Thunderstorms
http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/tstorm.html
It is late afternoon. The white puffy clouds that have been growing all day are replaced by a greenish sky.
A distant rumble is heard...then another. It starts to rain. A flash of light streaks the sky, followed by a
huge BOOM. Welcome to a thunderstorm.
Thunderstorms are one of the most thrilling and dangerous of weather phenomena. Over 40,000
thunderstorms occur throughout the world each day.
Thunderstorms have several distinguishing characteristics that can cause large amounts of damage to
humans and their property. Straight-line winds and tornadoes can uproot trees and demolish buildings.
Hail can damage cars and crops. Heavy rains can create flash floods. Lightning can spark a forest fire or
hurt you. Safety during a thunderstorm is really important.
Cumulus Stage: the sun heats the earth’s surface during the day. The warm
air starts to rise (known as an updraft). If the air is moist, the air forms a
cumulus cloud. The cloud grows as long as warm air continues to rise.
Mature Stage: when the cloud gets very large, the water gets
large and heavy. Raindrops fall when rising air can no longer
hold them up. Cool dry air starts to enter the cloud. The cool air sinks (known as
the downdraft), pulling the water with it, making rain. The cloud is now a
cumulonimbus. Lightning and thunder and heavy rain occur.
Dissipating stage: when the downdraft is stronger than the updraft, warm moist
air can no longer rise so no cloud droplets can form. The storm dies out with
light rain. The cloud disappears from the bottom up. This whole process takes
about an hour for an ordinary thunderstorm.
Lightning and Thunder
Lightning is the most spectacular element of a thunderstorm. In fact
it is how thunderstorms got their name. Lightning is a giant spark. A
single stroke of lightning can heat the air around it to 30,000 degrees
Celsius (54,000 degrees Farhenheit)! This extreme heating causes the
air to expand at an explosive rate. The expansion creates a shock
wave that turns into a booming sound wave, better known as thunder.
Thus the name thunderstorm.
The sky is filled with electric charge. In a calm sky, the + and charges are evenly interspersed thoughout the atmosphere, and therefore neutral.
Inside a thunderstorm, electric charge is spread out differently. A thunderstorm consists of ice
crystals and hailstones. The ice crystals, (+ charged) are pushed to the top of the thunderstorm
cloud by an updraft. Meanwhile, the hailstones (- charge) are pushed to the bottom of the
thunderstorm by its downdraft. Thus, the thunderstorm's + and - charges are separated into two
levels: the + charge at the top and the - charge at the bottom.
During a thunderstorm, the Earth's surface has a + charge. Because opposites attract, the - charge
at the bottom of the thunder cloud wants to link up with the + charge of the Earth's surface.
Once the amount of charge gets large enough to overcome air resistance, positives and negatives
will attract causing lightning.
Hail
Hail is frozen raindrops that are tossed around the cloud by wind.
The top of a cumulonimbus cloud is cold enough to freeze water. A
hailstone grows larger when supercooled water droplets freeze on
contact with a hailstone. The longer a hailstone stays within a cloud
the larger it becomes. When it’s too heavy it will fall to the ground.
Most hailstones are about the size of peas. The heaviest one in the
United States was the size of a cantaloupe and fell on Coffeyville,
Kansas in September of in 1970. Large hailstones can break
windows, dent cars, damage roof of homes, and cause damage to crops. One hailstorm can
destroy a farmer’s entire crop in minutes. In the United States, hail causes hundreds of millions
of dollars of damage each year.
Severe Thunderstorms
They are capable of producing baseball-sized hail, strong winds, intense rain, flash floods, and
tornadoes. Severe thunderstorms can last several hours and can grow 18 kilometers high. Several
phenomena are associated with severe thunderstorms. These include the gust front, microburst,
supercell thunderstorm, and the squall line.
Thunderstorm Safety
GO INSIDE! If you hear distant thunder or see a flash of light, get indoors immediately. Seek
shelter from sturdy buildings, not lean-tos or outhouses. You can stay in a car if that is your only
option, but do not touch any of the metal on the car. If you cannot find shelter, stay away from
tall, isolated objects such as trees, poles, or posts. Make sure that you are not the tallest object by
crouching down. Crouch down, bend forward, and grab your ankles. Keep your head down. Do
not lie flat on the ground and try to keep out of puddles or other standing water.
If you can get into a house or school, go down to either the basement or a room that is in the
center of the building. Stay away from windows. That way, if a tornado touches down, you'll be
safe.
Do not use a phone or a computer during a thunderstorm. Do not take a shower or wash dishes.
Lightning can strike the plumbing or electrical wires that connect to your house and give you an
electrical shock if you use these items.
tornadoes....Nature's
Most Violent Storms
http://www.nssl.noaa.gov/edu/safety/tornadoguide.html
Greg Stumpf
Although tornadoes occur in many parts of the world, these destructive
forces of nature are found most frequently in the United States east of
the Rocky Mountains during the spring and summer months. In an
average year, 800 tornadoes are reported nationwide, resulting in 80
deaths and over 1,500 injuries. A tornado is defined as a violently
rotating column of air extending from a thunderstorm to the ground.
