chapter_08 - Chapter 8 Chapter Air Pressure and Winds Winds...

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Unformatted text preview: Chapter 8 Chapter Air Pressure and Winds Winds Why Does the Wind Blow? Why What is wind? What Wind is horizontal Wind movement of air movement Why does air move? Air moves in response to a Air horizontal gradient in air pressure, attempting to equalize imbalances in pressure pressure The wind direction is the The direction from which the from wind is blowing wind I. Atmospheric Pressure I. Pressure is the force exerted Pressure on objects by the weight of tiny molecules of air How heavy is the pressure? How - 1 ton per square ft ton Measuring Air Pressure Measuring The instruments that measure air pressure are called barometers Mercury Barometer Aneroid Barometer Measuring Air Pressure –cont’d Measuring Sea level standard High altitude low pressure Bad weather low pressure Surface and Sea Level Pressures Surface • Why can’t we compare station pressures? • Pressure decreases with elevation by about 10 mb/100 m • We extrapolate pressure from altitude to sea level – a common level Sea Level Pressure Sea • Sample of sea level pressure map or chart The contours are isobars The numbers at stations are sea level pressure in millibars (mb). Pressure and Temperature Pressure Pressure relates to density; density relates temperature Colder air – molecules move more slowly and are closer together (more dense – column contracts) Warmer air – molecules move faster and are further apart (less dense – column expands) Pressure Differences and Wind Pressure Diurnal Pressure Changes Diurnal II. Surface and Upper Level Charts II. Sea level pressure chart Sea is a constant height chart chart Upper level charts are Upper constant pressure charts charts Temperature and Height Temperature Ridges and Troughs Ridges Aviation and Height Aviation Heights on a weather map Heights Elevation and Height Elevation Summary: Pressure Summary: Pressure is the weight of air above a given level It decreases with height rapidly; the higher elevation; the lower pressure Pressure is used to represent a vertical coordinate, instead of altitude 850mb, 700mb, 500mb, 300mb, as altitude. Pressure at different altitudes is converted to common sea level Warmer (lighter) air above results in lower surface pressure; and vice versa Higher pressure typically is associated with fair weather; the opposite is true with low pressure III. Newton’s Laws of Motion III. 1st Law – objects in motion remain in motion motion 2nd Law – the force exerted on an object equals its mass times the acceleration produced produced To determine wind direction we must To consider all the forces acting on it: consider Pressure Gradient Force Coriolis Force Centripetal Force Friction IV. Forces that Influence Wind IV. Pressure gradient – the amount of pressure change that occurs over a given distance that ∆p Example: PG = 4 mb / 100 km PG = If isobars are closer together If – steep (strong) pressure gradient gradient If isobars are further apart – If weak pressure gradient weak Pressure gradient force Pressure (PGF) is the force due to (PGF) pressure difference, pointing from higher to lower pressure from Magnitude of force proportional Magnitude to pressure gradient to d Pressure and Air Movement Pressure Pressure gradient Pressure force moves air from high to low pressure areas pressure Differential heating or cooling a column establishes horizontal pressure gradients that cause air movement, I.e., winds More Than Just Pressure Gradient Force More PGF should cause winds PGF to blow directly into the low center low Why do winds blow Why almost parallel to isobars? almost There must be other There forces that act on the air forces Coriolis Force Coriolis Watching outside Watching the disk – ball goes in straight line, But to an observer But on the spinning disc, the ball appears to veer to the right the The faster the ball The moves, the larger the deflection the Coriolis Force - cont’d Coriolis Because the Earth is rotating, air motions will Because appear to turn or deflect, similar to merry-goappear round round This deflection is an “apparent” force meaning This it would not exist if it were not for the rotation of the Earth of This apparent force is called the Coriolis force This (CF) (CF) Coriolis Force – cont’d Coriolis As an air parcel intends to move from south to north along a meridian, it actually moves towards northeast Similarly, a south-moving parcel would be deflected towards southwest In both cases, the parcel is deflected toward right in the Northern Hemisphere Factors Impacting Deflection Factors The amount of Coriolis deflection The depends on: depends The rotation of earth The latitude The object’s speed Summary: Forces Summary: Two main forces act on air aloft where the surface friction is