The Microwave Limb Sounder (MLS) on Upper Atmosphere Research Satellite (UARS) is sensitive to water vapor and ozone in the lower stratosphere. The Cryogenic Limb Array Etalon Spectrometer (CLAES), another of the instruments on UARS, has a spatial and temporal coverage similar to that of MLS and can be used to indicate the presence of cirrus. We examine the relationships among water vapor, ozone, and cirrus in the tropical region at 68 and 83 hPa during December--February 1991/1992, an El Ni¿o period. For most longitudes at 68 hPa, three-dimensional trajectories show that the cirrus form predominately in air which ascends slowly through the tropical tropopause layer. Cirrus form in air which has travelled a long way in the horizontal, in the westerly jet, before heading equatorward in subtropical anticyclones. In the Indonesian region, however, a greater proportion is found in air which is sinking than in air which is ascending. The western Pacific region sees the highest proportion of cirrus which may have been influenced by convection, and the possible contribution from convection is greater at 83 hPa than at 68 hPa. In cloudy regions, the air is likely to have a high relative humidity. Mixing ratios of ozone from MLS have a tendency to be lower, and this may indicate that ozone is destroyed on the ice particles that comprise the clouds.