Air masses and Fronts
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Transcript of Air masses and Fronts
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Air masses and Fronts
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• If we were to pick one term to help explain why we have weather, what do you think would be a good word?
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What causes weather?• Water vapor (moisture)• Composition of atmosphere• Solar heating• Air pressure • Wind
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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
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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
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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
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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.
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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
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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
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How does a warm front look on a weather map?A red-line with solid semi-circles
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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
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How does a cold front look on a weather map?• Solid blue line with triangles
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How does a stationary front form?• Surface positions of the front do not move• Often a region of clouds but no precipitation
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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
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How do occluded fronts form?• 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.
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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.
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Review question
• What type of front can be found close to point D ?
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Review question
• Which of these fronts would you expect to have greater precipitation, but be short lived as the front passes?
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Review question
• Give the name of the air mass that would have the following characteristics:• cool, moist, unstable
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Review question
• That important weather word that refers to the transfer of heat
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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
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Take a look at this
Notice the band of clouds around the equator ?• This is the ITCZ or
inter tropical convergence zone
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Why do you think there is this band of clouds near the equator?
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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!
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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?
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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
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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
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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
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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
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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.
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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
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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
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Equatorial doldrums (Low P)TRADEWINDS
TRADEWINDS
Global Winds
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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
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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
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Short Review1.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 air5. The Coriolis effect causes the air and water to be
deflected to the _____ of the equator6. 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 usuallycontrols our weather ?
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1. CONVECTION
2. COLD
3. ITCZ
4. LESS
5. RIGHT
6. FAIR
7. MORE
8. WESTERLIES