Download - Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

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Page 1: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Chapter 6: Air Chapter 6: Air Pressure and WindsPressure and Winds

Atmospheric pressureAtmospheric pressure Measuring air pressureMeasuring air pressure Surface and upper-air chartsSurface and upper-air charts Why the wind blowsWhy the wind blows Surface windsSurface winds Measuring and determining Measuring and determining

windswinds

Page 2: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Atmospheric PressureAtmospheric Pressure air pressure at a given level is the weight of the air above air pressure at a given level is the weight of the air above air pressure and temperatureair pressure and temperature

P = P = ρρRTRT

at constant P, cold parcel is denser;at constant P, cold parcel is denser;

at constant T, higher P means denser air;at constant T, higher P means denser air;

at constant density, higher P means higher air Tat constant density, higher P means higher air T

Q1: Because P = P = ρρRT, higher T always leads to higher PRT, higher T always leads to higher P a) true, b) falsea) 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

Page 3: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Fig. 6-2, p. 143

Stepped Art

Same density

Page 4: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

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

Page 5: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Measuring air pressureMeasuring air pressure mercury barometermercury barometer digital barometer in weather digital barometer in weather

observationsobservations

Standard atmospheric pressure:1013.25 mb = 1013.25 hPa = 29.92 in.Hg

Page 6: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Pressure ReadingsPressure Readings station pressure: surface P at specific location station pressure: surface P at specific location if mercury barometer is used, corrections ofif mercury barometer is used, corrections of temperature, gravity, and instrument error (surface temperature, gravity, and instrument error (surface

tension of mercury) are needed tension of mercury) are needed sea-level pressure: obtained from station P withsea-level pressure: obtained from station P with corrections of altitude usingcorrections of altitude using 1 mb pressure increase for 10 m elevation decrease1 mb pressure increase for 10 m elevation decrease IsobarsIsobars constant pressure contourconstant pressure contour

Q6: which statement is correct: a) 1 mb change for >10 m height change for warm air; b) 1 mb change for <10 m height change for warm air

Page 7: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.
Page 8: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Surface and Upper Air ChartsSurface and Upper Air Charts Surface map: isobars, high (H), low (L), cross-isobar flowSurface map: isobars, high (H), low (L), cross-isobar flow 500 mb map: height contour lines, ridges, troughs, 500 mb map: height contour lines, ridges, troughs,

flow parallel to height contoursflow parallel to height contours

Q7: Why do height contours decrease in value from south to north?Q7: Why do height contours decrease in value from south to north?

a) because temperature is higher in the south;a) because temperature is higher in the south;

b) because pressure is higher in the southb) because pressure is higher in the south

Page 9: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Figure 2, p. 150

Page 10: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

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

Page 11: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Why the Wind BlowsWhy the Wind Blows Newton’s first law of motionNewton’s first law of motion An object at rest (or in motion) will remain at rest (or in An object at rest (or in motion) will remain at rest (or in

motion) as long as no force is exerted on the objectmotion) as long as no force is exerted on the object Newton’s second law of motionNewton’s second law of motion F = ma (force = mass times the acceleration) F = ma (force = mass times the acceleration) acceleration could be change of speed or directionacceleration could be change of speed or direction

Four forces include pressure gradient force, Coriolis force, Four forces include pressure gradient force, Coriolis force, centripetal force (or its opposite, centrifugal force), and centripetal force (or its opposite, centrifugal force), and frictionfriction

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)

Page 12: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Forces that Influence the Forces that Influence the WindWind

net force and fluid movementnet force and fluid movement• Wind is the result of a balance of several forces.Wind is the result of a balance of several forces.

Page 13: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Pressure Gradient ForcePressure Gradient Force pressure gradient (pressure difference/distance)pressure gradient (pressure difference/distance) pressure gradient force (PGF) (from high to low pressure)pressure gradient force (PGF) (from high to low pressure) strength and direction of the pressure gradient forcestrength and direction of the pressure gradient force

• The The horizontalhorizontal (rather than the vertical) pressure (rather than the vertical) pressure gradient force is responsible for air movement. 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)