negligible Pressure gradient force (PGF) as an actual force; Coriolis force (CF) is caused by earth rotation and air movement PGF is proportional to gradient; steep gradient produces stronger force CF depend are wind speed and direction Direction: always perpendicular to wind deflection to the right in the northern hemisphere Magnitude: increases with wind speed; If air is still, CF deflection is zero Near ground surface, friction slows down air movement and deflects toward low pressure Straight Line Flow – Geostrophic Winds Straight Air aloft is away from the influence of friction due Air to surface objects to As soon as the PGF initiates movement, the CF As deflects the parcel to the right deflects As wind speed increases, so does CF Eventually CF balances PGF: parcel no longer Eventually accelerates – then it moves at constant speed accelerates Air flowing parallel Air to straight isobars is known as the geostrophic wind geostrophic How to Tell Wind Direction? How By knowing the isobar pattern, we can determine the By direction and speed of the geostrophic wind direction PGF PGF CF CF Friction Geostrophic Winds – cont’d Geostrophic Speed of geostrophic wind is directly related to the Speed pressure gradient; greater gradient results in stronger winds, similar to water flow in a stream stronger Curved Flow – Gradient Winds Curved Real isobaric contours are seldom straight, and winds rarely brow straight trajectories • Thus, we study circular wind pattern - gradient wind Gradient Winds – Cont’d Gradient Unlike geostrophic winds blowing parallel to Unlike straight lines, gradient winds blow along straight curved isobars curved For cyclonic flow, PGF>CF For anticyclonic flow, PGF<CF Surface Friction Surface Both geostrophic and gradient winds ignore friction Both The surface friction decreases the wind speed and The increases the angle between the wind and isobars Decreasing wind speed also reduces CF; CF is Decreasing smaller than PGF. The wind would blow toward low pressure side pressure Because of friction, winds seldom blow along Because straight lines or perfect circles straight They are only good approximation to real winds They aloft away from ground surface aloft Real Wind Patterns Real Winds flow in northern hemisphere counterclockwise and inward into a low (L) counterclockwise low clockwise and outward from a high (H) clockwise high Real Wind Patterns –cont’d Real Low (L) – low center High: H – high center Trough – elongated low Ridge – elongated high Isobar Isotherm Isoheight Wind barbs Wind 1 knot=1.15 mi/hr Ridge Trough V. Winds and Vertical Motions V. As air converges to a low-pressure center, it must As go somewhere – up! go The air aloft diverges to compensate for the The converging air; opposite is true for a high-pressure center at the surface center Convergence(Confluence) vs. Divergence(Difluence) Note: confluence is not the same as convergence! Confluence refers to air flowing towards an axis parallel to the airflow; convergence is the net inflow of air into a given area. Looks more like a confluent process to me! Confluence Difluence Hydrostatic Equilibrium Hydrostatic The upward directed PGF is balanced by The the downward force of gravity the The atmosphere is normally in The hydrostatic balance except within violent storms storms Case Study: 1994 Ice Storm Black circles indicate confluence zone Case Study: 1994 Ice Storm Winds and Vertical Motions Winds Recall that the vertical motion is very important Recall for weather for Rising air cools down and expands to form Rising clouds and precipitation – and that air must continue to rise to maintain this precipitation continue The stronger the vertical motion – the more The intense the precipitation (as a general rule) intense The vertical motion is supported by pressure The gradients that in turn are related to temperature distribution distribution Next chapter will study air masses and fronts Next which characterize horizontal temperature patterns patterns Summary: Wind Patterns Summary: Two main wind patterns are discussed: geostrophic wind and gradient wind Geostrophic wind results when pressure gradient force (PGF) and and Coriolis force (CF) balance It blows along straight isobars; its direction is such that when you stand with your back against wind, the high pressure in on your right in northern hemisphere. Gradient wind describes circular flow when PGF is larger or smaller than CF The surface friction, another force, causes flow to cross isobars towards the lower pressure side Convergence generated by cross-isobars flow generates upward motion and thus weather ...
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This note was uploaded on 04/18/2011 for the course EAS 107 taught by Professor Pan,z during the Spring '08 term at Saint Louis.

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