IX. Air Pressure

A. Measurements - pressure exerted by weight above

1. station measurement
1. barometer
2. units: millibars and inches of mercury
3. 1013.25mb = 29.92 in. Hg
4. elevation effect - sea level equivalent
2. constant height maps
1. corrected to sea level (or other constant height)
2. isobars - contour lines connecting points of equal pressure
3. pressure gradient
• rate of change of pressure
• spacing of contour lines (pressure difference and distance)
3. variables
1. gas law pressure = temperature x density x constant

B. Forces

1. pressure gradient - causes wind to blow
1. blows from high pressure to low pressure (perpendicular to isobars)
2. steep gradient - strong wind
3. gentle gradient - weak breeze
2. coriolis effect - only affects direction, not speed (fig. 6.11 & 6.12)
1. deflects wind to the right in N. hemisphere, left in S. hemisphere
2. amount of deflection increases with increased velocity
3. amount of deflection increases toward the poles
3. surface friction
1. slows wind and decreases Coriolis effect
2. inward directed

C. Wind patterns above 1000m - winds named for direction from which they blow

1. Geostrophic winds (fig 6-13)
1. balance pressure gradient forces and coriolis forces
2. high elevation winds blow in a straight path parallel to isobars
2. Cyclones (fig. 6.15)
1. Low pressure in center
2. ccw in N; cw in S
3. Anti-cyclones
1. High pressure in center
2. cw in N; ccw in S

D. Surface wind patterns (fig. 6.16)

1. frictional force is added
2. friction slows wind so reduces Coriolis force
3. resulting wind blow across isobars toward low pressure (about 30º angle)
4. air into Low pressure forces uplift and then divergence aloft, while air out of High pressure caused downdraft and convergence aloft.