThunderstormsThunderstorms Section 13.2: Severe WeatherSevere Weather Section 13.3: Tropical StormsTropical Storms Section 13.4: Recurrent

CH

Section 13.1: Thunderstorms

Section 13.2: Severe Weather

Section 13.3: Tropical Storms

Section 13.4: Recurrent Weather

The Nature of Storms

ObjectivesIdentify the processes that form thunderstorms.

Compare and contrast different types of thunderstorms.

Describe the life cycle of a thunderstorm.

ThunderstormsSection 13.1

Review Vocabulary

latent heat: stored energy in water vapor that is not released to warm the atmosphere until condensation occurs

The intensity and duration of thunderstorms depend on the local conditions that create them.


New Vocabulary

air-mass thunderstorm

mountain thunderstorm

sea-breeze thunderstorm

frontal thunderstorm

stepped leader

return stroke

At any given moment, nearly 2000 thunderstorms are in progress around the world.

Overview of Thunderstorms


Both geography and air mass movements make thunderstorms most common in the southeastern United States.




For a thunderstorm to form, three conditions must exist:

1.a source of moisture,

2.lifting of the air mass,

3.and an unstable atmosphere.- clouds can grow into a cumulonimbus cloud.


How thunderstorms form


For a thunderstorm to form, there must be an abundant source of moisture in the lower levels of the atmosphere.




For a thunderstorm to form, there must be some mechanism for moisture to condense and release its latent heat. This occurs when a warm air mass is lifted into a cooler region of the atmosphere.




If the surrounding air remains cooler than the rising air mass, the unstable conditions can produce clouds that grow upward. This releases more latent heat and allows continued lifting.




Because the rate of condensation diminishes with height, most cumulonimbus clouds are limited to about 18,000 m.

Thunderstorms are also limited by duration and size.


Limits to thunderstorm growth


Thunderstorms are often classified according to the mechanism that causes the air mass that formed them to rise.

There are two main types of thunderstorms:

1.air-mass and 2. frontal.

Types of Thunderstorms


Air-mass thunderstorms

When air rises because of unequal heating of Earth’s surface beneath one air mass, the thunderstorm is called an air-mass thunderstorm.

There are two kinds of air-mass thunderstorms.



1.Mountain thunderstorms occur when an air mass rises by orographic lifting, which involves air moving up the side of a mountain.

2.Sea-breeze thunderstorms are local air-mass thunderstorms that occur because land and water store and release thermal energy differently.

Air-mass thunderstormsTypes of Thunderstorms

During the day, the temperature of land increases faster than the temperature of water. At night, conditions are reversed.

Sea breeze thunderstorms




Frontal thunderstorms are produced by advancing cold fronts and, more rarely, warm fronts.

Thunder storms due to cold fronts are severe and hundreds of kilometers long.

Thunder storms due to warm fronts are less severe.

Frontal thunderstormsTypes of Thunderstorms


A thunderstorm usually has three stages: 1.the cumulus stage- air starts to rise vertically. This creates updrafts.

2.the mature stage,

3.and the dissipation stage.

The stages are classified according to the direction the air is moving.

Thunderstorm Development


In the mature stage, updrafts and downdrafts exist side by side in the cumulonimbus cloud. The updrafts and downdrafts form a convection cell which produces the surface winds and rain associated with thunderstorms.

Mature stage



In a thunderstorm, the cool downdrafts spread in all directions when they reach Earth’s surface. Downdrafts cool the areas from which the storm draws its energy, the updrafts cease, and clouds can no longer form. The storm is then in the dissipation stage.

Dissipation stage



Lightning is the transfer of electrical charge due to friction between the updrafts and downdrafts within a cumulonimbus caused by the rapid rushes of air.

Lightning


Friction between the updrafts and downdrafts within a cumulonimbus cloud removes electrons from some of the atoms in the cloud. Atoms that lose electrons become positively charged ions, and atoms that receive the extra electrons become negatively charged ions.

Lightning


Eventually, the differences in charges break down, and a branched channel of partially charged air, called a stepped leader, is formed between the positive and negative regions.

Lightning


When the stepped leader nears the ground, a branched channel of positively charged particles, called the return stroke, rushes upward to meet it and illuminates the connecting channel with about 100 million volts of electricity.

Lightning


A lightning bolt heats the surrounding air to about 30,000C, about five times hotter than the surface of the Sun.

The thunder you hear is the sound produced as this superheated air rapidly expands and contracts.

Thunder

Lightning


There are several names given to lightning effects: sheet lightning, heat lightning, spider lightning, ball lightning, blue jets, and red sprites.

Lightning variations

Lightning


Each year in the United States, lightning causes about 7500 forest fires and an average of 300 injuries and 93 deaths to humans.

Thunderstorm and lightning safety

Lightning

CH

Thunderstorms in the United States are most common in the Midwest.

a. true

b. false

13.1 Section Questions


CH



A mature thunderstorm has a region of updraft and a region of downdraft.

a. true

b. false

CH

What is the difference between air-mass thunderstorms and frontal thunderstorms?



Answer: Air-mass thunderstorms form as a result of uplift of air within one air mass. Frontal thunderstorms form as a result of uplift of air along frontal boundaries.

Documents

CH Section 13.1: ThunderstormsThunderstorms Section 13.2: Severe WeatherSevere Weather Section 13.3: Tropical StormsTropical Storms Section 13.4: Recurrent