GEO 203 Notes – 11/2/10 (Lecture #12 Review and #13…again)Pressure commonly measured in millibars (mb)Metric unit: hectoPascal (1 hP = 1 mb)Pressure always decreases with height (exponentially).In the lower atmosphere, pressure decreases by about 10 mb per every 100 m.Stations across US measure pressureWithout adjusting for height, pressure map would look identical to topographical mapWind blows due to differences in pressureLecture #13Four types of force that influence winds:Pressure gradient force: difference in pressureChange in pressure / unit distanceDenser isobars = larger PGFAlways points directly from high to low pressureOnly force that drives the windCoriolis force: apparent force due to rotation of the earthProportional to speed and latitudeHigher latitude = higher Coriolis forceFaster air = larger Coriolis forceDoes not affect wind speed—just direction——Geostrophic wind: balance between PGF and CoriolisGood approximation for winds above Earth’s surfaceBlows in straight line parallel to isobarsHowever, around the lows and highs, wind follows a curved pathGradient winds: wind that blows at a constant speed but parallel to curved isobarsThree-way balance between PGF, geostrophic, and centripetal windsBuys Ballot’s Law: With back toward wind, low pressure is on left; high on rightLow pressure center: wind blows counterclockwise (Northern Hemisphere)High pressure center: wind blows clockwise (Northern Hemisphere)Centripetal force: imbalance between pressure gradient force and Coriolis forceTwo attributes: velocity and directionAccelerated motion Directly proportional to wind speedFrictional force: force due to drag of Earth’s surfaceTwo attributes: magnitude and directionMagnitude depends upon speed of air parcel and roughness of surfaceDirection always acts in the direction opposite to the movement of the air parcelImportant in the friction layer (lowest ~1000m in atmosphere)Reduces wind speed (causes Coriolis force to be smaller)Force causes about 30 degree discrepancy to L/H pressure centers when standing with back to windForces that affect wind:PGF—only force that drives wind (other forces react)High to lowCoriolisTo right of motionCentrifugalProportional to wind speedFictionalIn opposite direction of windIf no wind, only PGF is presentWinds converging into a low pressure center generate upward winds that remove the accumulating air
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