Transcript Weather Patterns 2015

Weather Patterns
Air Masses
A huge body of air that
has similar temperature,
humidity, and air
pressure at any given
height. A single air mass
may spread over millions
of square kilometers and
be up to 10 kilometers
deep.
Classifying Air Masses
Wet
Dry
Warm
Maritime
tropical
Continental
tropical
Cold
Maritime
polar
Continental
polar
Maritime Tropical
• Warm, humid air masses form over
tropical oceans.
• In the summer they usually bring hot,
humid weather
• In the winter they can bring heavy rain or
snow.
Maritime Polar
• Cool, humid air masses form over the icy
cold North Pacific and North Atlantic
oceans.
• They affect the West Coast more than the
East Coast.
• Even in summer, these masses of cool,
humid air often bring fog, rain, and cool
temperatures to the West Coast.
Continental Tropical
• Hot, dry air masses form mostly in
summer over dry areas of the
Southwest and northern Mexico.
• They cover a smaller area than
other air masses.
• They occasionally move northeast,
bringing hot, dry weather to the
southern Great Plains.
Continental Polar
• Large continental polar air masses form over
central and northern Canada and Alaska.
• In winter they bring bitterly cold weather with
very low humidity to much of North America.
• In summer the air mass is milder.
• Storms may occur when they move south and
collide with maritime tropical air masses
moving north.
How Air Masses Move
Jet Stream
• The jet stream is a current of fast moving air
found in the upper levels of the atmosphere.
This rapid current is typically thousands of
kilometers long, a few hundred kilometers wide,
and only a few kilometers thick. Jet streams are
usually found somewhere between 10-15 km (69 miles) above the earth's surface. The position
of this upper-level jet stream denotes the
location of the strongest SURFACE temperature
contrast (as in the previous diagram).
• During the winter months, Arctic and
tropical air masses create a stronger
surface temperature contrast resulting
in a strong jet stream. However,
during the summer months, when the
surface temperature variation is less
dramatic, the winds of the jet are
weaker.
Prevailing Westerlies
• The prevailing westerlies, the major wind
belts over the continental United States,
generally push air masses from west to
east.
• For example, maritime polar air masses
from the Pacific Ocean are blown onto the
West Coast, bringing in low clouds and
showers.
Fronts
• As huge masses of air move across the
land and the oceans, they collide with
each other.
• However, they do not easily mix.
• The boundary where the air masses meet
becomes a FRONT.
• Storms and changeable weather often
develop along fronts.
Types of Fronts
• Colliding air masses can form four types of
fronts:
• Cold fronts
• Warm fronts
• Stationary fronts
• Occluded fronts
• The kind of front that develops depends on
the characteristics of the air masses and
how they are moving.
Cold Fronts
A cold front is defined as the transition
zone where a cold air mass is replacing a
warmer air mass. Cold fronts generally
move from northwest to southeast. The air
behind a cold front is noticeably colder and
drier than the air ahead of it. When a cold
front passes through, temperatures can
drop more than 15 degrees within the first
hour.
Symbolically, a cold front is represented by a solid line
with triangles along the front pointing towards the
warmer air and in the direction of movement. On
colored weather maps, a cold front is drawn with a
solid blue line.
Warm Fronts
• A warm front is defined as the transition
zone where a warm air mass is replacing a
cold air mass. Warm fronts generally move
from southwest to northeast and the air
behind a warm front is warmer and more
moist than the air ahead of it. When a
warm front passes through, the air
becomes noticeably warmer and more
humid than it was before.
• Symbolically, a warm front is represented by a
solid line with semicircles pointing towards the
colder air and in the direction of movement. On
colored weather maps, a warm front is drawn
with a solid red line.
Stationary Fronts
• When a warm or cold front stops moving, it
becomes a stationary front. Once this
boundary resumes its forward motion, it
once again becomes a warm front or cold
front.
• A stationary front is represented by alternating
blue and red lines with blue triangles pointing
towards the warmer air and red semicircles
pointing towards the colder air.
Occluded Fronts
A developing cyclone
typically has a preceding
warm front (the leading
edge of a warm moist air
mass) and a faster moving
cold front (the leading
edge of a colder drier air
mass wrapping around
the storm). North of the
warm front is a mass of
cooler air that was in
place before the storm
even entered the region.
• As the storm intensifies,
the cold front rotates
around the storm and
catches the warm front.
This forms an occluded
front, which is the
boundary that separates
the new cold air mass (to
the west) from the older
cool air mass already in
place north of the warm
front.
Symbolically, an occluded front is represented by a solid
line with alternating triangles and circles pointing the
direction the front is moving. On colored weather maps,
an occluded front is drawn with a solid purple line.
Cyclones
• A cyclone is an area of low pressure around which the winds flow
counterclockwise in the Northern Hemisphere and clockwise in the
Southern Hemisphere.
• A developing cyclone is typically accompanied by a warm front
pushing northward and a cold front pulling southward, marking the
leading edges of air masses being wrapped around a center of low
pressure, or the center of the cyclone. They are associated with
clouds, wind, and precipitation.
Anticyclones
• The opposite of a cyclone is an anticyclone. It is
a high pressure system with winds that blow out
from the center of the high in a clockwise
direction. High pressure systems usually bring
dry, clear weather. In anticyclones, the air
converges high in the atmosphere and then
sinks toward the ground. In the lower
atmosphere, the air spreads out, or diverges.
The sinking air contains little moisture, and
therefore brings dry weather.
A high pressure center is represented on a weather map by a blue H. Winds flow
clockwise around the high in the northern hemisphere. The opposite is true in the
southern hemisphere, where winds flow in a counterclockwise around an area of high
pressure.
Sinking air in the vicinity of a high pressure center tends to suppress the upward
motions required for the clouds and precipitation to develop, which is why fair weather
is commonly associated with an area of high pressure.
Homework
• Review notes
• Find or draw pictures of how the four
fronts form.
• Remember you need to be watching the
weather on TV.