Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Figure 3-4: Water vapor partial pressure and temperature measurements and calculations from various
contrail studies. Symbols indicate measured ambient conditions of temperature and H 2 O abundance behind
various identified aircraft. Measurements are grouped using visual observations of contrails into three
categories: (a) The existence of contrails is confirmed, (b) contrails were just at the limit of formation or
disappearance, and (c) contrails were not observed. Liquid and ice saturation pressure are given by full and
dashed curves, respectively. The thin line connected to each symbol represents the mixing line of plume
states between ambient and engine exit conditions. The mixing lines have altitude- and engine-dependent
slopes given by the expression EI(H 2 O) cp p [0.622 Q(1 - h)]-1, which includes the emission index of water
vapor, the specific heat capacity of air cp, the ambient pressure p, the ratio of molar masses of water and
air (0.622), and the effective specific combustion heat Q(1 - h), where h is the overall efficiency of
propulsion for the aircraft and Q is the fuel-specific heat of combustion. Because contrails are observed
only when the mixing line crosses [in (a)] or at least touches [in (b)] the liquid water saturation curve, these
observations are consistent with the modified Schmidt-Appleman criterion used to predict contrail
occurrence (from Kärcher et al., 1998a).
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