The Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) are both instruments on the Upper Atmosphere Research Satellite and are sensitive to water vapor and subvisible cirrus in the tropical tropopause layer. We analyze the longitudinal and vertical variations of water vapor in the tropical tropopause layer using a new version of MLS water vapor that has improved vertical resolution. The water vapor fields show similarity with subvisible cirrus and sea surface temperature (SST) on seasonal and intraseasonal timescales. The formation of cirrus is more prevalent over continental landmasses and over warm oceanic areas where the upper troposphere is also moist. The intraseasonal variability in cirrus is correlated with upper tropospheric water vapor and linked to a variability in SST of about 1¿C. The incidence of cirrus increases with SST until temperatures exceed about 29¿C. A further rise in sea surface temperature does not see a corresponding increase in cirrus. Similarly, the incidence of cirrus increases as 100 hPa temperatures fall, with clouds being more likely when temperatures are about 190K. At temperatures colder than this, when in situ formation of cirrus should be more likely, there is a decrease in the likelihood of finding cirrus. Thus evidence points strongly to a dominant role for the convective formation of these cirrus.