Chemical composition of the lower atmosphere
78% N2, 21% 02, 0.6-4% H20, 0.03% C02, noble and trace gases of natural and anthropogenic origin.
Water vapour pressure: The maximum amount of water vapour that can be held by the atmosphere is termed saturation pressure (e0), and which depends on temperature and pressure. The pressure of the atmosphere without water vapour, Pa, is derived from the measured pressure of the atmosphere, P, and the actual vapour pressure, e (Pa=P-e). At a constant air pressure the vapour pressure rises exponentially with a linear increase in temperature. For exact data on the composition of air, see List (1971).
Note: humid air is lighter than dry air, determined by the relationship between the molecular weights of H20 and N2, which is 0.62 (18/36). Hence, humid air in the lower atmosphere rises until it condenses on cooling, and forms fog or clouds. For the same reason, condensation forms on the ceilings of humid rooms or on the lids of Petri dishes.
The atmosphere is only rarely saturated with water vapour. The actual vapour pressure of the atmosphere (e) is generally lower than the saturation value (eQ). There are a number of terms that are used to describe the humidity of air (Fig. 2.1.1):
absolute vapour pressure: cw=(2.17/T) e, unit: g m~3 (Note: volume of air is temperature and pressure dependent); relative humidity: H=e/e0, unit % (Note: at constant vapour pressure this is dependent on T);
water vapour saturation deficit of the atmosphere: Da= (eQ-e)/pa, unit Pa Pa-1 (often shortened to VPD: vapour pressure deficit). Note: the value is independent of temperature and pressure; water vapour saturation deficit between leaf and atmosphere: Di= (eoL-e)/pa, where eoL is the vapour pressure deficit at leaf temperature and e the actual vapour pressure in the atmosphere; this term is important as it is the driving force for transpiration in plants (used to be abbreviated as WD: water vapour deficit); dew point temperature °C: Tj for e = eQ (temperature, t, below which condensation temperature is reached and therefore condensation follows);
wet bulb temperature Tw: This unit is used to measure the actual vapour pressure (psychrometer)
where y, the psychrometric constant, is 66.1 Pa K_1 for a ventilated thermometer at 100 kPa air pressure and 20 °C.
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