Air Masses, Fronts and Global Wind Patterns

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Air Masses, Fronts and Global Wind Patterns

• Meteorology
• CGS Earth Science

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Air Masses
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Definition

• Air mass - a large dome of air which has similar
  horizontal temperature and moisture
  characteristics throughout.
• Very similar to a balloon.

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Continental Arctic (cA)

• Frigid record low temperatures
• Dry - very low dew points
• Dense - very high barometric pressure
• Usually originate north of the Arctic Circle
• Siberian Express
• Usually once or twice a winter
• very rarely form during the summer
• because the sun warms the Arctic.

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Continental polar (cP)

• Cold and dry - stable
• Usually originates in NW Territory of Canada
• Influences mainly the northern USA
• Responsible for clear and pleasant weather during
  the summer
• Usually in winter
• Creates troughs in the polar jet stream
• Lake effect snow in Great Lakes areas

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Maritime polar (mP)

• Cool and moist - unstable
• Originate over N. Atlantic and N. Pacific
• Main Influence - the Pacific Northwest and the
  Northeast.
• can form any time of the year
• Generally not as cold as cP air masses

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Maritime tropical (mT)

• Warm and very moist unstable
• Originate in the Gulf of Mexico and the Southern
  Atlantic Ocean
• Influences the eastern USA
• Most prevalent during summer
• Responsible for hot, humid summer days across the
  South and the East.

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Continental Tropical (cT)

• Very Hot and very dry stable aloft
• Originates in Desert Southwest and northern
  Mexico
• Occurs in the summer, rarely in winter
• Usually keeps the Desert Southwest scorching
  above 100oF during summer
• Generally clear skies, hot, low humidity

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Source Regions
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Reasoning for Tornadoes

• Orographic Perfection
• Meeting of
• Moist - mT
• Hot - cT
• Cool cP

Rocky Mtn.
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Tornado Alley

• http//www.britannica.com/thunderstorms_tornadoes/
  video/ocliwea124v4.mov

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Fujita Scale
(NationalAtlas.com)
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Fronts and their symbols
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Fronts

• Boundary between two air masses
• Characterized by shift in weather
• Cold
• Warm
• Stationary
• Occluded

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5 Characteristics of a Front

• Sharp temperature changes over a relatively short
  distance.
• Changes in air moisture content
• Shifts in wind direction
• Pressure changes
• Clouds and precipitation

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Cold Fronts

• Temperature drops rapidly
• Pressure rises steadily
• Clouds Vertical building
• Precipitation Heavy along front
• Winds Strong and shifting
• Typically move faster than warm front

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Cold Front
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Cold Front
(Fozzy)
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Cold Front
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In the summer, cold fronts can trigger

• thunderstorms
• large hail
• dangerous winds
• tornadoes

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Graphic Depiction!
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Warm Fronts

• Temperature rises slowly
• Pressure slight rise, then fall
• Clouds strato- and cirro-
• Precipitation long, steady
• Winds variable and light
• Typically will have affect for days

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Warm Front
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Warm Front
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Warm Front
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Effects of warm fronts

• Slow-moving warm front can mean days of wet
  weather before warm air
• Sometimes water vapor in warm fronts condense to
  produce
• rain
• snow
• sleet
• freezing rain

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Stationary Front
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Stationary Fronts

• Temperature stagnent
• Pressure slightly fluctuates
• Clouds altocumulus
• Precipitation none
• Winds variable and light
• Can last for days weeks

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Occluded Front
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Occluded Fronts

• Temperature
• Warm gets milder
• Cold gets colder
• Pressure
• Warm - slight drop
• Cold slight rise
• Clouds cumulus
• Precipitation steady and light
• Winds variable and light

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Occluded Front
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Different Temperatures - Different Pressures
Less Dense Less Pressure
Cool Air
Denser More Pressure
Warm Air
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Pressure and Air Movement
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Pressure Gradient Force

• Difference in pressure over a given
  distance---between isobars
• Close together step pressure gradient
• STRONG winds
• Far apart gentle pressure gradient
• Light winds
• Just like contour lines

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Pressure Gradient Force
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Isobaric Maps
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Coriolis Effect

• Apparent force due to the rotation of the Earth
  (Think Merry-go-round)
• N. Hemisphere ? wind turns right
• S. Hemisphere ? wind turns left
• Strength depends on latitude and wind speed

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Coriolis Effect
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Coriolis Effect
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Centripetal Force

• In-ward directed force
• Allows an object to remain in circular motion
• Winds moving around high and low pressure areas
• Clockwise around Highs.
• Counter-clockwise around Lows.

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Friction (What a Drag)

• The resistance to movement
• Surface winds are affected by friction
• Why? Ground resistance
• trees, mountains, houses, buildings, etc.
• This drag causes winds to blow across pressure
  gradient at the surface.

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Pressures All Together
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General Planetary Circulation
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Winds

• Horizontal movements at surface
• Names from WHERE it came fromnot where it is
  going!!!

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Around Pressure Cells
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Pressure Cells

• High In and Up
• Converge at surface
• Ascend in center
• Diverge Aloft
• Low Down and Out
• Converge aloft
• Descend in center
• Diverge at surface

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X-section of Planetary Circulation
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January Global Pressure Map
January
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July Global Pressure Map
July
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Summer Highs
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Quiz Questions

• Where would you expect there to be the strongest
  winds? Why?
• Where would you expect there to be the calmest
  winds?
• Where would you expect clear, cool skies?
• Where would you expect cloudy skies with the
  greatest potential for precipitation?

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Work Cited (Incomplete)

• http//www.usatoday.com/weather/tg/wamsorce/wamsor
  ce.htm seen 1/03/06
• http//www.srh.weather.gov/srh/jetstream/synoptic/
  airmass.htm