SEVERE
WEATHER
Thunderstorms are small, intense
weather systems that produce
strong winds, heavy rain,
lightening, and thunder.
Thunderstorms form when warm
air is forced upward at a cold
front.
Cumulonimbus clouds also form
on hot, humid afternoons in the
sprig and summer.
In both cases, the warm, humid
air rises rapidly.
As the air rises, it cools, forming
dense thunderheads.
THUNDERSTORMS
2 atmospheric
conditions are required
to produce thunderstorms:
* warm and moist air near Earth’s surface
* an unstable atmosphere
The air is unstable when the surrounding air is
colder than the rising air mass. The air mass will
continue to rise as long as the surrounding air is
colder than the air mass.
When the warm rising air reaches its dew point, the
water vapor in the air condenses and forms cumulus
clouds.
If the air is extremely unstable, the air will continue to
rise which causes the cloud to grow into a dark,
cumulonimbus cloud.
Thunderstorms are very active electrically.
Lightning is an electric discharge that occurs between
a positively charged area and a negatively charged
area.
Lightning can happen
between:
•two clouds
•between earth and a
cloud
•two parts of the same
cloud
When lightning strikes, energy is released.
This energy is transferred to the air and causes the air
to expand rapidly and send out sound waves.
Thunder is the sound
that results from the
rapid expansion of air
along the lightning strike.
Severe Thunderstorms can
produce 1 or more of the following:
*
*
*
*
high winds
hail
flash floods
tornadoes
Flash flooding that results from heavy rains causes
millions of dollars in property damage annually.
Flash flooding is the leading
cause of weather-related
deaths.
Lightning is responsible for starting thousands of
forest fires each year and killing or injuring
hundreds of people each year
in the United States.
TORNADOES
Tornadoes happen in only 1% of all thunderstorms.
A tornado is a small, spinning column of air that
touches the ground.
A tornado starts out as
a funnel cloud and
hangs in the air.
The funnel cloud
becomes a tornado
when it makes contact
with the Earth’s surface.
About 75% of the world’s tornadoes happen in the
United States.
Most of these happen in the spring and early summer
when cold, dry air from Canada meets warm, moist
air from the Tropics.
Although they only last a
few minutes they can cause
a lot of damage due to
their strong, spinning winds.
Average tornado wind speeds
are between 120 and 180 km/h.
Tornado Facts
Texas holds the record
for the most tornados –
178 in 2004
Ohio averages 16
tornadoes a year and
five tornado-related
deaths.
Ohio’s worst Tornado
was in Xenia, Ohio in
1974 – 33 people were
killed
HURRICANES
A large, rotating weather system that has wind speeds
at least 120 km/h is called a hurricane.
Hurricanes are the most powerful storms on Earth.
Hurricanes have different names is different parts of the
world.
In the western
Pacific Ocean = typhoons
Over the
Indian Ocean = cyclones
Most hurricanes form in the areas between 5° and 20°
north latitude and between 5° and 20° south latitude
over warm, tropical oceans.
At higher altitudes, the water is too cold for hurricanes
to forms.
A hurricane begins as a group of thunderstorms
moving over tropical ocean waters.
Winds traveling in 2 different directions meet and
cause the storm to spin.
Because of the Coriolis Effect, the storm turns
counterclockwise in the Northern hemisphere and
clockwise in the Southern hemisphere.
Defining a Hurricane
According to the National Hurricane Center,
"hurricane" is a name for a tropical cyclone that
occurs in the Atlantic Ocean. "Tropical cyclone" is
the generic term used for low-pressure systems that
develop in the tropics.
"Tropical cyclones with maximum sustained surface
winds of less than 17 meters per second (39 mph)
are called tropical depressions. Once the tropical
cyclone reaches winds of at least 17 meters per
second, it is typically called a tropical storm and
assigned a name. If winds reach 33 m/s (74 mph),
then it is called a "hurricane."
Hurricanes are defined by the following characteristics:
They are tropical, meaning that they are generated in tropical areas of the ocean near the
Equator.
They are cyclonic, meaning that their winds swirl around a central eye. Wind direction is
counterclockwise (west to east) in the Northern Hemisphere and clockwise (east to west) in the
Southern Hemisphere (more about this later).
They are low-pressure systems. The eye of a hurricane is always a low-pressure area. The lowest
barometric pressures ever recorded have occurred inside hurricanes.
The winds swirling around the center of the storm have a sustained speed of at least 74 mph.
HOW A HURRICANE FORMS
A hurricane gets energy from the condensation of
water vapor.
Once formed, ,the hurricane is fueled through
contact with the warm ocean water.
Moisture is added to the warm air by evaporation
from the ocean.
As the warm, moist air rises, the water vapor
condenses and releases large amounts of energy.
The hurricane continues to grow as long as its over
the source of warm, moist air.
When it moves into colder waters or onto land, it
begins to die because it has lost its source of energy.
Katrina's eyewall swirls in a photograph by a National
Oceanic and Atmospheric Administration (NOAA) P-3
hurricane hunter pilot on August 28, 2005, a day
before the powerful storm slammed into the United
States Gulf Coast.
During the last 35 years, the number of Category 4 or 5 Hurricanes
have doubled, which researchers have linked to global warming.
You would think that with the overwhelming evidence of
increased temperatures affecting our planet, there would be no
longer any doubts that this is being caused by global warming.
Yet there are still a large number of people (and scientists) who
either rule this out completely, or reluctantly admit that maybe
there is some connection or link, but usually insist on telling us that
there is nothing to worry about.
Hurricane Damage
THUNDERSTORM SAFETY
Lightning is one of the most
dangerous parts of a
thunderstorm.
Lightning is attracted to tall
objects – so if outside, stay away
from trees. If you are in the open
– crouch down.
Stay away from bodies of water!
TORNADO SAFETY
A watch = a weather alert that lets people know that a
tornado may happen
A warning = a weather alert that lets people know that
a tornado has been spotted
Find shelter quickly
FLOOD SAFETY
A flash flood is a flood that rises and falls very suddenly.
Best thing to do = find a high place to wait out the flood!
HURRICANE SAFETY
People living in the area may be asked to evacuate.
You and your family should have
A disaster supply kit that
Includes enough water and
food to last a few days.
To protect your windows –
You should cover them with
Plywood.
Stay inside your home!