Air pressure and 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 • Warm air aloft is normally associated with high atmospheric pressure and cold air aloft with low atmospheric 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

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. 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

23. Surface and Upper Level Charts • Observation: Constant Pressure Surface • Pressure altimeter in an airplane causes path along constant pressure not elevation • May cause sudden drop in elevation • Radio altimeter offers constant elevation

24. Newton’s Law of Motion • AN object at rest will remain at rest and an object in motion will remain in motion as long as no force is executed on the object. • The force exerted on an object equals its mass times the acceleration produced. • Acceleration: speeding up, slowing down, change of direction of an object.

25. Forces that Influence Winds • Pressure Gradient Force: difference in pressure over distance • Directed perpendicular to isobars from high to low. • Large change in pressure over s short distance is a strong pressure gradient and vice versa. • The force that causes the wind to blow.

30. Forces that Influence Winds • Coriolis Force • Apparent deflection due to rotation of the Earth (the rotation rate of Venus is so slow that the Coriolis force is extremely small on Venus) • Right in northern hemisphere and left in southern hemisphere • Stronger wind = greater deflection • No Coriolis effect at the equator greatest at poles. • Only influence direction, not speed • Only has significant impact over long distances

34. Forces that Influence Winds • Geostrophic Winds • Earth turning winds • Travel parallel to isobars • Spacing of isobars indicates speed; close = fast, spread out = slow • Topic: Math & Geostrophic Winds Vg = 1 x Δp fρ d

38. Forces that Influence Winds • Gradient Winds Aloft • Cyclonic: counterclockwise • Anticyclonic: clockwise • Gradient wind parallel to curved isobars • Cyclostrophic near Equator • Observation: Estimates Aloft • Clouds indicate direction of winds, place pressure in location consistent with cloud location.

43. Stepped Art Fig. 8-29, p. 214

44. Forces that Influence Winds • Winds on Upper-level Charts • Winds parallel to contour lines and flow west to east • Heights decrease from north to south • Surface Winds • Friction reduces the wind speed which in turn decrease the Coriolis effect. • Winds cross the isobars at about 30° into low pressure and out of high pressure • Buys-Ballots Law

48. Winds and Vertical Motion • Replacement of lateral spreading of air results in the rise of air over a low pressure and subsidence over high pressure • Hydrostatic equilibrium and equation • Topic: Hydrostatic equation Δp = -ρg Δz