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METR 104 Background for Lab #7

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 104 Background for Lab #7

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  1. METR 104Background for Lab #7

  2. 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

  3. Isotherm map – 00Z 14 Nov 2012

  4. Where are the strong (or weak) temperature gradients?

  5. (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?

  6. (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

  7. (3) Units of pressure • Millibars(mb) • Inches of mercury (“in Hg”) (U.S. only)

  8. (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?

  9. 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

  10. 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

  11. (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.

  12. (6) Visualizing pressure patterns • Hard to to with pressure values plotted on a map • See example

  13. Sea level pressure map example

  14. 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)

  15. Fig. 6-11, p. 159

  16. Sea level pressure (isobar) map00Z 14 November 2012

  17. (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

  18. 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.

  19. (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.

  20. Fig. 6-12, p. 159

  21. Where are the large and small pressure gradients on this map?

  22. In what direction is the Pressure Gradient Force (PGF) pushing on air at different locations on this map?

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