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METR 104 Background for Lab #7. Remember isotherms (contours of temperature)? They are lines on a temperature map that connect places with the same temperature. In Lab #6, if your contour analysis was correct, you could have colored in areas between contours in your map.

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slide2
Remember isotherms(contours of temperature)? They are lines on a temperature map that connect placeswith the sametemperature
  • In Lab #6, if your contour analysis was correct, you could have colored in areas between contours in your map
1 what is wind what creates wind
(1) What is wind? What creates wind?
  • Wind is air in motion
  • What causes air to move? a net forcemust push or pull on it.
  • A force is any push or pullthat can change the motion of an object
  • So, for wind to blow, some force must push air into motion.
  • So, what forces create wind?
2 to understand wind we need to understand pressure
(2) To understand wind we need to understand pressure
  • What is air pressure?
  • Consider a “parcel” (small blob) of air:

Pressure is the collective force exerted by a parcel's molecules in random motion, colliding with the parcel's surroundings (or vice-versa)

  • Visualizing pressure:The Ideal Gas Law
3 units of pressure
(3) Units of pressure
  • Millibars(mb)

• Inches of mercury (“in Hg”) (U.S. only)

4 what values of pressure do we typically observe
(4) What values of pressure do we typically observe?

• What is the global, long-term average pressure at sea level?

1013.25 mb(29.92 in Hg)

• How much does pressure change over distance?

change in atmospheric pressure over distance pressure gradient
Change in Atmospheric Pressureover Distance (Pressure Gradient)

Vertical pressure gradient: Very large!

  • Tropopausepressure is ~20% of sea level pressure.
  • At 00Z November 14, 2012:
    • Sea-level pressure at Oakland: 1020 mb
    • At tropopause (~11 km above Oakland): 200 mb
    • Vertical pressure gradient between them

= (1020 mb – 200 mb)/11 km

=74 mb per km vertically

change in atmospheric pressure over distance pressure gradient1
Change in Atmospheric Pressureover Distance (Pressure Gradient)

Horizontal pressure gradient: Much smaller!

  • Typical sea-level pressure at center of midlatitude cyclone (relatively low): ~ 990 mb
  • Typical sea-level pressure at center of high pressure area 1000 km away: ~ 1030 mb
  • Average pressure gradient between them:

(1030 mb – 990 mb)/1000 km = 40 mb/1000 km

= 0.04 mb/km

5 what does atmospheric pressure have to do with wind
(5) What does atmospheric pressure have to do with wind?
  • Wind is the horizontal movement of air
  • The net force exerted on air parcels due to pressure differences between places at the same level can push air into motion.
  • The net force is in the direction from higher toward lower pressure.
6 visualizing pressure patterns
(6) Visualizing pressure patterns
  • Hard to to with pressure values plotted on a map
  • See example
isobars lines that connect all points with the same pressure
Isobars:Lines that connect all points with the samepressure
  • They work just like isotherms (lines that connect points with the same temperature)
  • In the US, sea level pressure isobars are drawn every 4 mb
  • On one side of the isobar the pressures are higher (than they are on the contour itself)
  • On the other side of the isobar the pressures are lower (than on the contour itself)
6 how strong is the force on air due to horizontal pressure differences
(6) How strong is the force on air due to horizontal pressure differences?
  • It depends on the horizontal pressure gradient(a measure of how rapidly the pressure varies from place to place horizontally).
  • Large pressure gradients are found where isobars are close together
the pressure gradient force
The Pressure Gradient Force
  • The pressure-gradient force (PGF) pushes on an air parcel from the higher pressure side of the parcel toward the lower pressure side
    • It pushes in a direction perpendicular to the isobar through that spot (toward lower pressure)
  • The larger the pressure gradient is, the greater the PGF will be.
7 visualizing connection between pressure patterns and winds on weather maps
(7) Visualizing connection between pressure patterns and winds on weather maps
  • To understand wind patterns, we must see patterns of pressure-gradient force (PGF).
  • To see patterns of PGF we must see patterns of pressure.
slide22
In what direction is the Pressure Gradient Force (PGF) pushing on air at different locations on this map?