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Pressure, Forces and Motion Readings A&B: Ch. 4 (p. 93-114) CD Tutorials: Pressure Gradients, Coriolis, Forces & Winds Topics 1. Review: What is Pressure? 2. Horizontal Pressure Gradients 3. Depicting Pressure Gradients a. Constant Height Charts b. Constant Pressure Charts 4. Newton’s 2nd Law 5. Forces & Winds a. Pressure Gradient Force b. Coriolis Force → Geostrophic Wind → Gradient Wind c. Friction Force 6. Anticyclones & Cyclones G109: Weather and Climate Review: What is Pressure? • Pressure = Force / Area • Atmospheric Pressure: weight of air above an area • “Weight above” decreases steadily with height ∴Pressure decreases steadily with height • Can be used to “measure” height in the atmosphere G109: Weather and Climate 10: Pressure & Motion Review: The Ideal Gas Law The Ideal Gas Law: P: ρ: T: Rd: • P = ρ T Rd pressure density abs. temperature spec. gas constant for dry air [Pa = kg m-1 s-2] [kg m-3] [K] 287 [J kg-1 K-1] If Pressure (P) is constant Increased T → Decreased Density → Increased Volume Decreased T → Increased Density → Decreased Volume G109: Weather and Climate 10: Pressure & Motion Horizontal Pressure Gradients • When surface under one air column is heated the air column expands, following: P = ρ Rd T • Example: Two columns of air with equal ρ, P, and T G109: Weather and Climate 10: Pressure & Motion Horizontal Pressure Gradients • After heating and expansion of right-hand column: • Height at which 500 mb pressure is reached is now • At 5640 m the pressure in the warm column is now • OR: The height of 500 mb level column G109: Weather and Climate cool 10: Pressure & Motion Horizontal Pressure Gradients • Gradual poleward decrease in mean temperature Denser air at higher latitude More rapid decrease of pressure with height Horizontal changes in pressure G109: Weather and Climate 10: Pressure & Motion Horizontal Pressure Gradients • As a result of horizontal temperature differences: A given pressure (e.g. 500 mb) occurs at different At a given height (e.g. 5640 m) in the atmosphere, • (If no air is flowing away or into the air column, the surface pressure does not vary horizontally … yet) G109: Weather and Climate 10: Pressure & Motion Horizontal Pressure Gradients • With a horizontal pressure gradient aloft (created by T differences), air can start to flow from to →The weight of the entire air column (surface pressure) changes as well, because of the air flow →There are now at the surface and at height • These give rise to → Pressure differences cause → Pressure differences occur because of G109: Weather and Climate 10: Pressure & Motion Depicting Horizontal Pressure Variation • Pressure varies Spatial & temporal patterns Need a way to depict variations • Two types of charts or maps At sea-level, show horizontal variation of pressure as isobars = pressure variation at For a given level of pressure (e.g. 500 mb), show at what height this occurs, as contours of geopotential = height variation of G109: Weather and Climate 10: Pressure & Motion Depicting Horizontal Pressure Variation 1. Shows variations of pressure at the surface Altitude corrections: all pressures are corrected to same level (usually sea level) • Prevents mountainous areas from appearing with lower pressure because of height G109: Weather and Climate 10: Pressure & Motion Depicting Horizontal Pressure Variation 2. Shows variation of height along an Isobaric Surface, i.e., the height at which a certain pressure occurs • Isobaric Surface: a surface of constant pressure G109: Weather and Climate 10: Pressure & Motion Newton’s 2nd Law of Motion • Acceleration of an air parcel is equal to the Acceleration: change of velocity over (unit) time • i.e., a change in and/or Net force: vector sum of all component forces G109: Weather and Climate 10: Pressure & Motion Newton’s 2nd Law of Motion • Forces in the atmosphere (the most important ones): Pressure Gradient Force: FPG • Driving force – affects speed and direction • Accelerates air from High to Low pressure Coriolis Force: FC • Deflecting force – affects direction only Friction Force: Ff • Retarding force – affects speed only • Friction Increases with increasing wind speed G109: Weather and Climate 10: Pressure & Motion Pressure Gradient Force (FPG) • Force resulting from the horizontal difference in pressure Proportional to i.e., depends on FPG H 1008 • 1004 L 1000 (mb) FPG goes from At right angles to the isobars G109: Weather and