9. Air Pressure and winds

9. Air Pressure and winds 9.1 Atmospheric pressure 9.2 Surface and upper-level charts 9.3 Newton’s laws of motion 9.4 Forces that influence the winds 9.5 Winds and vertical air motions

9.1 Atmospheric pressure Horizontal pressure variations- A tale of two cities Warm, less-dense air, cold, more dense air

A complete circulation of air It is the horizontal differences in air pressure that cause the wind to blow

Pressure measurements Barometers: instrument that detect and measure pressure changes 1bar=1000mb 1hPa=1mb

Pressure reading Stations pressure: the barometer reading at a particular locations and elevation Sea-level pressure: pressure adjusted to a level of mean sea level Isobar: line connecting points of equal pressure drawn at interval of 4 mb on surface map

9.2 Surface and upper-level charts Sea-level pressure chart: constant height chart Upper-level charts: constant pressure chart or iosbaric chart

High heights on an isobaric chart correspond to higher-than-normal Pressures at any given altitude, and low height on an isobaric chart Correspond to lower-than-normal pressures

9.3 Newton’s law of motion ---Fundamental force (기본힘) • Pressure gradient force (기압경도력) • Gravitational force (만유인력) • Viscous force (점성력) ----apparent force (겉보기 힘) • Coriolis force (코리올리힘) • Centripetal acceleration and centrifugal force (구심가속도와 원심력)

9.4 Forces that influence the winds Pressure gradient force

Gravitational force m r M

Viscous force • Any real fluid is subject to internal friction , which causes it to resist the tendency to flow. shearing stress + z=l u(l)=u0 u(0)=0 z=0

Coriolis force The coriolis force causes the wind to deflect to the right of its intended path in the Northern Hemisphere and to the left of its intended path in S. H. Coriolis force depends on 1) the rotation of the earth, 2) the latitude 3) The object’s speed

Centripetal acceleration and centrifugal force

Geopotential(지오퍼텐셜):the work required to raise a unit mass to height z from mean sealevel

Natural coordinates

Straight-line flow aloft- geostrophic winds (지균풍) PGF=CF The geostrophic wind blows parallel to the straight isobar in the N. H. with lower pressure to its left.

Cyclostrophic flow(선형류) Ex) Tornadoes

Curved winds around lows and highs aloft - gradient winds Gradient wind (경도풍) The wind blows at a constant speed parallel to curved isobar Difference between CF and PGF = centripetal force

In a high The pressure field near the center of a high is always flat and the wind gentle compared to the region near the center of a low

정상저기압 정상고기압

Winds on upper-level charts

Surface winds Planetary boundary layer: the atmospheric layer that is influenced directly by surface forcing such as friction, heating or cooling etc. Friction reduces wind speed-> reduce the Coriolis force-> Wind blow across the isobars toward lower pressure PGF=CF+frictional force

If you stand with your back to the wind, then turn clockwise about 30°, The center of lowest pressure will be to your left. : Buys-Ballot’s law

9.5 Winds and vertical air motions Continuity equation (연속방정식) : mass conservations In the planetary atmosphere Boussinesq approximation

Convergence over surface low -> upward motion Divergence over surface high -> downward motion

9. Air Pressure and winds