Earth Science, 11e...• To alert the public to the possibility of tornadoes • Issued when the...

Earth Science, 11e

Weather Patterns and Severe Storms

Chapter 19

Middle-latitude cyclone Role of air aloft

• Cyclones and anticyclones• Generated by upper-level air flow• Maintained by upper-level air flow• Typically are found adjacent to one another

• Cyclone • Low pressure system • Surface convergence• Outflow (divergence) aloft sustains the low pressure

Middle-latitude cyclone Role of air aloft

• Anticyclone • High pressure system• Associated with cyclones• Surface divergence• Convergence aloft

Anticyclone (High) vs. Cyclone (Low)High pressure center(anticyclone): winds aloft converge, produce downdraft and fair weather. Low pressure center (cyclone): surface winds converge, produce updraft and storms

Thunderstorms, Tornadoes, LightningNational Weather Service, part of the National Oceanographic & Atmospheric Administration (NOAA)https://www.weather.gov/media/owlie/ttl6-10.pdf

Michael and Sean McQuilken were struck by lightning bolts shortly after this photo was taken while climbing Moro Rock in California’s Sequoia National Park on August 20, 1975.

Michael was 18 at the time and his brother was 12.

Though they survived the lightning strikes in the Sierra Nevada mountains, another hiker was killed during the storm.

Michael said all he remembered of the incident was a flash of white, a huge explosion and being lifted off the ground by the force of the lightning strike.

Danger! About to be hit by lightning!

Severe weather types Thunderstorms

• Features • Cumulonimbus clouds• Heavy rainfall• Lightning• Occasional hail

• Occurrence • 2000 in progress at any one time• 100,000 per year in the United States• Most frequent in Florida and eastern Gulf Coast

region

Average number of days per year with thunderstorms

Figure 19.15

Severe weather types Thunderstorms

• Stages of development • All thunderstorms require

• Warm air• Moist air• Instability (lifting)

• High surface temperatures• Most common in the afternoon and early

evening

Severe weather types Thunderstorms

• Stages of development • Require continuous supply of warm air and moisture

• Each surge causes air to rise higher• Updrafts and downdrafts form

• Eventually precipitation forms • Most active stage• Gusty winds, lightning, hail• Heavy precipitation

• Cooling effect of precipitation marks the end of thunderstorm activity

Stages in the development of a thunderstorm

Figure 19.17

Severe weather types Tornadoes

• Local storm of short duration • Features

• Violent windstorm• Rotating column of air that extends down from a

cumulonimbus cloud• Low pressures inside causes the air to rush into the

tornado• Winds approach 480 km (300 miles) per hour• Smaller suction vortices can form inside stronger

tornadoes

Severe weather types Tornadoes

• Occurrence and development • Average of 770 each year in the United States• Most frequent from April through June• Associated with severe thunderstorms• Exact cause of tornadoes formation is not known• Conditions for the formation of tornadoes

• Occur most often along a cold front• During the spring months• Associated with huge thunderstorms called

supercells

Severe weather types Tornadoes

• Characteristics • Diameter between 150 and 600 meters (500 and

2000 feet)• Speed across landscape is about 45 kilometers (30

miles) per hour, sometimes as fast as 73 MPH• Cut about a 10 km (6 miles) long path• Most move toward the northeast• Maximum winds range beyond 500 kilometers (310

miles) per hour• Intensity measured by the Fujita intensity scale

Dr. Ted Fujita(1920-1998)

University of ChicagoPioneered tornado research using wind tunnels, smoke,

high speed photography, and computer analysis

Average annual tornado incidence per 10,000 square

miles for a 27 year period

Figure 19.20

Three Stages of Tornado Formation

Tornado is still white because it has just touched down

EF-4 Tornado Damage

Andover, Kansas EF-5 Tornado (April 26, 1991)

EF-5 Tornado Damage

• Top photo shows a brick school building before the tornado struck

• Bottom photo shows the entire school completely leveled to the ground after being struck by the EF-5 tornado

Waterspouts• tornadoes that form over

water, or move from land to water.

• have same characteristics as land tornadoes

• associated with severe thunderstorms, often accompanied by high winds and seas, large hail, and frequent lightning.

• Found mostly in warm oceans

• Sometimes form over Lake Michigan

Two waterspouts over Lake Michigan

Tornadoes That Have Hit the Chicago AreaMarch 4, 1961: A rare tornado hits Chicago. And EF-2 ripped through the South Side—starting at 91st and Western and traveling to 68th Street and Lake Michigan. One person was killed, 115 hurt. The storm caused $7 million in damage.

April 11, 1965: An EF-4 hit McHenry, Crystal Lake and Wauconda, killing six and injuring 75.

May 26, 1965: An EF-2 tornado touched down in Skokie and tore away almost all of the roof of Old Orchard Junior High School.

April 21, 1967: A massive EF-4 storm cut a 28 mile path from Cherry Valley, to Belvidere and Woodstock. A total of 24 people were killed and 500 injured. Thirteen were killed and 300 injured as 12 buses were tossed around Belvidere High School at dismissal time.

May 26, 1965: An EF-2 tornado touched down in Skokie, IL, and tore away almost all of the roof of Old Orchard Junior High School.

