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THUNDERSTORMS
Severe Thunderstorms
Most thunderstorms include heavy
rains, strong winds, lightning, and
sometimes hail.
Storm clouds and darkening skies
indicate that a thunderstorm is
approaching.
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
As the moisture condenses, it releases latent heat
The release of latent heat keeps the cloud warmer
than the air around it, which is crucial in
maintaining the upward motion of the cloud
#2
Some mechanism (orographic
lifting, temperature differences,
fronts) must lift the air so that
the moisture can condense and
release heat.
#3
The portion of the atmosphere through
which the cloud grows must be
unstable. (It must continue to cool with
increasing altitude for the growing cloud
to stay warmer than the surrounding
air.)
Most Important!
Two of the most important
ingredients for thunderstorm
formation are instability (unstable
air) and moisture.
Types of Thunderstorms
There are three main types of
thunderstorms; orographic, air
mass, and frontal.
Air mass
thunderstorms are
the result of
localized convection
in an unstable air
mass.
Frontal
thunderstorms
occur along
the boundaries
of weather
fronts (e.g.
cold front).
Orographic
thunderstorms
are caused by air
that is forced up
by a mountain or
hillside.
Stages of Development
A thunderstorm usually has three stages:
◦The cumulus stage
◦The mature stage
◦The dissipation stage
(the stages are classified according to the
direction in which the air is moving)
Click here for Thunderstorm formation video
The Cumulus Stage
The cumulus stage when the updraft
reaches the
condensation point
begins to form the
cumulus cloud.
The Mature Stage
The mature stage begins when the first drop of
precipitation from the cumulus clouds reaches the
ground. In the convection cycle, this is when the water
droplets become too heavy for the updraft to hold
aloft.
During the mature stage, cloud tops begin to exceed
60,000 feet. Strong winds at these altitudes cause the
tops of the clouds to level off, and take an anvil shape.
The "anvil" is so high and temperatures are so low that
the top of the cloud is composed entirely of ice
crystals.
The mature stage continued
A thunderstorm is strongest toward the end
of the mature stage. Rain will be the heaviest
and lightning is abundant. This is when hail,
strong winds and even tornadoes may form.
The Dissipation Stage
The end of a thunderstorm. This is
when precipitation falls through the
cloud, breaking it up. During the
dissipating stage, the humidity in the air
drops and the precipitation ends.
The cumulus stage of a thunderstorm is characterized mainly
by updrafts. The mature stage is characterized by strong
updrafts and downdrafts. The storm loses energy in the
dissipation stage.
What causes a storm to be severe?
Greater temperature differences form
between upper and lower parts of the storm
This causes air to become more unstable.
As instability of the air increases, strength of
the storm’s updrafts/downdrafts intensifies
and the storm is said to be “severe”
Supercells
Severe thunderstorms may develop
into self-sustaining, extremely
powerful storms called “supercells,”
which are characterized by intense,
rotating updrafts.
click here for "How do Thunderstorms Form?" video
Click here for supercell video
Results of severe thunderstorms
Lightning
Thunder
Wind
Hail
Floods
Tornados
Lightning
The sky is filled with electric charge.
The positively and negatively charged
particles are separated by updrafts
and downdrafts, separating the
charges into two levels.
Click here for “Bluemoon Lightning Storm” video
Click here for “How Does Lightning Form?” video
Thunder
Lightning is a giant spark. A single stroke of
lightning can heat the air around it to
54,000 degrees Fahrenheit! 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.
Wind
During strong or severe thunderstorms, a burst of
intense winds often flatten buildings and knock
down trees.
These winds, known as "downbursts," are often
mistaken for tornadoes due to the severity of the
damage.
A downburst (or microburst) features air diving
toward the surface, whereas a tornado is
composed of rising air.
Click here for “Tucson Wet Microburst” video
Hail
Hail forms when: strong currents of
rising air, known as updrafts, carry
water droplets high enough in a
thunderstorm for the water droplets
to freeze.
Click here for “How Does Hail Form?” video
Floods
When a storm dumps its rain over a limited
location, rather than spreading it over a large
area.
If rain falls faster than the ground can absorb
it, or faster than streams and rivers can
transport it out of the area, flooding can
occur.
Click here for “Floods 101” video
Hurricanes
The most severe of all thunderstorms
are hurricanes.
Sept. 20, 2005: Hurricane Katrina hits
the southern coast of Louisiana. It
became the most destructive hurricane
in U. S. history.
Click here for “How Hurricane Katrine Formed” video