Page 14: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

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

Surface map

Page 15: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Coriolis ForceCoriolis Force real and apparent forcesreal and apparent forces Coriolis force is an apparent force due to earth’s rotationCoriolis force is an apparent force due to earth’s rotation Its strength increases with the object’s speed, earth rotation, Its strength increases with the object’s speed, earth rotation,

and latitude and latitude Its direction: Its direction:

perpendicular to wind, perpendicular to wind,

to the right-hand side over to the right-hand side over

Northern Hemisphere (NH), Northern Hemisphere (NH),

and to the left over SH and to the left over SH

Page 16: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

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

Q15: The Coriolis effect is stronger ifQ15: The Coriolis effect is stronger if a) wind speed is faster; b) latitude is higher;a) wind speed is faster; b) latitude is higher; c) both a) and b)c) both a) and b)

Page 17: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Straight-line Flow AloftStraight-line Flow Aloft

balance of the pressure gradient balance of the pressure gradient and Coriolis forcesand Coriolis forces

geostrophic wind: parallel to geostrophic wind: parallel to

isobars with low pressure to its isobars with low pressure to its

left (or right) in NH (or SH)left (or right) in NH (or SH) good approximation for flow good approximation for flow

aloftaloft

• Geostrophic winds can be Geostrophic winds can be observed by watching the observed by watching the movement of clouds.movement of clouds.

Page 18: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Curved Winds Around Lows and Highs Curved Winds Around Lows and Highs AloftAloft

cyclonic flow (with low P center) and anticyclonic flow (with cyclonic flow (with low P center) and anticyclonic flow (with high P center): direction opposite in NH versus SHhigh P center): direction opposite in NH versus SH

clockwise and anticlockwise: same direction in NH and SHclockwise and anticlockwise: same direction in NH and SH centripetal force (opposite to centrifugal force) centripetal force (opposite to centrifugal force) gradient wind: balance of PGF, Coriolis and centrifugal forcesgradient wind: balance of PGF, Coriolis and centrifugal forces

PGF = Co + Cen Co = PGF + Cen

Page 19: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

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

Page 20: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Winds on Upper-level Winds on Upper-level ChartsCharts meridional and zonal windsmeridional and zonal winds

wind is nearly parallel to the height contourwind is nearly parallel to the height contour higher air T yields greater height contour valuehigher air T yields greater height contour value

• Height contours on upper-level charts are interpreted Height contours on upper-level charts are interpreted in the same way as isobars on surface charts.in the same way as isobars on surface charts.

Page 21: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Figure 4, p. 157

West wind over midlatitudes in NH and SH

Page 22: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Surface WindsSurface Winds planetary boundary layer: bottom 1 km above surfaceplanetary boundary layer: bottom 1 km above surface Friction: opposite to wind in direction; increases with windFriction: opposite to wind in direction; increases with wind frictional effects on the wind: slow down windfrictional effects on the wind: slow down wind Wind rotates clockwise from near surface to free atmosphere Wind rotates clockwise from near surface to free atmosphere

in the NHin the NH

Page 23: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

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 Wind rotates anticlockwise from near surface to free atmosphere in the SHatmosphere in the SH

Page 24: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

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

Page 25: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Winds and Vertical MotionsWinds and Vertical Motions divergence and convergencedivergence and convergence hydrostatic equilibrium (vertical PGF = gravity)hydrostatic equilibrium (vertical PGF = gravity)

Q23: Vertical PGF is much larger than horizontal PGF.Q23: Vertical PGF is much larger than horizontal PGF.

a) true; b) falsea) true; b) false

Page 26: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Measuring and Determining Measuring and Determining WindsWinds

wind direction: the direction where wind comes fromwind direction: the direction where wind comes from prevailing wind: wind direction that occurs most frequentlyprevailing wind: wind direction that occurs most frequently wind rosewind rose

Q24: If the wind is southwesterly, the wind direction isQ24: If the wind is southwesterly, the wind direction is

a) 45a) 45oo; b) 135; b) 135oo; c) 225; c) 225oo; d) 315; d) 315oo

Page 27: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Wind Wind InstrumentsInstruments

wind vanewind vane cup anemometercup anemometer aerovaneaerovane rawinsonderawinsonde wind profilerwind profiler

• By observing flags and smoke By observing flags and smoke plumes, our eyes are also plumes, our eyes are also effective wind instruments.effective wind instruments.

Page 28: Chapter 6: Air Pressure and Winds Atmospheric pressure Atmospheric pressure Measuring air pressure Measuring air pressure Surface and upper-air charts.

Fig. 6-29, p. 163

WindPower

Q25: at 14:00 local time, the surface wind is

a) westerly; b) southerly; c) southwesterly;d) northeasterly