Recall that gases in our atmosphere are most dense in the troposphere. The troposphere is the region where greenhouse gas concentrations are highest. Infrared radiation from both the sun and the Earth can be absorbed by greenhouse gases in the troposphere.

Also recall that air is 78% nitrogen (N₂) and 21% oxygen (O₂). On a hot day, air feels hot because these molecules have more kinetic energy than on a cold day. However, neither N₂ nor O₂ molecules can absorb IR radiation because they have evenly distributed charges. How, then, do they gain energy on a hot day?