Transcript Air Masses

And The Weather’s Good…
AP Env Sci 2011-2011
Weather Basics – and a little more
• Air Takes on the Temperature and Moisture Characteristics
of the Surface it is Over.
– Does this happen instantaneously? NO
– Needs to be Stationary or Slow Moving Air
• Air Mass:
– Large body of air (~1600 km across)
– Perhaps several kilometers thick
– Homogeneous Temperature and Moisture Content
– When the Air Mass moves on, it can carry Temperature
and Moisture properties with it.
Air Masses Important for North America
Source: NOAA.gov
Air Masses: Who’s Who
• cA: Continental Arctic
– Source: Arctic basin and Greenland ice cap
– Bitterly cold and very dry in winter
• cP: Continental Polar
– Source: Interior Canada and Alaska
– Very cold and dry in winter, cool and dry in summer
• mP: Maritime Polar
– Source: North Pacific
– Mild (cool) and humid entire year
Air Masses: Who’s Who
• mT: Maritime Tropical
– Source: Gulf of Mexico, Caribbean, western Atlantic
– Warm and humid entire year
• cT: Continental Tropical
– Source: Interior Mexico, Southwestern US
– Hot and dry
• Air Mass Weather: entire region is covered by single air
mass – can last for days. Conditions set by air mass.
• More interesting weather happens when air masses
MEET.
When Air Masses Meet: Cold Front
Source: NOAA.gov
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Cold Air Mass meets Warm Air Mass
Cold Air Mass is Moving (fast moving)
“Pushes” Warm Air Up (warm air is less dense)
Result Can Be Violent: Thunderstorms
After: Cooler, Often Drier Conditions
When Air Masses Meet: Warm Front
Source: NOAA.gov
• Warm Air Mass meets or follows Cold Air Mass
• Warm Air Mass is Slower
• Warm Air Rises as it Overtakes Cold (warm air is less
dense, air then cools)
• Large Area of Moderate Precipitation for a Long Period
• After: Warmer Conditions
When Air Masses Meet: Stationary and
Occluded Fronts
• Stationary Front:
– Cold and Warm Air Masses Meet with Flow Parallel to
Front
– Little or No Movement
– Gentle to Moderate Precipitation
• Occluded Front:
– Two Cold Air Masses, Warm Air Mass in the Middle
– Warm Air Rises
– Precipitation
Source: NOAA.gov
Know: Symbols Point In Direction The Front is Moving
Wind
• ALL wind is caused by horizontal differences in
air pressure
• Important points about air pressure:
– Gravity on an air mass results in air pressure
– Measured in millibars, inches of mercury or
hectopascals (hPa).
– Air pressure decreases with altitude
– Low pressure produces cloudy, stormy weather
– High pressure contains cool, dense air that descends
towards Earth and becomes warmer. Brings fair
weather.
• Higher Pressure Difference, More Wind
What causes the pressure differences?
• Uneven Heating of the Earth’s Surfaces
• Result is sea and land breezes
• http://www.classzone.com/books/earth_sci
ence/terc/content/visualizations/es1903/es
1903page01.cfm
• Temperature Variation -> Pressure Variation -> Wind
If Pressure Acted Alone…
• If Earth did not rotate
• If there was no friction
• Then, air would always flow directly from High pressure
to Low pressure
• In Reality, three factors control wind:
– Pressure Differences
– Coriolis Effect
– Friction
Earth without Rotation…
•
http://teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-039
Rotating Earth – What Does it Do?
• Coriolis Effect:
– All free moving objects (wind, planes) are deflected
right of their path of motion in Northern Hemisphere,
left of path of motion in Southern Hemisphere
• http://www.youtube.com/watch?v=mcPs_OdQOYU
• http://www.youtube.com/watch?v=1PWjOMrPkrI
• To know: In the Northern Hemisphere, winds spiral clockwise out
from high pressure areas and spiral counterclockwise in toward low
pressure areas.
Results of the Coriolis Effect
Hadley, Ferrel, and Polar Air
Circulation Cells
• Hadley Air Circulation Cells (0-30 degrees)
– Hot air near the equator rises. Land area characterized by high
humidity, large high clouds, and heavy rains. (Tropical rain
forests)
– Area on land between 0-30 degrees characterized by high
temperture and low humidity. (Deserts)
• Ferrel Air Circulation Cells (30-60 degrees)
– Some of the air from the falling hadley air cells gets pulled north
by the Westerlies and mixes with Polar cells. Causes defined
seasons of winter and summer. (Forests)
• Polar Air Circulation Cells
– Cold, dry dense air falls from the troposphere to the ground,
meeting warm air from midlatitudes, returning to the poles and
then sinking. Sinking air supresses precipitation. (Tundra and
taiga)
Friction
• Most important for air contacting a surface
• Winds aloft – not affected
• At surface, friction slows winds
Some “Local” Winds
• Land and Sea Breezes
• Mountain and Valley Breezes
– Valley breeze: Sun heats air along slopes more
intensely, rises
– At night, mountain breeze
• Chinook: warm, dry air descending mountain
– Pulled over mountain by low pressure system
– Air is heated by compression as it flows downward
• Katabatic Winds:
– Cold air over ice-covered highland “falls” downhill
under influence of gravity
– Antarctica: up to 66 ft/sec
Global Winds to Know
Image/Text/Data from the University of Illinois WW2010 Project