Air masses and
Fronts
• If we were to pick one term to help explain why
we have weather, what do you think would be a
good word?
What causes weather?
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•
•
•
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Water vapor (moisture)
Composition of atmosphere
Solar heating
Air pressure
Wind
What is the distribution of
Insolation?
• Insolation: incident solar radiation
• Tropics receive more concentrated
insolation due to Earth’s curvature
• Tropics receive 2.5X more than the poles
What is an air mass?
• Because air and moisture move in the
atmosphere, weather is constantly changing
• Air pressure – measured by barometer
• An air mass is a large body of air whose
temperature and moisture are fairly similar at a
given altitude; properties like the part of Earth’s
surface over which it formed
• There are 5 different air masses that affect the
United States
How do we classify air masses?
• Classified via moisture and temperature
• cP( continental polar) : cold, dry stable
• cT( continental tropical) : hot, dry, stable air
aloft, unstable at the surface
• mP( maritime polar) : cool, moist, unstable
• mT( maritime tropical) : warm, moist, unstable
• A (artic):same as cP but colder
What is air mass modification?
• As air masses move from their source
region, their temperature and moisture
characteristic change with the regions they
come in contact with.
What is a front?
• Fronts are boundaries separating different air
masses of two different densities caused by
temperature, pressure and humidity.
• clouds, precipitation, and storms occur at frontal
boundaries
How do warm fronts work?
• A warm front is warm air displacing cool air
• Shallow leading edge warm air must “overrun”
cold air
• These are usually slow moving
• Hot, humid weather to follow
How does a warm front look on
a weather map?
A red-line with solid semi-circles
How do cold fronts work?
• Cold air advances into region of warm air
• Intensity of precipitation greater, but short lived
• Clearing conditions after front passes. Fair and cool
weather
• Usually approaches from W or NW
How does a cold front look on a
weather map?
• Solid blue line with triangles
How does a stationary front
form?
• Surface positions of the front do not move
• Often a region of clouds but no precipitation
How does a stationary front
look on a weather map?
• When the front starts moving again it
returns to either being a cold or warm
front
• Red semi-circles and blue triangles
How do occluded fronts form?
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Cold front overtakes warm front
Involves three air masses of different temperatures
Often found close to the low pressure center
Strong winds and precipitation on both sides of the
front.
How does a occluded front
look on a weather map?
• This occurrence usually results in storms
over an area
• In U.S., the colder air usually lies to the
west
• Purple semi circles and triangles.
Review question
• What type of front can be found close to point D
?
Review question
• Which of these fronts would you expect to have
greater precipitation, but be short lived as the
front passes?
Review question
• Give the name of the air mass that would have
the following characteristics:
• cool, moist, unstable
Review question
• That important weather word that refers to the
transfer of heat
What are pressure systems?
• Created by air molecules, product of motion, size and number.
• Since warmed air has more space between the
molecules, it’s less dense and rises
• Cooled air is more dense and tends to sink
• In general, air near the equator tends to rise and air
near the poles tends to sink
Take a look at this
Notice the band
of clouds around
the equator ?
• This is the ITCZ
or inter tropical
convergence
zone
Why do you think there is this band of
clouds near the equator?
Did you figure it out?
• Warm, moist air in the tropics rises
• Cold air can hold less moisture than warm air
• As the moist air rises, it condenses and forms
clouds!
Now What?
• Ok, so we know that the weather moves around on
these highways and that warm air rises and cold
air sinks.
• But why is it sunny one day, and rainy the next?
Let’s take another look at
the weather map
• Notice that there are H’s and L’s on the map
• There are also blue lines with spikes and red lines
with half circles
What is air pressure?
• Air weight that varies over Earth’s
surface
• Warmer air is less dense and exerts less
pressure
• Cooler air is more dense and exerts
more pressure
How does a high pressure
system form?
• When cooler air sinks and is warmed, the air can
hold more moisture
• This usually means sunny skies
• Winds tend to move clockwise around a high in
the Northern hemisphere.
• Anti-cyclones
• Diverging and descending winds
How do low pressure systems
form?
• When warm air rises and is cooled, the air can not
hold as much moisture
• Often, these areas are associated with
precipitation and stormy weather
• Winds tend to move counter clockwise around the
low in the northern hemisphere.
• Rising and converging air
What to expect on a weather
map?
• If you see a big L in your area you should expect stormy
weather and if you see a big H in your area expect clear, sunny
skies.
What else affects weather?
• Wind: Movement of air from one temperature or pressure
area to another
• Different areas of Earth receive different amounts of the
Sun’s energy
• Equator’s warm air, being less dense, is pushed
upward by denser, colder air
• Poles’ cold air, being more dense, sinks and moves
along Earth’s surface
• CORIOLIS EFFECT: spinning of the Earth
causes moving air to turn to the right in the
northern hemisphere and to the left in the
southern hemisphere
How do global wind patterns
affect weather?
• Wind patterns, caused by convection currents combined with
the Coriolis effect, of Earth that affect the world’s weather
• Near equator, very little wind and daily rain patterns
called the doldrums
• Surface winds:
• Between equator and 30 degrees N and S latitude
are steady trade winds
• Between 30 and 60 degrees N and S latitude, the
westerlies blow in opposite direction from the
trade winds. Affect weather in the United States.
• The polar easterlies blow from northeast to
southwest near the north pole and from southeast
to northwest near the south pole
Global Winds
TRADEWINDS
Equatorial doldrums (Low P)
TRADEWINDS
How does the Coriolis effect
create weather highways?
• The rotation of the earth creates the Coriolis
effect.
• The Coriolis effect causes the air and water to
be deflected to the right north of the equator.
• This creates global weather highways
What is a jet stream?
• An irregular, concentrated band of wind
occurring at several different locations that
supports surface weather conditions.
• Occur at the boundaries between wind zones.
• polar jet stream and subtropical jet stream
Short Review
1.Transfer of heat in liquids or gases_____
2. _____ air is dense and tends to sink.
3. Band of clouds found around the equator______
4. Cold air holds _____ moisture than warm air
5. The Coriolis effect causes the air and water to be
deflected to the _____ of the equator
6. If there is a big H on the weather map where
you live, would you expect fair or stormy
weather?
7. Warm air holds ( more or less ) moisture than
cold air
8. Which of the weather highways usually
controls our weather ?
1.
CONVECTION
2. COLD
3. ITCZ
4. LESS
5. RIGHT
6. FAIR
7. MORE
8. WESTERLIES