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 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 (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

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 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