Transcript Weather Patterns Chapter 17

Weather Patterns
Chapter 17
Air Masses
•Air masses: a huge
body of air with similar
temperature, humidity,
and pressure.
•In America, we have
four air masses that
affect us. They are
based on temperature
and humidity.
Types of Air Masses
•Tropical: form over the
tropics, so they are warm
and will have low air
pressure.
•Polar: form north/south of
50º latitude. So they are
cold and will have high
pressure.
•Maritime: form over the
oceans so they are humid.
•Continental: form over the
middle of continents (land)
so they are dry.
Air Masses Compared
Type
Temp
Humidity
Pressure
Maritime
tropical (mT)
Warm
Humid
Low
Maritime
polar (mP)
Cold
Humid
High
Continental
tropical (cT)
Warm
Dry
Low
Continental
polar (cP)
Cold
Dry
high
Air Masses and Global Winds
•In America air masses are pushed by the
prevailing westerlies.
•So they tend to move west to east.
Weather Fronts
•Fronts: where two
air masses meet
and don’t mix.
•Front is a military
term where two
armies meet to
fight.
•The collision of
fronts often causes
storms and
changeable weather.
•There are four
kinds of fronts.
Cold Fronts
•Cold fronts have cold,
dense air.
•They will slide under
warm, less dense air.
•So the warm air is
pushed up, which causes
it to cool.
•This means it can hold
less water vapor, so
clouds form as it
reaches its dew point.
•This can cause rain
and storms behind the
front.
•They move quickly, and
bring cool, dry air
after the front passes.
Cold Front Details
Warm Fronts
•Moving warm air
collides with cold
air.
•Tend to move more
slowly than cold
fronts.
•Can bring cloudy
skies, showers, and
light rain preceding
the front.
•Weather will be
warm and humid
after it passes.
Warm Front Details
Stationary Fronts
•Occur when cold and
warm fronts meet
and neither is strong
enough to force the
other to move.
•It is a “standoff.”
•The warm air will
condense and form
rain, fog, snow, or
clouds.
•Can cause many days
of wet weather.
Occluded Fronts
•Occur when a warm air
mass is trapped between
two cold air masses.
•The warm air is pushed
up when the two cold air
masses meet.
•It is cut off from the
ground, or occluded.
•Tend to produce strong
thunderstorms.
Weather Map Symbols
•These are the
symbols you would
see on a weather
map.
•The arrows or
bumps point in the
direction the
front is moving.
Low Pressure Areas
•Low pressure areas will
have warm, rising air.
•Cool air around it will
spiral in counter
clockwise (Coriolis
effect).
•As the air rises, it cools
and reaches its dew
point.
•So clouds and rain will
form.
•Called cyclones from a
Greek word meaning
wheel.
High Pressure Areas
•High pressure centers of
dry air.
•Spiral outward towards
areas of lower pressure.
•Coriolis effect causes
them to spiral clockwise
in the Northern
Hemisphere.
•Made of cool, falling air,
so its relative humidity
drops.
•Produces clear, dry
weather.
•Called anticyclones.
Storms
Storms
•A violent disturbance in
the atmosphere.
•Involve sudden changes
in air pressure, which
cause rapid air
movement.
Thunderstorms
•Heavy rainfall with
thunder and lightning.
•Form within
cumulonimbus clouds.
•Warm air rapidly rises,
causing updrafts that
bring moisture to the
tops of the cloud.
•The moisture
condenses, and
eventually forms rain.
Thunderstorm Stages
Thunder and Lightning
•Friction between updrafts
and downdrafts cause
electrons to be stripped
from some atoms and given
to others. Just like
walking across a carpet in
your socks.
•So the clouds develop
charges.
•Lightning is the spark
when the discharge occurs.
•Lightning is one big static
electricity discharge!
Thunder
• Lightning heats the air as it passes through it.
• This air is hotter than the sun, so it moves
rapidly.
• This causes the loud, rumbling sound.
• Light travels about a million times faster than
sound.
• Sound travels roughly 1-km in 3 seconds or 1mile in 5 seconds.
• So if you see the flash of lightning, and it takes
10 seconds for the thunder to reach you, the
lightning was about 2 miles away.
Tornadoes
•Rapidly rotating, funnel
shaped cloud.
•Most locally destructive
of all storms.
•Usually touch the
ground for only a few
minutes.
•Wind speeds can be
over 200mph!
Tornado Formation
• Develop in thunderstorms.
• If the rising updraft in a thunderstorm
changes direction, it can start to rotate.
• As this rotation tightens, it gets stronger
but narrower.
Tornado Damage
•Tornadoes are usually
less than a 100 yards
wide.
•The damage they
inflict is severe but
localized.
Fujita Wind Damage Scale
• F-5’s are the strongest, most damaging
tornadoes.
New
enhanced
scale is
very
similar.
Tornado Safety
•Tornado Watch:
conditions are
favorable for
tornadoes.
•Tornado Warning:
Tornado has been
sighted. Take cover if it
is in your area.
•Basements are the
best place if there is a
tornado.
Tornado Alley
•The area over which
tornadoes occur most
often in North America is
called Tornado Alley.
•It covers the Great Plains
witch is between the Rocky
Mountains and the
Appalachian Mountains. It
includes all or half of the
13 mid-west states.
Hurricanes
• A tropical cyclone that occurs in the
Atlantic.
• Also a generic term for low pressure
systems that develop in the tropics.
• Starts as a tropical depression (winds less
than 39mi/hr).
• Becomes a tropical storm and is given a
name when the winds exceed 39mi/hr.
• Finally becomes a hurricane when the
winds reach 74mi/hr.
Hurricane Names
• Since at least 1945, the US Navy and
later the Air Force started naming tropical
cyclones.
• At first they used exclusively English
female names, but since 1978 have
started to alternate male and female
names.
• Different areas of the world tend to use
local names for their areas.
Atlantic Hurricane Names
How Hurricanes Form
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•
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•
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Form in warm, tropical waters.
Water must be at least 80°F(27°C).
Needs warm, moist air and converging winds.
Has a large difference in air pressure.
Formed by the heat energy and as long as the
water is warm are self-sustaining.
• The moist, warm air circulates around a well
defined center.
• The lower the pressure at the center, the
faster the winds will rush in to try to fill it.
Parts of a hurricane
Parts Continued
• Eye: center of the hurricane. Weather is
calm, may be clear, and will have no rain.
• Winds will come from the opposite
direction after the eye passes.
• Rain bands will move counter-clockwise
around the eye. These bands with
hurricane force winds can extend over 300
kilometers from the eye. So the storms
can affect a wide area.
Hurricane Movement
•Hurricanes are steered
by the global winds.
•So the storms in the
tropics are steered to
the west by the trade
winds. When they get
far enough north the
westerlies take over and
steer them east.
•Once over land, they
lose strength as they no
longer have a source of
warm water to draw
energy from.
•Friction with the land
can slow the winds down
also.
Hurricane Damage
•Hurricane can cause
enormous damage when they
come ashore.
•While high winds do a lot
of damage, flooding is more
serious.
•Heavy rains cause flooding,
especially if the hurricane is
slow moving.
•Storm surge is even more
serious. It is a dome of
water caused by low
pressure and high winds. If
it coincides with the high
tide, many coastal areas will
be devastated.
Katrina descends on Alabama
Katrina descends on Alabama