  Understanding surface temperature is important for habitability. Recent work
on Mars has found that the dependence of surface temperature on elevation
(surface lapse rate) converges to zero in the limit of a thin CO2 atmosphere.
However, the mechanisms that control the surface lapse rate are still not fully
understood. It remains unclear how the surface lapse rate depends on both
greenhouse effect and surface pressure. Here, we use climate models to study
when and why "mountaintops are cold". We find the tropical surface lapse rate
increases with the greenhouse effect and with surface pressure. The greenhouse
effect dominates the surface lapse rate transition and is robust across
latitudes. The pressure effect is important at low latitudes in moderately
opaque atmospheres. A simple model provides insights into the mechanisms of the
transition. Our results suggest that topographic cold-trapping may be important
for the climate of arid planets.
