Air Pressure and Wind

Daily Web Guide

| Syllabus | Schedule | Weather Links | Email Kerry Magruder |

Reading Assignment	Browse through the Audubon Field Guide to the Weather pp. 46, 47-53. Review notes on temperature, clouds, and air pressure to determine what you can do for weather observations for your meteorological journal.
Reminders	Finish the Web Worksheet. Complete the Model of the Troposphere homework assignment. Turn both assignments in with your first journal.

Class notes:

Earth's atmosphere composition

• What is the most abundant gas in our atmosphere?
• Nitrogen, N₂ = 78%
• Oxygen, O₂ = 21%
• Carbon dioxide, CO₂ and Water vapor, H₂O are present in trace amounts

We live beneath an ocean of air

• What happens to a deep-sea fish like the Coelecanth if brought to the surface?
• Does air have weight?
• An air column 1 sq. ft. weighs....
• Would a toilet plunger stick to the floor on the Moon?
• Jan van Helmont, Magdeburg experiment

Barometer

• Evangelista Torricelli, 1644 (Blaise Pascal)
• Weight of a column of mercury balances the weight of the column of air
• Typical air pressure: 760 mm Hg, 30 inches Hg, 1000 millibars
• When pressure falls, bad weather occurs; when it rises, weather improves.
• Balloon lid on top of jar, straw pointer

Which weighs more?

• Cold air or warm air?
  • Temperature is a measure of average kinetic energy. When KE is high, molecules are farther apart and the air pressure is lower.
• Moist air or dry air?
  • Water vapor is less dense.
• Air at New Orleans or air at Denver?
  • Decrease 1 inch of mercury every 900 feet.

Low pressure air masses

• Warm
• Cloudy
  • Air currents rise, cool as they rise, form clouds
  • High pressure areas tend to be clear; compression of air inhibits condensation at low altitudes
• Receive winds from high pressure areas
• Often produce storms
  • Air rises at center of low pressure area
  • Clouds, storms
  • Winds rush in from neighboring highs, add to updraft
  • Jet stream, if directly overhead & very fast, perpetuates low pressure

Wind direction

• Direction named for place wind comes from, not where it's blowing to
  • Northerly wind is from the north, blowing south
• Wind vane
  • Straw pointer, rotating on a pin stuck in a spool
• Wet finger
• Change of wind direction means change of weather

Wind speed

• Anemometer
  • Paper plate with paper cones on rim
• Beaufort wind scale, AFG p. 608-609.
• Windiest place on earth? Commonwealth Bay, Antarctica, regularly over 200 mph

Jet stream

• Illustration AFG, p. 46
• Fast, high-altitude, wavy, west wind
• 40 mph in summer, 80 mph in winter
• 6-8 miles high
• Boundary between northern cold air and equatorial warm air.
• Storms originate on southern curves of the jet stream and are driven eastward.

Warm front

• Boundary for warm air mass, low pressure area, advancing slowly from south.
• AFG p. 50 bottom
• Jet stream: Northward flowing ridges pull warm equatorial air masses up from tropics, creating a warm front.

1. Preceded by high-level cirrus clouds, halos.
2. Steady precipitation, clouds get lower.
3. Temperatures rise.

Cold front

• Boundary for cold air mass, high pressure area, advancing quickly from north.
• AFG p. 50 top.
• Jet stream: Southward flowing troughs pull cold polar air masses down, creating a cold front.
• Sudden thunderstorms, brief squalls, as warm air is forced upward, followed by cooler temperatures, clear skies.

Surface low pressure areas

• "Cyclones"
• Develop beneath northward-moving ridges of jet stream
• Rotation: counterclockwise (p. 46)
• Storm track: to the northeast
• Leading edge is a warm front
• Precipitation

Surface high pressure areas

• "Anti-cyclones"
• Develop beneath southward-moving troughs of jet stream
• Rotation: clockwise (p. 46)
• Track: to the southeast
• Leading edge is a cold front
• Clear skies