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Vertical Structure of the Atmosphere
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VERTICAL STRUCTURE OF THE ATMOSPHERE
Measurement of atmospheric pressure with the mercury barometer: vacuum A B h Atmospheric pressure P = PA = PB = rHg gh Mean sea-level pressure: P = 1.013x105 Pa = 1013 hPa = 1013 mb = 1 atm = 760 mm Hg (torr)
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“SEA LEVEL” PRESSURE MAP THIS MORNING (2/4/14, 15Z)
…and the forecast: weather.unisys.com
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SEA-LEVEL PRESSURE CAN’T VARY OVER MORE THAN A NARROW RANGE: 1013 ± 50 hPa
Consider a pressure gradient at sea level operating on an elementary air parcel dxdydz: P(x) P(x+dx) Pressure-gradient force Vertical area dydz Acceleration For DP = 10 hPa over Dx = 100 km, g ~ 10-2 m s-2 a 100 km/h wind in 3 h! Effect of wind is to transport air to area of lower pressure a dampen DP On mountains, however, the surface pressure is lower, and the pressure-gradient force along the Earth surface is balanced by gravity: P(z+Dz) P-gradient This is why weather maps show “sea level” isobars even over land; the fictitious “sea-level” pressure assumes an air column to be present between the surface and sea level gravity P(z)
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MASS ma OF THE ATMOSPHERE
Radius of Earth: 6380 km Mean pressure at Earth's surface: 984 hPa Total number of moles of air in atmosphere: Mol. wt. of air: 29 g mole-1 = kg mole-1
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QUESTIONS What is the fractional increase in volume of water-soluble aerosol particles when relative humidity increases from 90% to 95%? (assume that the particles are mainly water, so neglect the contribution of the solute to particle volume). Assuming that visibility degradation is proportional to the cross-sectional area of the particles, what is the resulting percentage decrease in visibility? 2. Why does it take longer to boil an egg in Denver than in Boston?
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CLAUSIUS-CLAPEYRON EQUATION: PH2O, SAT = f(T)
A = 6.11 hPa B = K To = 273 K PH2O,SAT (hPa) DEMO: the freezing experiment. ask for student predictions. Explain about nucleation of the bubbles. Why are bubbles formed in the liquid and what are they made from? freezing: more energy in the ordered lattice, latent heat of "fusion" = 0.34 x 106 J/kg (less than evaporation, but still considerable). relate to boiling – what is boiling? condensation—as in a shower stall. Is heat released on condensation? To evaporate 1 kg of ice, = 2.6 kJ/kg. Clausius-Clapeyron relationship— Psat = A exp [B( 1/ – 1/T)] A=6.11 mbar, B= 5308K. A=water vap. pressure at 0C. T (K)
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VERTICAL PROFILES OF PRESSURE AND TEMPERATURE Mean values for 30oN, March
Stratopause Tropopause
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Barometric law (variation of pressure with altitude)
Consider elementary slab of atmosphere: P(z+dz) P(z) unit area hydrostatic equation Ideal gas law: Assume T = constant, integrate: Barometric law
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Application of barometric law: the sea-breeze effect
add A30 sea breeze
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