Severe Weather Thunderstorms, Tornadoes, and Hurricanes

Unit Essential Question: What are the different features of the atmosphere that characterize our weather. How does the atmosphere influence life and how does life influence the atmosphere?

Presentation Objectives: § Explain how a thunderstorm forms. § Define latent heat of vaporization. § Describe how lightning and tornadoes form. §

What Do You Need?• For a thunderstorm to occur, you need:

– Lots of moisture in the lower levels of the atmosphere. – Unstable air – A mechanism to lift the air so that moisture can

condense.

Q1

Energy Required• To evaporate, water molecules need enough energy

to change to water vapor. - The amount of energy needed for evaporation is known

as the latent heat of vaporization.

Q2

• When condensation occurs, this energy is released.

- Heat

+ Heat

Creating Instability• When an air parcel rises, it cools and begins to

condense. – Latent heat that is released keeps the air parcel

warmer than the surrounding air. • Allows it to keep rising!

Q2

Latent heat provides the energy for the wind associated with thunderstorms and hurricanes!

There Are Limits You Know!• Thunderstorm clouds can only be a certain height

because of two factors: – It reaches stable air and can no longer rise. – As air rises, the rate of condensation decreases.

• Not enough latent heat to keep the air rising. – Limits the height of cumulonimbus clouds to around 18,000 meters.

These limits give the thunderstorm the characteristic anvil shape!

Q3 Stages of Development• Cumulus Stage

– As moist air moves upward, it condenses to form a cumulus cloud.

– Updraft

• Mature Stage – Cloud spreads out to form

characteristic anvil shape. – Precipitation begins to fall.

– Air around the cloud cools, causing some air to move downward

– Downdrafts

Q4

Stages of Development• Dissipating Stage

– Cool, downdrafts take over and cut off the supply of moist upward moving air.

– Eventually the cloud runs out of raindrops.

The entire process takes about 30 minutes to an hour!

Q4 Thunder and Lightning• In a thunder cloud, positive and negative charges

separate.• The negatives in the bottom

of the cloud repel the negatives in the ground and attract the positives.

– A streamer of negative charges moves toward the ground.

• When they connect, electrons rapidly flow to the ground. – Creates light, which we see as the lightning. – Creates heat that rapidly expands the air that we hear as thunder.

Q5

– A streamer of positive charges moves toward the cloud.

Tornadoes• A tornado is a violent, whirling column of air in

contact with the ground. – Tornadoes form when there is a difference in wind

speed with height. – This causes horizontal rotation.

• Updrafts that form the thunderstorm cloud pushes the rotating air column vertical.

Most common in thunderstorms known as

super cells!

Q6 More About Tornadoes• Tornadoes are ranked by the Fujita Intensity Scale.

• Storms F3 or more are rare. – Only occur in the

United States.

Q7

95% of the tornadoes in the US are below F3.

– F2 & F3 • 110 - 205 mph • Last 20 min

– F4 & F5 • 200 mph + • Last more than 20 min

– F0 & F1 • 60 - 115 mph • Last 1-10 min

Hurricanes• A hurricane is a large storm that is associated

with a low pressure system. – Form in tropical waters that are at least 80°F. – Hurricanes that affect the United States form off the

coast of Africa near the Canary Islands.

Q8 Scary Winds!• Named after the Aztec god Huracán.

– Same characteristic shape that demonstrate the counterclockwise winds.

• Classification By Wind Speed – Tropical Depression

• 38 mph or less – Tropical Storm

• 39 mph - 74 mph

– Hurricane • Category 1: 74-95 mph • Category 2: 96-110 mph • Category 3: 111-130 mph • Category 4: 131-155 mph • Category 5: >155 mph

Q9

What’s In A Name?• Hurricanes are named according to the list developed by the

World Meteorological Organization. – Atlantic and Pacific Names

• Ana, Bill, Claudette, Danny, Erika, Fred… • http://www.nhc.noaa.gov/aboutnames.shtml

– When particularly destructive, names are retired for 10 yrs so as not to cause emotional distress or confusion.

• Katrina, Denis, Wilma, Rita, etc.

The prediction for 2015 is 7 named storms, 3 hurricanes, only 1 of which will be major.

Mass Destruction• Hurricanes cause wind and water damage.

– Spawn tornadoes – Debris – Storm Surge

Q10

New England Hurricanes• Even though hurricanes are unlikely to hit New

England, we have experienced a few.

The Great Colonial Hurricane of 1635

The Great September Gale of 1815

The September Gale of 1869

The Long Island Express of 1938

The Great Atlantic Hurricane of 1944Hurricane Dog in 1950

Hurricane Carol in 1954

Hurricane Edna in 1954

Hurricane Diane in 1955

Hurricane Donna in 1960

Hurricane Gloria in 1985

Hurricane Bob in 1991

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