Air Pressure, Forces and Winds

Geography 103 - Weather Summary notes. Air pressure, forces and winds (Chpts 5 and 8 of text) Pressure pressure is exerted by weight of air above – on a global scale atmosphere is in hydrostatic balance, i.e. pressure at every altitude is exactly that needed to balance the weight of air above (so that air doesn’t fall to ground or take off into space) it takes a shorter column of cold, more dense air to exert the same surface pressure as a tall column of warm, less dense air atmospheric pressure decreases more rapidly with elevation in a cold column of air warm air aloft is associated with high pressure aloft and cold air aloft is associated with cold pressure aloft difference in temperature of air aloft creates a horizontal pressure gradient force which pushes air from warm to cold column. This adds air to cold column, increases surface pressure and generates a pressure difference at the surface between the cold column and warm column of air. At the surface the air is pushed in the opposite direction. (Highs aloft correspond to Lows at surface and vice-versa.) Thermal circulation cell is set up which is responsible for winds on 3 scales – local (e.g. land-sea breeze), synoptic/regional (e.g. monsoons), global (circulation of air between equator and poles). air sinks above surface high – clear weather air rises above surface low – cloudy weather pressure charts are used for sea-level (i.e. 0 m), all surface data is converted to 0 m altitude. lines of constant pressure are isobars (know typical surface pressures and their range) at higher altitudes charts show height variations on surfaces of constant pressure (isobaric charts) low heights on isobaric charts are equivalent to low pressures on constant height charts and are associated with cold air aloft high heights on isobaric charts are equivalent to high pressures on constant height charts and are associated with warm air aloft ‘H’ = center of high pressure = anticyclone ‘L’ = center of low pressure = cyclone elongated highs = ridges elongated lows = troughs Winds wind is caused by horizontal differences in pressure – which are caused by unequal heating if earth did not rotate and there was no friction, air would go directly from high -> low pressure pressure gradient force: spacing of isobars shows pressure gradient = difference in pressure/distance. The closer the isobars the greater the pressure gradient, and the greater the pressure gradient force, and the stronger the wind. pressure gradient force is perpendicular to isobars – is in a direction from high to low pressure. Coriolis force: apparent force due to rotation of earth (if we were fixed out in space we would not see this force. It is ‘apparent’ because it exists only because we are on a rotating earth and so to us free-moving objects are deflected from straight-line travel.) in NH objects (winds) are deflected to the right in SH objects (winds) are deflected to the left CF=2uΩsinϕ (u=wind speed, Ω=rotation speed of earth, ϕ=latitude) CF – is at right angles to direction of movement affects only direction, not speed of an object the stronger the wind, the stronger the force strongest at poles, 0 at equator Geostrophic wind = wind generated aloft by balance between PGF and CF. flows parallel to isobars stronger for closely-spaced isobars Stand with lower pressure will be on your left and higher on your right then wind will be at your back (you will be looking in the direction the wind is going) in NH and at your face in the SH For a High, flow is clockwise in NH, anticlockwise in SH Surface wind = wind at/close to earth’s surface where friction is important friction caused by trees, buildings, topography PGF now balanced by CF and friction PGF directed from H to L CF at right angles to wind friction directly opposes wind causes wind to blow at an angle to isobars (in towards L, out away from H) – angle depends on roughness of terrain, 10° over smooth water, 35° near hills because of friction, surface winds blow slower than geostrophic winds aloft Vertical air motion surface lows (cyclones) cause air to blow inwards = convergence -> air must rise -> divergence aloft. Therefore surface lows are associated with rising air, clouds, precipitation. surface highs (anticyclones) cause air to blow outwards = divergence at surface -> convergence aloft -> subsidence of air. Therefore surface highs are associated with sinking (and warming) air and fair weather. Santa Ana conditions characterized by high winds, and very dry conditions. Also typically warm air. Clear skies. initiated by high pressure center over Great Basin (usually close to Salt Lake City) – forces air downslope from elevated desert plateau. If pressure is very high, winds will be strong (high pressure gradient). Winds are from NE due to flow around anticyclone (see NH wind direction around surface H).