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Metr 104 Lab

Metr 104 Lab. November 16 , 2012. Remember isotherms?  Lines that joint points with the same temperature. In last week’s lab, if your contour analysis was correct, you could have colored areas in your map. Isotherm map – 00Z 14 Nov 2012.

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Metr 104 Lab

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  1. Metr 104 Lab November 16, 2012

  2. Remember isotherms? Lines that joint points with the same temperature • In last week’s lab, if your contour analysis was correct, you could have colored areas 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 pressure? • Consider a parcel (blob) of air • Pressure is the collective force exerted by a parcel's molecules colliding with the parcel's surroundings • Visualizing pressure:The Ideal Gas Law

  7. (3) Units of pressure • Millibars(mb) or hectoPascals (hPa) (Two names for the same unit of pressure, used by meteorologists) • In the US they often report pressure in terms of “inches of mercury”

  8. (4) What values of pressure do we typically observe? • What is the average pressure at sea level? Average value of pressure at sea level is 1013.25 mb or 29.92 inches of mercury • How much does pressure change over distance?

  9. Change in Atmospheric Pressure over Distance Vertical change: Very large! Tropopause pressure is about 20% of sea level pressure. At 00Z November 14 2012 Pressure at Oakland at ground level (3 meters above sea level): 1019hPa Pressure at Oakland 12 km above sea level was 200 hPa = a 819 hPa difference in 12 km or More than 68 hPa/km

  10. Change in Atmospheric Pressure over Distance Horizontal change: Much smaller! At 00Z November 14 2012 Pressure at Oakland at ground level (3 meters above sea level): 1019hPa Pressure at Hilo Hawaii (11 meters above sea level): 1016 hPa Distance beween Oakland and Hilo: 3746 km or 0.0008 hPa/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 joint points with the same pressure • They work just like isotherms (lines that connect points with the same temperature) • In the US, sea level pressure isobars are drawn every 4 hPa • On one side of the isobar the pressures are higher • On the other side of the isobar the pressures are lower

  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, the difference in pressure across each unit of distance (horizontally) • It is a measure of how rapidly the pressure varies from place to place horizontally, expressed in hPa/km. • Strong pressure gradients are found where isobars are close together (similar to strong temperature gradients)

  18. The Pressure Gradient Force • The pressure-gradient force (PGF) pushes on an air parcel from the higher pressure side toward the lower pressure side. • 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 strong/weak pressure gradients on this map?

  22. Where is the Pressure Gradient Force pushing air at different locations on this map?

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