The most violent tornadoes are capable of tremendous destruction with
wind speeds of 250 mph or more. Damage paths can be in excess of
one mile wide and 50 miles long. Once a tornado in Broken Bow,
Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas!
What causes tornadoes?
Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These
thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and
early spring are often associated with strong, frontal systems that form in the Central States and
move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern.
Several states may be affected by numerous severe thunderstorms and tornadoes.
During the spring in the Central Plains, thunderstorms frequently develop along a "dryline,"
which separates very warm, moist air to the east from hot, dry air to
the west. Tornado-producing thunderstorms may form as the dryline
moves east during the afternoon hours.
Along the front range of the Rocky Mountains, in the Texas
panhandle, and in the southern High Plains, thunderstorms
frequently form as air near the ground flows "upslope" toward
higher terrain. If other favorable
conditions exist, these thunderstorms can produce tornadoes.
Jim Ladue
Tornadoes occasionally accompany tropical storms and
hurricanes that move over land. Tornadoes are most common to
the right and ahead of the path of the storm center as it comes
onshore.
Colorado Tornado (David Blanchard)
Dr. Joseph Golden (NOAA)
Tornado Variations
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Some tornadoes may form during the early stages of rapidly developing thunderstorms. This type
of tornado is most common along the front range of the Rocky Mountains, the Plains, and the
Western States.
Tornadoes may appear nearly transparent until dust and debris are picked up.
Occasionally, two or more tornadoes may occur at the same time.
Waterspout
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Waterspouts are weak tornadoes that form over warm water.
Waterspouts are most common along the Gulf Coast and southeastern states. In the western
United States, they occur with cold late fall or late winter storms, during a time when you least expect
tornado development.
Waterspouts occasionally move inland becoming tornadoes causing damage and injuries.
How Do Tornadoes Form?
Before thunderstorms
develop, a change in wind
direction and an increase in
wind speed with increasing
height creates an invisible,
horizontal spinning effect in
the lower atmosphere.
Rising air within the
thunderstorm updraft tilts
the rotating air from
horizontal to vertical.
An area of rotation, 2-6
miles wide, now extends
through much of the storm.
Most strong and violent
tornadoes form within this
area of strong rotation.
Tornadoes Take Many Shapes and Sizes
Weak Tornadoes
Strong Tornadoes
Violent Tornadoes
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69% of all tornadoes
Less than 5% of tornado
deaths
Lifetime 1-10+ minutes
Winds less than 110 mph
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29% of all tornadoes
Nearly 30% of all tornado
deaths
May last 20 minutes or
longer
Winds 110-205 mph
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Only 2% of all
tornadoes
70% of all tornado
deaths
Lifetime can exceed 1
hour
Lifetime can exceed 1
hour
Weather Radar Watches the Sky
Meteorologists rely on weather radar to provide
information on developing storms. The National
Weather Service is strategically locating Doppler radars
across the country which can detect air movement
toward or away from the radar. Early detection of
increasing rotation aloft within a thunderstorm can allow
life-saving warnings to be issued before the tornado
forms.
Doppler Radial Velocity
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TORNADO WATCH: Tornadoes are possible in your area. Remain alert for approaching
storms.
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TORNADO WARNING: A tornado has been sighted or indicated by weather radar. If a tornado
warning is issued for your area and the sky becomes threatening, move to your pre-designated place of
safety.
SEVERE THUNDERSTORM WATCH: Severe thunderstorms are possible in your area.
SEVERE THUNDERSTORM WARNING: Severe thunderstorms are occurring.
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Look out for:
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Dark, often greenish sky
Wall cloud
Large hail
Loud roar; similar to a freight train
Tornado Safety
If a Warning is issued or if threatening weather approaches:
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In a home or building, move to a pre-designated shelter, such as a basement.
If an underground shelter is not available, move to an interior room or hallway on the lowest floor
and get under a sturdy piece of furniture.
Stay away from windows.
Get out of automobiles.
Do not try to outrun a tornado in your car; instead, leave it immediately.
Mobile homes, even if tied down, offer little protection from tornadoes and should be abandoned.
In Schools
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Develop a severe weather action plan and have frequent drills,
Make sure someone knows how to turn off electricity and gas in
the event the school is damaged.
Keep children at school beyond regular hours if threatening
weather is expected. Children are safer at school than in a bus or car.
Students should not be sent home early if severe weather is
approaching.
Gymnasiums, cafeterias, and auditoriums offer no protection from tornado-strength winds.
Move students quickly into interior rooms or hallways on the lowest floor. Have them assume the
tornado protection position (shown at right).
Hurricanes (also known as Tropical Cyclones)
As a strong hurricane heads towards the coast, people
prepare - boarding up houses, packing the car, and
evacuating. These storms can spell disaster for people
in hurricane prone areas, so they are taken very
seriously. They are the most powerful of all weather
systems and they are huge - an average of 340 miles
across.
