On April 21, 1996 the HIS aboard the ER-2 observed the spectral radiance of a contrail from a jet aircraft flying beneath at a pressure altitude of 250 mb. According to the MAS imagery the contrail was relatively diffuse when the ER-2 passed over it some four to five minutes after the passage of the D.C. 8. The amazing result was that the radiating temperature of the cirrus across the 10-12 micron window varied over 30 degrees K (figure 1) corresponding to an emissivity variation of 32% (figure 2) (i.e. the transmissivity of the contrail decreased from 74% at 10 microns to about 40% at 12 microns). The spectral character of this large variation of optical depth is a signature of very small ice crystals.
