Figure 3-4: Water vapor partial pressure and temperature
measurements and calculations from various contrail studies. Symbols indicate
measured ambient conditions of temperature and H2O 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(H2O) 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 K�rcher et al., 1998a).
