Download The Nature of Storms

The Nature of Storms
Chapter 13
13.1
THUNDERSTORMS
At any given moment, nearly 2000 thunderstorms
are occurring around the world.
1. Abundant source of moisture in the lower levels of the
atmosphere
2. A mechanism to lift the air to allow moisture to
condense and release latent heat
3. The portion of the atmosphere where clouds grow must
be unstable, to allow rising and cooling of air necessary
to stop the cloud growth.
Conditions of Formation of
Thunderstorms
13.2
SEVERE WEATHER
Occasionally weather events come together in such a
way that there is a continuous supply of surface
moisture, causing more severe storms.
Lightning is electricity caused by
the rapid rush of air in a
cumulonimbus cloud.
A lightning bolt forms when
friction between updrafts and
downdrafts separates electrons
from some of their atoms
creating positive ions and
negative ions.
This creates regions of air with
opposite charges.
A channel of negatively charged
air moves toward the ground and
a channel of positively charged
ions rushes upward from the
ground to meet it, creating an
illumination called lightning.
Lightning
• Lightning heats the air to around 30000°C, which is five
times HOTTER than the surface of the Sun!
• As the super-heated air rapidly expands and contracts, it
creates a sound called thunder.
• Because sound travels more slowly than light, you
typically see lightning before you hear thunder even
though they are actually generated at the same time!
Thunder
• Hail: precipitation in
the form of balls or
lumps of ice,
occurring most
frequently in the
central US. Most
common during
Spring
Other Dangers of
Thunderstorms
• Flood: occur when a
thunderstorm moves
slowly over one
location, dumping all
its rain in one place
Tornadoes
A tornado is a
violent, whirling
column of air in
contact with the
ground
Characteristics
• Before reaching the ground,
it’s called a funnel cloud
• Often associated with a super
cell, which is the most severe
thunderstorm
• Air is made visible by dust and
debris drawn into the swirling
column or by the condensation
of water vapor into a visible
cloud
• Over the area it covers, few
storms on Earth can match a
tornado’s violence
Formation
• A tornado is caused by wind
shear, which is when wind speed
and direction change suddenly
with height – causing a
horizontal rotation near the
Earth’s surface
• An updraft can shift this rotating
air into the vertical position
• As updrafts speed up the
rotation, air pressure in the
center decreases, creating a
pressure gradient between the
inside and outside of the tornado
– creating violent winds
• Although devastating, tornadoes
typically last only a few minutes.
The Fujita Tornado Intensity Scale ranks tornadoes according to the path
of destruction, wind speed, and duration
• Bottom range is F0 - winds up to 118km/hr
• Top range is F5 – winds of more than 500km/hr
• Most tornadoes do not exceed the F1 category
Tornadoes can occur at any time and at any place, although some times and
locations are more conducive to their formation
• Most occur in the spring, during late afternoon and evening, when
temperature differences are greatest
• Most common in the central United States due to colliding cP and mT air
masses
• More than 700 tornadoes touch down each year in the United States
Classification and Distribution
13.3
TROPICAL STORMS
The most violent storm on Earth is within the
calm, sunny tropics.
Tropical Cyclones
• Large, rotating, low-pressure storm
• The strongest tropical cyclones are known in the United
States, Mexico, and the Caribbean as hurricanes
• In the western Pacific Ocean, these storms are referred to
as typhoons
• People living near the Indian Ocean refer to these storms
as cyclones
Characteristics
Formation
Tropical Cyclones / Hurricanes
require two basic conditions to
form:
1. Abundant supply of warm
water
2. Some sort of disturbance to lift
the warm air and keeprising it
1. As water evaporates from the
ocean, heat is stored in the form
of latent heat
2. The heat is released as the air
rises and water vapor condenses
into clouds and rain
3. The rising air creates a low
pressure system at the ocean
surface and more air moves in
to replace it
4. The Coriolis Effect causes the
air to turn counterclockwise,
resulting in the rotation of the
cyclone
5. Air moving towards center rises
and rotates faster, further
decreasing pressure in the
center
These conditions exist in all
tropical oceans except the South
Atlantic Ocean and the Pacific
Ocean west of the South American
coast because waters in these areas
are cooler.
Tropical Disturbance - beginning stages, causes the air to rise
Tropical Depression - when a disturbance acquires a cyclonic rotation
around a center of low pressure
Tropical Storm - when winds of the depression reach 65km/hr
Hurricane - when winds of the storm reach 120km/hr
Eye - calm center of the storm
Eyewall - a band immediately surrounding the eye where the winds are
the strongest
Stages of a Hurricane
• Saffir-Simpson Hurricane
Scale - classifies hurricanes
according to wind speed, air
pressure in center, and
potential for property damage
• Ranges from Category 1
(74 mph winds) to
Category 5 (+155 mph winds)
• At Category 3 it is classified
as a major hurricane
Classifying Hurricanes
Hurricane
Hazards
• Hurricanes cause a lot of
damage, especially along
coastal areas
• Much of the damage results
from violent winds
• Strong winds move onshore
and are responsible for
another major threat, storm
surges, which is when winds
move a mound of water over
land
• Storm surges can be as high
as 6m above normal sea
level
• Heat released through
condensation of vast
amounts of water vapor fuels
hurricanes. This
condensation produces great
amounts of rain
13.4
RECURRING WEATHER
Persistent or repetitive weather can negatively
affect agriculture, transportation, and recreation.
Floods
• Natural occurrence in which water spills over the sides of a
stream’s banks onto land
• Can be caused by long thunderstorms, hurricanes, and mild
storms that stay in the same area for extended times
• The main cause of thunderstorm-related deaths in the United
States each year
Droughts
• A drought is an extended period of below average rainfall
• Caused by large high-pressure systems in an area for an
extended period of time
• One of the most extreme droughts occurred during the 1930’s
in the central United States (Dust Bowl)
Heat Wave
• A Heat Wave is an extended period
of above normal temperatures
• Can result from long term highpressure systems
Cold Wave
• A Cold Wave is an extended
period of below normal
temperatures, which can be
caused by high pressure systems
of continental polar (cP) or
Arctic air
• Few clouds to block the blazing Sun
• System barely moves because the
air currents guiding the highpressure system are weak
• Increases rate of humidity
• Can cause heatstroke, sunstroke,
and/or death
• Wind-Chill: because wind
transports heat away from the
body, the temperature may feel
cooler than it really is
• The wind-chill factor tells how
cold it actually feels to the
human body
Due to humidity, air
may feel warmer than
the real temperature
The heat index tells
how warm it feels to the
human body
Heat Index