  A radiative-convective climate model is used to calculate stratospheric
temperatures and water vapor concentrations for ozone-free atmospheres warmer
than that of modern Earth. Cold, dry stratospheres are predicted at low surface
temperatures, in agreement with recent 3-D calculations. However, at surface
temperatures above 350 K, the stratosphere warms and water vapor becomes a
major upper atmospheric constituent, allowing water to be lost by
photodissociation and hydrogen escape. Hence, a 'moist greenhouse' explanation
for loss of water from Venus, or some exoplanet receiving a comparable amount
of stellar radiation, remains a viable hypothesis. Temperatures in the upper
parts of such atmospheres are well below those estimated for a gray atmosphere,
and this factor should be taken into account when performing 'inverse' climate
calculations to determine habitable zone boundaries using 1-D models.
