Pressure and wind Ch. 4
Air Pressure • If you take a package of air and squeeze or compress it into a smaller area, that air will become higher in pressure Air will push against sides of package
Barometers • Aneroid • Mercury
Air Pressure • Measured in • millibars (mb) • Force per sq. meter • kilopascals (kPa) 1kPa=10mb • Normal sea-level pressure is 1013.2mb, 29.92 inches/Hg, 14.7 psi
‘Sea level pressure’ vs. ‘Station pressure’ Kent: mean SLP=1013mb mean station P=974mb Pressure corrections for altitude
Wind • Wind is the general horizontal motion of air from one location to another based on differences in air pressure • Air speed is measured by an anemometer
Moving the atmosphere • Primary forces: • Pressure gradient force (PGF) • Gravity (g) • Coriolis • Friction
Gets things started PGF = DP/Dx Pressure gradient force
Gravity (g) • Why doesn’t the atmosphere collect into a small layer on the ground? • Hydrostatic equilibrium: DP / Dz = -rg • What does this say about how pressure changes vertically?
The Coriolis Effect • When wind moves from zones of high pressure to zones of low pressure it veers due to earth rotation • To the right in the northern hemisphere • To the left in the southern hemisphere • http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm?chapter_no=visualization
Remembering the direction of bend in the coriolis effect Bends to the right Greatest at poles Least at equator Increases with increasing speed Bends to the left
Friction • Slows down wind speed • In what part of the atmosphere is this most significant?
Balance of forces in upper atmosphere Pressure Gradient + Coriolis = GEOSTROPHIC FLOW
Balance of forces near surface • Pressure Gradient + Coriolis + Friction= • SURFACE FLOW
Upper air vs. surface H L Pressure gradient