Hurricanes form in the tropics over warm ocean water
and die down when they move over land or out of the
tropics. These storms are called hurricanes in the Atlantic and typhoons or tropical cyclones in
other areas of the world. In the Northern Hemisphere the storms rotate counterclockwise and in
the Southern Hemisphere they rotate clockwise because of the Coriolis Effect. At the center of
the rotating storm is a small area of calm weather and clear skies called the eye.
Hurricane damage in coastal areas is often due to storm surge, which floods coastal areas. Strong
waves and wind also batter coastal areas. Hurricanes also cause a tremendous amount of rain.
Not all storms are the same. Large and strong storms cause much more damage than small
storms. In the Atlantic and Eastern Pacific, the Saffir-Simpson Hurricane Scale is used to
describe the size of a hurricane.
Hurricanes usually happen at a particular time of year called hurricane season. The timing of
hurricane season is different in different regions of the world. In the North Atlantic, hurricane
season is from June 1st to November 30th each year.
How Hurricanes Form
A tropical thunderstorm can grow
into a massive hurricane under
certain conditions. Sometimes
several thunderstorms start rotating
around a central area of low
pressure. This is called a tropical
depression. If the depression
strengthens so that winds reach at
least 39 mph, it is called a tropical
storm. And if wind speeds increase
to more than 74 mph, it is called a
tropical cyclone or hurricane.
Once formed, hurricanes take energy
from the warm ocean water to
become stronger. A storm will
strengthen if there is a supply of
warm, moist air to feed it. Warm,
moist air is found above warm,
tropical ocean waters. While a
hurricane is over warm water it will continue to grow. A hurricane dies when it moves away
from the tropics. When a hurricane moves into areas with cooler ocean water, it weakens. It will
also weaken if it travels over land.
The rotation of the storm is due to the Coriolis Effect, a product of the Earth's rotation. This
causes the air being drawn into the central low pressure to curve. The air rises as it rotates. This
rising air, which is saturated with water, cools and condenses, forming clouds. Hurricanes do not
occur within 300 miles (500 kilometers) of the equator because there is no Coriolis Effect at the
equator.
The Eye of a Hurricane
At the center of a fierce tropical
storm, there is a small area where
the weather is calm, the sky is
clear, and the winds are just light
breezes. This area is called the eye
of the storm.
As a hurricane strengthens and
wind speeds increase, an eye
begins to form at the center of the
storm. Usually this happens once
winds reach about 80 mph. The eye
is usually circular when viewed
from above, and about 20 to 40 miles is diameter. It is roughly cylinder shape in cross section,
extending through the center of the storm like a chimney. Air from above the storm sinks down
in the center of the eye.
Surrounding the eye is the eyewall, which is the most violent part of a hurricane. The eyewall is a
ring of dense cumulonimbus clouds – thunderstorm clouds. There, the strongest winds in the
hurricane are found and warm, moist air is pulled into the storm, rising along the eyewall where
it cools and forms more cumulonimbus clouds.
Storm Surge
One of the most dangerous parts of a hurricane
isn’t the rain or the wind. It’s the flooding
caused by storm surge.
As a hurricane or other tropical storm moves
towards a coast, it can cause sea level to rise as
much as 20 or 30 feet higher than normal. The
sea level rise only lasts a short time, usually
just a few hours, but it can cause a huge
amount of damage. The rising water may
totally submerge low-lying areas and towns
along the coast. Huge ocean waves cause
damage too, demolishing docks, houses, roads, and eroding beaches.
Most storm surge is caused when a storm’s winds push ocean water towards the land. When the
water is pushed into the shallow parts of the ocean, it piles up, flooding the coast. Some storm
surge is caused by the low pressure of the storm too. When storm surge happens at high tide,
there is even more flooding.
Safety
BEFORE A HURRICANE: Have a
disaster plan and a pet plan ready. Before a
storm threatens, contact your veterinarian or
local humane society for information on
preparing your pets for an emergency.
Board up windows and bring in outdoor
objects that could blow away. Make sure
you know which county or parish you live
in and know where all the evacuation routes
are.
Prepare a disaster supplies kit for your home
and car. Have enough food and water for at
least 3 days. Include a first aid kit, canned
food and a can opener, bottled water,
battery-operated radio, flashlight, protective
clothing and written instructions on how to
turn off electricity, gas, and water. Have a
NOAA weather radio handy with plenty of
batteries, so you can listen to storm
advisories. Have some cash handy as well,
because following a hurricane, banks and
ATMs may be temporarily closed. Make
sure your car is filled with gasoline.
DURING A HURRICANE: Stay away from low-lying and flood prone areas. Always stay
indoors during a hurricane, because strong winds will blow things around. Leave mobile homes
and to go to a shelter. If your home isn’t on higher ground, go to a shelter. If emergency
managers say to evacuate, then do so immediately.
AFTER A HURRICANE: Stay indoors until it is safe to come out. Check for injured or trapped
people, without putting yourself in danger. Watch out for flooding which can happen after a
hurricane. Do not attempt to drive in flooding water. Stay away from standing water. It may be
electrically charged from underground or downed power lines. Don’t drink tap water until
officials say its safe to do so.