Air Masses, Fronts and Global Wind Patterns Meteorology CGS – Earth Science

Air Masses

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): 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): 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): 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

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): 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

Source Regions

Reasoning for Tornadoes Orographic Perfection Meeting of Moist - mT Hot - cT Cool – cP Rocky Mtn.

Tornado Alley http://www.britannica.com/thunderstorms_tornadoes/video/ocliwea124v4.mov

Fujita Scale (NationalAtlas.com)

Fronts and their symbols

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

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

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

Cold Front

Cold Front (Fozzy)

Cold Front

In the summer, cold fronts can trigger: thunderstorms large hail dangerous winds tornadoes

Graphic Depiction!

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

Warm Front

Warm Front

Warm Front

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

Stationary Front

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

Occluded Front

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

Occluded Front

Different Temperatures - Different Pressures Less Dense Pressure Cool Air Denser More Pressure Warm Air

Pressure and Air Movement

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

Pressure Gradient Force

Isobaric Maps

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

Coriolis Effect

Coriolis Effect

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.

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.

Pressures All Together

General Planetary Circulation

Winds Horizontal movements at surface Names from WHERE it came from…not where it is going!!!

Around Pressure Cells

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

X-section of Planetary Circulation

January Global Pressure Map

July Global Pressure Map

Summer Highs

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?

Work Cited (Incomplete) http://www.usatoday.com/weather/tg/wamsorce/wamsorce.htm seen 1/03/06 http://www.srh.weather.gov/srh/jetstream/synoptic/airmass.htm