Chapter 8 AIR PRESSURE AND WINDS Atmospheric Pressure

Chapter 8 AIR PRESSURE AND WINDS

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

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

Atmospheric Pressure • Special Topic: Gas Law P is proportional to T x ρ P = pressure T = temperature ρ = density

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. 25 mb – Aneroid barometers • Altimeter, barograph

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

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

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

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.

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.

Forces that Influence Winds • Coriolis Force – Apparent deflection due to rotation of the Earth – 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

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

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.

Stepped Art Fig. 8 -29, p. 214

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

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