Weather, Air Masses, Fronts and Global Wind Patterns Meteorology
Weather is what conditions of the atmosphere are over a short period of time. Climate is how the atmosphere "behaves" over long periods of time. Weather forecasting is predicting what the weather will be like at a certain time in the near future, in a given location. The weather map tells the distribution patterns of atmospheric pressure, wind, temperature and humidity at the different levels of the atmosphere.
The Weather Symbols:
Air Pressure- This is the weight or pressure the air exerts on the ground and is measured in millibars.
Air Masses
Definition: Air mass - a large dome of air which has similar horizontal temperature and moisture characteristics throughout. Very similar to a balloon.
List of Air Masses: Continental Arctic Continental Polar Maritime Polar Maritime Tropical Continental Tropical
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 100 o F during summer Generally clear skies, hot, low humidity
Source Regions
Reasoning for Tornadoes Orographic Perfection Meeting of Moist - mt Hot - ct Cool cp
Tornado Alley http://www.britannica.com/thunderstorms _tornadoes/video/ocliwea124v4.mov
Fujita Scale (NationalAtlas.com)
Fronts and their symbols
Fronts: 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 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 Less Pressure Cool Air Denser Warm Air More Pressure
Pressure and Air Movement
Pressure Gradient Force Difference in pressure over a given distance---between isobars Isobars-lines connecting areas of the same air pressure Close together = STRONG winds Far apart = light winds Just like contour lines
Pressure Gradient Force
Isobars are lines on a weather map joining together places of equal atmospheric pressure. On the map the isobar marked 1004 represents an area of high pressure, while the isobar marked 976 represents an area of low pressure.
Isobaric Maps
Coriolis Effect Apparent force due to the rotation of the Earth (Think Merry-goround) 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!!! For example: While standing in the RHS parking lot, you experience a northenly wind. Is that wind coming from Knightdale (heading north to Rolesville) OR Coming from Louisburg/Bunn (heading south to Rolesville)
Around Pressure Cells
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
X-section of Planetary Circulation
January January Global Pressure Map
July July Global Pressure Map
Summer Highs