Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure...

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Transcript of Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure...

  • Chapter 6: Air Pressure and WindsAtmospheric pressureMeasuring air pressureSurface and upper-air chartsWhy the wind blowsSurface windsMeasuring and determining winds

  • Atmospheric Pressureair pressure at a given level is the weight of the air above air pressure and temperature P = RT at constant P, cold parcel is denser; at constant T, higher P means denser air; at constant density, higher P means higher air T

    Q1: Because P = RT, higher T always leads to higher P a) true, b) false

    Q2: When we say warmer air parcel is less dense and hence would rise, the implicit assumption isa) Parcel pressure is the same as the environment;b) Parcel pressure is higher; c) parcel pressure is lower

  • Fig. 6-2, p. 143Stepped ArtSame density

  • Q3: Which statement is correct? a) Warm air leads to high pressure in the atmosphere; b) Cold air lead to high pressure in the atmosphere

    Q4: Which statement is correct? a) It takes a shorter column of colder air to exert the same surface pressure b) It takes a taller column of colder air to exert the same surface pressure

    Q5: Air flows from high pressure to low pressure at the same altitude. a) true, b) false

  • Measuring air pressuremercury barometerdigital barometer in weather observationsStandard atmospheric pressure:1013.25 mb = 1013.25 hPa = 29.92 in.Hg

  • Pressure Readingsstation pressure: surface P at specific location if mercury barometer is used, corrections of temperature, gravity, and instrument error (surface tension of mercury) are needed sea-level pressure: obtained from station P with corrections of altitude using 1 mb pressure increase for 10 m elevation decreaseIsobars constant pressure contourQ6: which statement is correct: a) 1 mb change for >10 m height change for warm air; b) 1 mb change for
  • Surface and Upper Air ChartsSurface map: isobars, high (H), low (L), cross-isobar flow500 mb map: height contour lines, ridges, troughs, flow parallel to height contoursQ7: Why do height contours decrease in value from south to north? a) because temperature is higher in the south; b) because pressure is higher in the south

  • Figure 2, p. 150

  • Q8: Assuming pressure at point A is higher than that at B at the same height (e.g., around 5500 m), a) 500 mb height at A is greater than that at B; b) 500 mb height at A is less than that at B; c) 500 mb height at A is the same as that at B

    Q9: Assuming 500 mb height at A is greater than that at B, pressure at the same height (e.g., around 5500 m) would be a) higher at A; b) higher at B; c) equal at A and B

    Q10: Assuming pressure at point A is higher than that at B at the same height (e.g., around 5500 m), air temperature is a) higher at A; b) higher at B; c) equal at A and B

  • Why the Wind BlowsNewtons first law of motion An object at rest (or in motion) will remain at rest (or in motion) as long as no force is exerted on the objectNewtons second law of motion F = ma (force = mass times the acceleration) acceleration could be change of speed or direction

    Four forces include pressure gradient force, Coriolis force, centripetal force (or its opposite, centrifugal force), and friction Q11: if F = 0, does the object still move? a) yes, if it was moving; b) no, if it was at rest; c) both a) and b)

  • Forces that Influence the Windnet force and fluid movementWind is the result of a balance of several forces.

  • Pressure Gradient Forcepressure gradient (pressure difference/distance)pressure gradient force (PGF) (from high to low pressure)strength and direction of the pressure gradient forceThe horizontal (rather than the vertical) pressure gradient force is responsible for air movement. Q12: how to increase PGF? a) increasing pressure difference; b) decreasing distance between isobars; c) both a) and b)

  • AQ13: What is the wind speed at point A? a) 40 knots; b) 40 miles/hour; c) 40 km/hourSurface map

  • Coriolis Forcereal and apparent forcesCoriolis force is an apparent force due to earths rotationIts strength increases with the objects speed, earth rotation, and latitude Its direction: perpendicular to wind, to the right-hand side over Northern Hemisphere (NH), and to the left over SH

  • Q14: The claim that water swirls down a bathtub drain in opposite directions in the northern and southern hemispheres a) is true; b) is false

    Q15: The Coriolis effect is stronger if a) wind speed is faster; b) latitude is higher; c) both a) and b)

  • Straight-line Flow Aloftbalance of the pressure gradient and Coriolis forcesgeostrophic wind: parallel to isobars with low pressure to its left (or right) in NH (or SH)good approximation for flow aloftGeostrophic winds can be observed by watching the movement of clouds.

