Air Masses, Fronts and Global Wind Patterns Meteorology CGS – Earth Science
Dec 29, 2015
Definition:Definition: Air mass - a large dome of air
which has similar horizontal temperature and moisture characteristics throughout.
Very similar to a balloon.
Continental Arctic (cA): 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.
Continental polar (cP): 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
Maritime polar (mP):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 cPcP air masses
Maritime tropical (mT): 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.
Continental Tropical (cT):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
Reasoning for TornadoesReasoning for Tornadoes
Orographic Perfection
Meeting of• Moist - mT
• Hot - cT
• Cool – cP
Roc
ky M
tn.
Tornado AlleyTornado Alley http://www.britannica.com/thunderstorms_tornadoes/video/ocli
wea124v4.mov
Fronts:Fronts: Boundary between two air masses Characterized by shift in weather
Cold Warm Stationary Occluded
5 Characteristics of a 5 Characteristics of a FrontFront
Sharp temperature changes over a relatively short distance.
Changes in air moisture content Shifts in wind direction Pressure changes Clouds and precipitation
Cold FrontsCold Fronts
• Temperature – drops rapidly• Pressure – rises steadily• Clouds – Vertical building• Precipitation – Heavy along front• Winds – Strong and shifting
• Typically move faster than warm front
In the summer, cold In the summer, cold fronts can trigger:fronts can trigger:
thunderstorms large hail dangerous winds tornadoes
Warm FrontsWarm 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
Effects of warm frontsEffects of warm fronts Slow-moving warm front can mean
days of wet weather before warm air Sometimes water vapor in warm fronts
condense to producerainsnowsleet freezing rain
Stationary FrontsStationary Fronts
• Temperature – stagnent• Pressure – slightly fluctuates• Clouds – altocumulus• Precipitation – none• Winds – variable and light
• Can last for days weeks
Occluded FrontsOccluded 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
Different Temperatures - Different Temperatures - Different Pressures Different Pressures
CoolAir
WarmAir
Denser
MorePressure
Less Dense
LessPressure
Pressure Gradient ForcePressure Gradient Force
Difference in pressure over a given distance---between isobarsClose together = step pressure gradient
STRONG winds
Far apart = gentle pressure gradientLight winds
• Just like contour lines
Coriolis EffectCoriolis 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
Centripetal ForceCentripetal ForceIn-ward directed forceAllows an object to remain in
circular motionWinds moving around high and
low pressure areasClockwise around Highs.Counter-clockwise around Lows.
Friction Friction (What a Drag)(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.
Winds Winds
Horizontal movements at surface
Names from WHEREWHERE it came from…not where it is going!!!
Pressure CellsPressure Cells High – In and Up
Converge at surfaceAscend in centerDiverge Aloft
Low – Down and OutConverge aloftDescend in centerDiverge at surface
Quiz Questions:
1. Where would you expect there to be the strongest winds? Why?
2. Where would you expect there to be the calmest winds?
3. Where would you expect clear, cool skies?
4. Where would you expect cloudy skies with the greatest potential for precipitation?