Thunderstorms & Severe Weather

Biblical Reference

His thunder announces the coming storm.

Job 36:33

Thunderstorms• At any given time, nearly 2000 thunderstorms

are occurring around the world.

• Some are capable of producing hail the size of baseballs, swirling tornadoes, and surface winds of more than 160 km/h.

• All thunderstorms, regardless of intensity, have certain characteristics in common.

• Thunderstorms usually form within large cumulonimbus clouds along cold fronts.

How Thunderstorms Form• For a thunderstorm to form, three conditions

must exist:

1. There must be an abundant source of moisture in the lower levels of the atmosphere.

2. Some mechanism must lift the air so that the moisture can condense and release latent heat.

3. The portion of the atmosphere through which the cloud grows must be unstable.

Limits to Growth• The air in a thunderstorm keeps rising until:

1. It meets a layer of stable air that it cannot overcome.

2. The rate of condensation, which diminishes with height, is insufficient to generate enough latent heat to keep the cloud warmer than the surrounding air.

• Typical thunderstorms last only about 30 minutes and individual storms are only about 24 km in diameter.

Types of Thunderstorms• Thunderstorms are classified according to the

mechanism that caused the air to rise.• Air-mass thunderstorms result from the air

rising because of unequal heating of Earth’s surface within one air mass.– Mountain thunderstorms occur when an air

mass rises as a result of orographic lifting, which involves air moving up the side of a mountain.

– Sea-breeze thunderstorms are local air-mass thunderstorms caused, in part, by extreme temperature differences between the air over land and the air over water.

Air-Mass Thunderstorms

Frontal Thunderstorms• Frontal thunderstorms are thunderstorms

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

– Cold-front thunderstorms get their initial lift from the push of the cold air which can produce a line of thunderstorms along the leading edge of the cold front.

–

– Because they are not dependent on daytime heating for their initial lift, cold-front thunderstorms can persist long into the night.

Thunderstorm Formation• A thunderstorm usually has three stages,

classified according to the direction in which the air is moving:– Cumulus Stage: Dominated by cloud formation

and updrafts

– Mature Stage: Heavy Winds, Rain and Lightning

– Dissipation Stage: Updrafts stop, Winds die down, Lightning stops and precipitation slows down

Cumulus Stage– In the cumulus stage,

air starts to rise nearly vertically upward.

– Transported moisture condenses into a visible cloud and releases latent heat.

–

– As the cloud droplets coalesce, they form larger droplets, which eventually fall to Earth as precipitation.

Mature Stage– As precipitation falls, it cools

the air around it which becomes more dense than the surrounding air, so it sinks creating downdrafts.

– The updrafts & downdrafts form a convection cell.

– In the mature stage, nearly equal updrafts & downdrafts exist side by side in the cumulonimbus cloud.

Dissipation Stage– The supply of warm, moist

air runs out because the cool downdrafts cool the area from which the storm draws energy.

– Without the warm air, the updrafts cease and precipitation can no longer form.

– The dissipation stage is characterized primarily by lingering downdrafts.

James Pollard

Lightning• Lightning is an electrical discharge caused

by the friction of falling and rising ice crystals within strong drafts of a cumulonimbus cloud.– Some atoms lose electrons and become

positively charged ions, while other atoms receive the extra electrons and become negatively charged ions.

– This creates regions of air with opposite charges. – To relieve the electrical imbalance, an invisible

channel of negatively charged air, called a stepped leader, moves from the cloud toward the ground.

Lightning• When the stepped leader

nears the ground, a channel of positively charged ions, called the return stroke, rushes upward to meet it.

• The return stroke surges from the ground to the cloud, illuminating the channel with about 100 million V of electricity.

Thunder– A lightning bolt heats the surrounding air to

about 30,000°C.

– Five times hotter than the surface of the Sun.

– Thunder is the sound produced as the superheated air rapidly expands and contracts.

Lightning Variations• Sheet Lightning is reflected by clouds.

– Heat Lightning is Sheet lightning near the horizon.

• Spider Lightning can crawl across the sky for up to 150 km.

• Ball Lightning is a hovering ball the size of a pumpkin that disappears in a fizzle or a bang.

Lightning Variations

Sheet Lightning Heat Lightning

Spider Lightning Ball Lightning

The Power of Lightning– Each year in the United States, lightning

accounts for about 7500 forest fires, which result in the loss of millions of acres of forest.

– Lightning strikes in the United States cause a yearly average of 300 injuries and 93 deaths to humans.

Identify whether the following statements are true or false.

