Understanding Atmospheric Pressure Patterns and their Impact on Winds

1. Chapter 8 Air pressure and winds

2. Atmospheric Pressure • What causes air pressure to change in the horizontal? • Why does the air pressure change at the surface?

3. Atmospheric Pressure • Horizontal Pressure Variations • It takes a shorter column of dense, cold air to exert the same pressure as a taller column of less dense, warm air • A Warm air column is normally associated with a low surface pressure • A Cold air column is normally associated with a High surface pressure • At surface, horizontal difference in temperature = horizontal pressure in pressure = wind

6. Atmospheric Pressure • Special Topic: Gas Law P is proportional to T x ρ P = pressure T = temperature ρ = density http://www.youtube.com/watch?v=17wGIdMvJfM (youtube video of gas law)

7. Atmospheric Pressure • Daily Pressure Variations • Thermal tides in the tropics • Mid-latitude pressure variation driven by transitory pressure cells • Pressure Measurements • Barometer, barometric pressure • Standard atmospheric pressure 1013.25mb • Aneroid barometers • Altimeter, barograph

13. Atmospheric Pressure • Pressure Readings • Instrument error: temperature, surface tension • Altitude corrections: high altitude add pressure, 10mb/100m above sea level

15. To get sea level pressure Add 10 mb * (elevation/ 100 m) to the station pressure measurement. Example: A station at 700 meters elevation records a pressure of 940 mb. The sea level pressure for this station would be 940 mb + 70 mb= 1010 mb

16. Surface and Upper Level Charts • Sea-level pressure chart: constant height • Upper level or isobaric chart: constant pressure surface (i.e. 500mb) • High heights correspond to higher than normal pressures at a given latitude and vice versa