  • Curved Winds Around Lows and Highs Aloftcyclonic flow (with low P center) and anticyclonic flow (with high P center): direction opposite in NH versus SHclockwise and anticlockwise: same direction in NH and SHcentripetal force (opposite to centrifugal force) gradient wind: balance of PGF, Coriolis and centrifugal forcesPGF = Co + Cen Co = PGF + Cen

  • Q16: what is the direction of PGF? a) from high P to low P; b) from low P to high P; c) depending on NH or SH

    Q17: what is the direction of Coriolis force? a) to the right of movement in NH; b) to the left of movement in NH; c) to the right of movement in SH

    Q18: what is the direction of centrifugal force? a) always outward; b) always inward; c) depending on NH or SH

    Q19: what is the balance of PGF, Co, and Cen for SH cyclonic flow? a) PGF = Co + Cen; b) Co = PGF + Cen

  • Winds on Upper-level Chartsmeridional and zonal windswind is nearly parallel to the height contourhigher air T yields greater height contour valueHeight contours on upper-level charts are interpreted in the same way as isobars on surface charts.

  • Figure 4, p. 157West wind over midlatitudes in NH and SH

  • Surface Windsplanetary boundary layer: bottom 1 km above surfaceFriction: opposite to wind in direction; increases with windfrictional effects on the wind: slow down windWind rotates clockwise from near surface to free atmosphere in the NH

  • Fig. 6-21, p. 160 Wind always moves cross isobars toward the low pressure center in both NH and SH; it moves outward for the high pressure center. Wind rotates anticlockwise from near surface to free atmosphere in the SH

  • Q20: draw the three force (PGF, Co, Cen) balance and wind direction for a NH low pressure center.

    Q21: draw the three force (PGF, Co, Cen) balance and wind direction for a SH low pressure center.

    Q22: if surface wind is southwest in Tucson, the wind at 3000 m would be a) southerly; b) westerly; c) southwesterly; d) northeasterly

  • Winds and Vertical Motionsdivergence and convergencehydrostatic equilibrium (vertical PGF = gravity)Q23: Vertical PGF is much larger than horizontal PGF. a) true; b) false

  • Measuring and Determining Windswind direction: the direction where wind comes fromprevailing wind: wind direction that occurs most frequentlywind roseQ24: If the wind is southwesterly, the wind direction is a) 45o; b) 135o; c) 225o; d) 315o

  • Wind Instrumentswind vanecup anemometeraerovanerawinsondewind profilerBy observing flags and smoke plumes, our eyes are also effective wind instruments.

  • Fig. 6-29, p. 163WindPowerQ25: at 14:00 local time, the surface wind isa) westerly; b) southerly; c) southwesterly;d) northeasterly

    Figure 6.2: (a) Two air columns, each with identical mass, have the same surface air pressure. (b) Because it takes a shorter column of cold air to exert the same surface pressure as a taller column of warm air, as column 1 cools, it must shrink, and as column 2 warms, it must expand. (c) Because at the same level in the atmosphere there is more air above the H in the warm column than above the L in the cold column, warm air aloft is associated with high pressure and cold air aloft with low pressure. The pressure differences aloft create a force that causes the air to move from a region of higher pressure toward a region of lower pressure. The removal of air from column 2 causes its surface pressure to drop, whereas the addition of air into column 1 causes its surface pressure to rise. (The difference in height between the two columns is greatly exaggerated.) Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login.Figure 2: The area shaded gray in the diagram represents a surface of constant pressure. Because of the changes in air density, a surface of constant pressure rises in warm, less-dense air and lowers in cold, more-dense air. These changes in elevation of a constant pressure (500-mb) surface show up as contour lines on a constant pressure (isobaric) 500-mb map.Figure 6.14: On nonrotating platform A, the thrown ball moves in a straight line. On platform B, which rotates counterclockwise, the ball continues to move in a straight line. However, platform B is rotating while the ball is in flight; thus, to anyone on platform B, the ball appears to deflect to the right of its intended path.Figure 4: Upper-level chart that extends over the Northern and Southern hemispheres. Solid gray lines on the chart are isobars.Figure 6.21: (a) Surface weather map showing isobars and winds on a day in December in South America. (b) The boxed area shows the id