Measurement of atmospheric pressure with the mercury barometer: vacuum AB h CHAPTER 2. VERTICAL STRUCTURE OF THE ATMOSPHERE Atmospheric pressure P = P A = P B =  Hg gh Mean sea-level pressure: P = 1.013x10 5 Pa = 1013 hPa = 1013 mb = 1 atm = 760 mm Hg (torr)

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

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) Vertical area dydz Pressure-gradient force Acceleration For  P = 10 hPa over  x = 100 km,  ~ m s -2  100 km/h wind in 3 h! Effect of wind is to transport air to area of lower pressure  dampen  P On mountains, however, the surface pressure is lower, and the pressure-gradient force along the Earth surface is balanced by gravity: P(z) P(z+  z) P-gradient gravity  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

MASS m a 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

QUESTIONS 1.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?

CLAUSIUS-CLAPEYRON EQUATION: P H2O, SAT = f(T) A = 6.11 hPa B = K T o = 273 K P H2O,SAT (hPa) T (K)

VERTICAL PROFILES OF PRESSURE AND TEMPERATURE Mean values for 30 o N, March Tropopause Stratopause

Barometric law (variation of pressure with altitude) Consider elementary slab of atmosphere: P(z) P(z+dz) hydrostatic equation Ideal gas law: Assume T = constant, integrate: Barometric law unit area

Application of barometric law: the sea-breeze effect