Something that has bothered me a long while regarding the atmosphere of Earth, Mars and Venus is whether the differences in temperature is to due to the amount of green house gases or to atmosphere density. Venus's atmosphere is very hot, which is normally attributed to its atmosphere being largely CO2, but its atmosphere is also very dense. Mars' atmosphere is cold. Its atmosphere is also largely CO2, but it is very thin. Even though the atmosphere is by proportion largely CO2, in absolute terms there is not so much CO2 because the atmosphere is so thin. So is it the density of the atmosphere that matters or the amount of CO2 that matters? In Earth's atmosphere, air cools as it rises and becomes less dense in accordance with Boyle's Law, PV = nRT (or P = ρRT). However at ground level, the Earth is heated by sunlight which transfers heat to the air via convection (and by other complicated routes via absorption, re-radiation and phase change). Therefore it should not matter how dense the atmosphere was at ground level because it is heated by the ground, which is heated by absorbed light. This is largely why it is so much colder at the poles than at the equator. The density of sunlight falling at the poles is much less than at the equator, and more of it is reflected away. So even though atmospheric density at the poles is similar to that at the equator, it is much colder. This is not the end of the story as air temperatures are evened out somewhat by advection, the movement of air and water currents. However, if it is the case that the air at ground level is heated by the ground, which is heated by solar radiation, why is it that the temperature on high plains are cooler than at sea level? For example, Mexico City is situated on a plain several thousand feet up. It's climate is relatively cool compared to other places in Mexico, but the solar radiation it receives is the same. Is this because there is a breeze coming from lower lying areas, which cools as it rises and becomes less dense? Is is because there is proportionally less greenhouse gas above? Or is it just due to the air being thinner up there.