Pop Quiz

_____ Latent heat is crucial in maintaining the upward motion of a cloud.

_____ Thunderstorms are more likely to develop along a warm front instead of a cold front.

_____ A mountain thunderstorm is an example of an air-mass thunderstorm.

_____ In the mature stage of a thunderstorm, updrafts are roughly equal to downdrafts.

true

false

true

true

Severe Weather• Occasionally, weather events come together

in such a way that there is a continuous supply of surface moisture.– This happens along a cold front that moves into

warmer territory and can lift and condense a continuous supply of warm air.

– Cold fronts are usually accompanied by upper-level, low-pressure systems that are marked by pools of cold air, which cause the air to become more unstable.

– When the strength of the storm’s updrafts and downdrafts intensifies, the storm is considered to be severe.

Super Cells• Supercells are self-sustaining, extremely

powerful severe thunderstorms, which are characterized by intense, rotating updrafts.

• Only about 10% of the roughly 100,000 thunderstorms that occur each year in the US are considered to be severe

– Even fewer become supercells.

The Fury of the Wind• Instead of dispersing over a large area

underneath a storm, downdrafts sometimes become concentrated in a local area.

– Downbursts are violent downdrafts that are concentrated in a local area and can contain wind speeds of more than 160 km/h.

– Macrobursts can have wind speeds of more than 200 km/h, can last up to 30 minutes, and cause a path of destruction up to 5 km wide.

– Microbursts affect areas of less than 3 km wide but can have winds exceeding 250 km/h.

Hail• Hail is precipitation in the form of lumps of

ice that can do tremendous damage.

• Hail forms because of two characteristics common to thunderstorms:

– Water droplets exist in the liquid state in the parts of a cumulonimbus cloud where the temperature is actually below freezing,

– The abundance of strong updrafts and downdrafts existing side by side within a cloud.

Hail• The supercooled water droplets in the cloud

freeze on contact with other ice pellets and are caught alternately in the updrafts & downdrafts.

• The ice pellets are constantly encountering more supercooled water droplets and growing.

• Eventually they become too heavy for the updrafts to keep aloft and fall to Earth as hail.

• Some thunderstorms produce heavy rains or snow packs produce lots of water as they melt.

– Water levels in streams and rivers rise.

– Excess water spills over the banks and floods the surrounding areas.

• Floods are the main cause of thunderstorm-related deaths in the US each year.

Floods

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 are often associated with supercells.

– The air in a tornado is made visible by dust and debris drawn into the swirling column, or by the condensation of water vapor into a visible cloud.

Tornadoes• A tornado forms when wind speed and

direction change suddenly with height, a phenomenon known as wind shear.– Under the right conditions, this can produce a

horizontal rotation near Earth’s surface. – A thunderstorm’s updrafts can tilt the twisting

column of wind from a horizontal to a vertical position.

– Air pressure in the center drops as the rotation accelerates.

– The extreme pressure gradient between the center and the outer portion of the tornado produces the violent winds associated with tornadoes.

Classifying Tornadoes• The Fujita Scale classifies tornadoes

according to their path of destruction, wind speed, and duration.

– The scale ranges from F0, characterized by winds of up to 118 km/h, to an F5, which can pack winds of more than 500 km/h.

– Most tornadoes do not exceed the F1 category.

– Only about 1% ever reach the violent categories of F4 and F5.

Classifying Tornadoes

Ted Fujita

Tornado Distribution• While tornadoes can occur at any time or

place, some places are more conducive to their formation.

– Most tornadoes form in the spring during the late afternoon and evening, when the temperature contrasts between polar air and tropical air are the greatest.

– More tornadoes occur in the U.S. than any other country.

• Cold air from Canada collides with warm, moist air from the Gulf of Mexico

Tornado Alley

Tornado Safety• In the US, an average of 80 deaths and 1,500

injuries result from tornadoes each year.– The National Weather Service issues tornado

watches and warnings before a tornado strikes. – The NWS stresses that despite advanced

tracking systems, advance warnings may not be possible.

– Signs of an approaching or developing tornado include the presence of dark, greenish skies, a towering wall of clouds, large hailstones, and a loud, roaring noise similar to that of a freight train.

Pop Quiz

Match the following terms with their definitions.___ supercell

___ macroburst

___ microburst

___ tornado

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

B. self-sustaining, extremely powerful thunderstorms that are characterized by intense, rotating updrafts

C. downburst causing a path of destruction up to 5 km wide

D. downburst causing a path of destruction up to 3 km wide

B

C

D

A

Why do so many tornadoes form in “Tornado Alley”?

Pop Quiz

Large temperature contrasts occur most frequently in the Central US, where cold continental polar air collides with maritime tropical air moving northward from the Gulf of Mexico.