April 21, 1967: A violent EF-4 tornado formed in Palos Hills in Cook County and traveled through Oak Lawn and the South Side of Chicago. A total of 33 people died, and 500 people were injured by this 200 yard wide tornado that traveled 16 miles and caused over $50 million in damage.

June 13, 1976: An EF-4 tornado hit Lemont and traveled for eight miles, leaving $13 million in damage. Two people were killed and 23 injured.

August 28, 1990: A total of 29 people died and 353 people were injured when an EF-5 tornado pulverized Plainfield. It is believed to be the only EF5 tornado to strike in Chicago area.

Tornadoes That Have Hit the Chicago Area

Paths of Illinois tornadoes

(1916 – 1969)Figure 19.21

Severe weather types Tornadoes

• Tornado forecasting • Difficult to forecast because of their small size• Tornado watch

• To alert the public to the possibility of tornadoes• Issued when the conditions are favorable• Covers 65,000 square km (25,000 square miles)

• Tornado warning is issued when a tornado is sighted or is indicated by weather radar

• Use of Doppler radar helps increase the accuracy by detecting the air motion

STORM TYPE TORNADOES HURRICANES

WIND SPEED 65 to >200 MPH(Highest: 301 MPH)

75 to >150 MPH(Highest: 190 MPH)

SIZE 500 to 2000 feet (typical)Largest: 2.6 miles

(El Reno, OK, 2013)

300 miles (typical)Largest: 1380 miles(Typhoon Tip, 1979)

DURATION 10 minutes (typical)Longest: 3.5 hours

12 to 24 hours (typical)Longest: 30 days

GEOGRAPHICRANGE

Mostly occur in the U.S.(on all continents except

Antarctica)

All tropical oceans, except South Atlantic and Eastern

South PacificHAZARDS Blowing debris, flooding Blowing debris, flooding,

storm surge

Contrasts: Tornadoes vs. HurricanesThe only similarities they share are high wind speeds and circular motion; they are otherwise very different kinds of storms.

Severe weather types Hurricanes

• Most violent storms on Earth• To be called a hurricane

• Wind speed in excess of 119 kilometers (74 miles) per hour

• Rotary cyclonic circulation• Profile

• Form between the latitudes of 5 degrees and 20 degrees

Severe weather types Hurricanes

• Profile • Known as

• Typhoons in the western Pacific• Cyclones in the Indian Ocean

• North Pacific has the greatest number per year• Parts of a hurricane

• Eyewall • Near the center• Rising air• Intense convective activity

Severe weather types Hurricanes

• Profile • Parts of a hurricane

• Eyewall • Wall of cumulonimbus clouds• Greatest wind speeds• Heaviest rainfall

Severe weather types Hurricanes

• Profile • Parts of a hurricane

• Eye • At the very center• About 20 km (12.5 miles) diameter• Precipitation ceases• Winds subsides• Air gradually descends and heats by

compression• Warmest part of the storm

Cross section of a hurricane

Figure 19.25

Severe weather types Tropical Storms

• Wind speeds 38 to 74 miles/hour

Hurricanes • Profile

• Wind speeds 75 to 190 miles/hour• Generate 50 foot waves at sea

• Hurricane formation and decay• Form in all tropical waters except the

• South Atlantic and• Eastern South Pacific

Severe weather types Hurricanes

• Hurricane formation and decay• Energy comes from condensing water vapor• Develop most often in late summer when warm

water temperatures provide energy and moisture• Initial stage is not well understood

• Tropical depression – winds do not exceed 61 kilometers (38 miles) per hour

• Tropical storm – winds between 61 to 119 km (38 and 74 miles) per hour

Severe weather types Hurricanes

• Hurricane formation and decay• Diminish in intensity whenever

• They move over cooler ocean water• They move onto land• The large-scale flow aloft is unfavorable

Severe weather types Hurricanes

• Destruction from a hurricane • Factors that affect amount of hurricane damage

• Strength of storm (the most important factor)• Size and population density of the area affected• Shape of the ocean bottom near the shore

• Saffir-Simpson scale ranks the relative intensities of hurricanes

Saffir-Simpson Hurricane Scale

• The secret energy source of a hurricane is the large latent heat of water. Air over the tropical oceans is drier than you might think. Although both the air and water may be warm and calm, evaporation can take place because the air is not at 100 percent relative humidity.

• The heat release of a hurricane is equivalent to a 10-megaton nuclear bomb exploding every 20 minutes (10,000 nukes per storm). According to the 1993 World Almanac, the entire human race used energy at a rate of 1013 watts in 1990, a rate less than 20% of the power of a hurricane.

• Not only can you not stop a hurricane with a nuclear bomb, you would be spreading deadly radiation all over a wide area.

Immense Power of Hurricanes Comes from Water

Severe weather types Hurricanes

• Destruction from a hurricane • Categories of hurricane damage

• Storm surge - large dome of water 65 to 80 kilometers (40 to 50 miles) wide sweeps across the coast where eye makes landfall

• Wind damage• Inland flooding from torrential rains

Storm Surge of a Hurricane

Aerial satellite image of a hurricane

Low Atmospheric Pressure of Hurricane Fran (954 millibars to 980 mb as it moves inland)

End of Chapter 19