http://www.hpc.ncep.noaa.gov/dailywxmap/pdffiles.html

http://weather.uwyo.edu/surface/front.html

Mass Density (ρ, Greek “rho”) = Mass/Volume For example, ρwater = 1.0 gm/cm3 ρair = 1.3 x 10-3 gm/cm3 (varies with T and p) = 1.3 kg/m3 ρMercury= 13.6 gm/cm3

Mass density, ρ ρ = number x molecular per volume mass ρ = n x m Near sea level, nair = 2.72 x 1019 cm-3 and mair = 4.81 x 10-23 gm so ρair = 1.3 x 10-3 gm/cm3 NOTE: ρair depends on pressure (or altitude) and the air temperature.

Gas Law p = n k T where n = number density (or number/volume) What is connection between the pressure of a gas and temperature? Gas Law p = n k T where n = number density (or number/volume) and k = Boltzmann’s constant = 1.38 x 10-16 °K (cgs units) = 1.38 x 10-23 °K (mks units) and the temperature, T, is in degrees Kelvin Basically, p = (Number/Volume) x Temperature or p x V = (Constant) x T

Sketches by C. Weidman

Units of pressure Pressure = Force/Area p = (Mass x Acceleration) /Area p = (Mass x gravity)/Area p = Mass/Area x gravity p = (M/A) x g Pressure = (Mass of liquid/Area) x g where g = acceleration of gravity

Units of pressure - 2 Now, consider a water barometer with a 1 cm2 cross-sectional area Many experiments near sea level show that the average height, H, of a water barometer is about 10.33 meters or 1033 cm. Water has a mass density of 1.0 gm per cm3. Therefore, the total mass of water in a 1033 m column (above an area of 1 cm2) is Volume = Area x Height V = 1 cm2 x 1033 cm V = 1033 cm3 (per cm2) Now, since the density of water is 1 gm per cm3, the total mass above a 1 cm2 area is Mass per cm2 = Density x Volume (per cm2) M/A = 1 gm/cm3 x 1033 cm3 (per cm2) M/A = 1033 gm/cm2

Units of pressure - 3 Now, since the average acceleration of gravity at sea level is g = 9.806 m/sec2 = 980.6 cm/sec2, we can multiply the mass (per cm2) by g to get the force per area (or pressure) in physical units p = M/A x g = 1033 gm/cm2 x 980.6 cm/sec2 = 1.013 x 106 gm cm/sec2 per cm2 or = 1.013 x 106 dynes per cm2 DEFINITION: 1 bar ≡ 106 dynes/cm2 which makes p = 1.013 bars = 1013 x 10-3 bars or p = 1013 millibars (mb) near sea level.

Units of pressure - 4 In the meter-kilogram-sec (or mks) system of units, p = M/A x g = 1033 gm/cm2 x 10-3 kg/gm x 104 cm2/m2 x 9.8 m/sec2 p = 1.013 x 105 kg m/sec2/m2 p = 1.013 x 105 Nt/m2 p = 1.013 x 105 Pascals (Pa) Note: 1 mb = 10-3 bars = 100 Pa = 1 hectoPascals (hPa)

SUMMARY Units of pressure - 5 One atmosphere, patm = 33.9 feet water = 10.3 m = 1033 cm water = 1033 gm/cm2 (atmospheric depth) = 1.033 kg/cm2 =1.033 x 104 kg/m2 = 760 mm Mercury (Hg) = 76 cm Hg = 29.92 inches Hg

Units of pressure - 6 In cgs units, patm = 1.013 bars = 1013 millibars (mb) where 1 bar = 106 dynes/cm2 and 1 mb = 103 dynes/cm2.

Units of pressure - 5 In mks units, patm = 1.013 x 105 Pascals (Pa) = 101.3 kiloPascals (kPa) = 1013 hectoPascals (hPa) Note: 1 mb = 1 hPa

Average Sea Level Pressure June July August December January February

Rawinsonde measures pressure, temperature, dew point, wind speed, and wind direction as function of altitude. Every 12 hours, worldwide at 00:00 and 12:00 GMT

Air pressure decreases rapidly (exponentially) with altitude.

Graph of balloon measurements of the upper atmosphere Temperature (red) Dew Point (blue) (both in ºC)