And The Weather’s Good… AP Env Sci

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.

Source: NOAA.gov Air Masses Important for North America

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 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 Source: NOAA.gov

When Air Masses Meet: Warm Front 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 Source: NOAA.gov

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

Know: Symbols Point In Direction The Front is Moving Source: NOAA.gov

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 ence/terc/content/visualizations/es1903/es 1903page01.cfmhttp:// 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…

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 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