Large-scale, sub-tropical cloud activity near Titan's 1995 equinox

We report observations of Titan using 190 images from the Hubble Space Telescope WFPC2 and NICMOS over 4 oppositions of Titan 1994-1997. Analysis of these images yields maps of surface brightness at 0.67, 0.94, and 1.08 mu m that compare well with Cassini data, plus some transient features that are interpreted as large cloud systems. These systems indicate either two clouds covering 2 x 10(6) km(2) (similar to 10% of Titan's observable disk and 2-3% of Titan's surface) in September 1995 or a repeat observation of a single cloud. The clouds are estimated to be upper-tropospheric, with tops between 27 and 39 km altitude. This pre-Cassini observation of prominent cloud activity near 30 N and 40 N shortly before equinox (L-s = 177 degrees) mirrors more recent Cassini observations and serves as an important constraint on the seasonal variation of convective activity with latitude on Titan.