METR 104 Background for Lab #7

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# METR 104 Background for Lab #7 - PowerPoint PPT Presentation

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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### METR 104Background for Lab #7

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?
• 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
• 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
• Millibars(mb)

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

(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?

• 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

• 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?
• 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
• Hard to to with pressure values plotted on a map
• See example
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?
• 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