High-resolution (1 Hz at true airspeeds near 200 m s-1) observations of methane, ozone, water vapor, and temperature taken between the surface and 18 km from the WB57F aircraft near San Jos¿, Costa Rica (10¿N, 84¿W), are used to examine processes influencing the maintenance of these profiles. There was a clearly defined thermal tropopause on each of the eight profiles, accompanied by structure on methane-ozone scatterplots on the five profiles having methane observations. There was a well-defined decrease in methane mixing ratio between approximately 12 and 15 km in each of these five profiles, 2--5 km beneath the thermal tropopause, correlated with sharp changes in water vapor and equivalent potential temperature. The methane observations are interpreted as meaning that air is recirculated between the lower stratosphere and the upper tropical troposphere. At the point on each vertical profile where the water vapor had its minimum value, the air was never saturated or apparently supersaturated, although apparent supersaturation with respect to ice was observed in vertically extensive, near-adiabatic layers with tops some 200--300 m below the water vapor minimum on all profiles. One of the profiles also exhibited apparent supersaturation above its water vapor minimum, near 18 km. We examine the decrease in water vapor to minimal values as a four-stage process in which its mixing ratio was lowered from ~10 to ~3 ppmv, consider the role of solar and thermal evaporation of the smaller ice particles in the final stage of the dehydration process, and correlate features separating near-adiabatic layers above 150 hPa pressure altitude with local sea surface temperatures.