1. Vertical Structure of the Atmosphere

2. 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)

3. “SEA LEVEL” PRESSURE MAP THIS MORNING (2/4/14, 15Z)
…and the forecast:
weather.unisys.com

4. 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)

5. 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

6. 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?

7. 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)

8. VERTICAL PROFILES OF PRESSURE AND TEMPERATURE Mean values for 30oN, March
Stratopause
Tropopause

9. 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

10. Application of barometric law: the sea-breeze effect
add A30 sea breeze