Climate 10: Pressure & Motion Pressure Gradient Force (FPG) • The closer the isobars, the stronger the pressure gradient and thus the FPG FPG = H ∆P change in PRESSURE = d DISTANCE FPG L FPG H 1008 1004 1000 1008 1004 G109: Weather and Climate L 1000 10: Pressure & Motion Pressure Gradient Force (FPG) • • • Can set a stationary air parcel in motion Mostly responsible for the If FPG were the only force, winds Air would move at • But the earth rotates → G109: Weather and Climate 10: Pressure & Motion Coriolis Effect • Coriolis effect: • An unaccelerating object moves in a straight line (due to balanced forces) Because of Earth’s rotation, a “straight line motion” on Earth as viewed from (e.g.) another planet, leaves a curved trace on Earth: the motion appears to be accelerated (curved!) • G109: Weather and Climate 10: Pressure & Motion Coriolis Force (FC) • • • Apparent acceleration accounted for by apparent force: Coriolis force FC Northern Hemisphere: (counter-clockwise rotation) Southern Hemisphere: (clockwise rotation) G109: Weather and Climate 10: Pressure & Motion Coriolis Force (FC) • FC (on Earth) dependent on: • Latitude Strongest twisting motion at poles. No twisting motion at the equator. FC increases • Velocity: proportional to velocity. FC increases with … the greater the distance traveled per unit time the greater the deflection G109: Weather and Climate 10: Pressure & Motion Coriolis Force (FC) • • Always at right angles to the direction of air flow Affects only the not • Affected by • Stronger the wind speed the greater the force Strongest at the and weakest at the FC = 2 ν Ω sin φ ν - wind speed Ω - earth's angular rate of spin (constant ) φ - latitude G109: Weather and Climate 10: Pressure & Motion Geostrophic Wind • Wind aloft (above a few km) above the effects of friction • Assume evenly spaced, and straight isobars → Idealized model (an approximation of winds aloft) • balances the and directs the airflow • FPG = FC • Net force = 0 G109: Weather and Climate 10: Pressure & Motion Geostrophic Wind • • FPG = FC Wind flows in a • Proportional to the pressure gradient force steep gradient strong winds; weak gradient light winds • With back to wind, Low is (in northern hemisphere) G109: Weather and Climate 10: Pressure & Motion Gradient Wind → A second idealized model of winds aloft • Above the level of frictional influence wind, with isobars that are curved • Wind blows at constant but with constantly changing … therefore has acceleration G109: Weather and Climate 10: Pressure & Motion Gradient Wind • • Above the level of frictional influence wind, when isobars are NOT parallel Wind blows G109: Weather and Climate 10: Pressure & Motion Troughs and Ridges • • • In upper atmosphere pressure height variations are distributed as a series of ridges and troughs: Ridge: elongated zone of pressure, extending toward the pole: associated with weather Trough: elongated zone of pressure, extending toward the equator: associated with weather Greatest surface instability (T-storms) is usually ahead of (to the right of) the 500 mb trough G109: Weather and Climate 10: Pressure & Motion Friction Force (Ff) • • As we move toward the surface (away from aloft) friction slows down the movement of air Roughness of the surface retards the airflow Wind speed reduced • Impacts Coriolis force (FC) – • Pressure gradient force (FPG) is not affected → • • Wind crosses isobars Angle depends on friction Smooth ocean: • Slight angle: 10-20o Rough terrain: • Greatest angle: 45o G109: Weather and Climate . 10: Pressure & Motion Anticyclones and Cyclones • • • • • Anticyclone: enclosed area of with circular isobars or height contours Northern Hemisphere Winds rotate as FPG is outward and FC deflects to right NH surface Southern Hemisphere Winds rotate as FPG is outward and FC deflects to left SH surface NH upper atm SH upper atm Near surface: wind is not parallel to isobars but spirals → Upper atmosphere: flow is parallel to isobars G109: Weather and Climate 10: Pressure & Motion Anticyclones and Cyclones • • • • • Cyclone: enclosed area of Low pressure, with circular isobars or height contours Northern Hemisphere Winds rotate as FPG is inward and FC deflects to right Southern Hemisphere Winds rotate as FPG is inward and FC deflects to left NH surface SH surface NH upper atm SH upper atm Near surface: wind is not parallel to isobars but → spirals Upper atmosphere: flow is parallel to isobars G109: Weather and Climate 10: Pressure & Motion