Is it always colder on top of mountains?
It’s a common misconception that the air is colder at the top of a mountain rather than at the valley below. In the bottom ten or so miles of the atmosphere, air mixes vertically–cold air sinks, and hot air rises. As the ground bakes in the sun, the air right above it heats up and starts to rise. As it gets higher, it experiences a decrease in pressure that allows it to expand and lose heat.
The more air expands, the colder it becomes. The exception to this is if the air contains a lot of moisture. In this case, all that cooling will result in condensation and cloud formation, a process that gives off heat and prevents the air from cooling as quickly. While all of this provides for one chilly mountain top, sunlight plays an important role in warming the mountain’s tip.
The temperature on top of a mountain also depends on the way sunlight hits the mountain. If the mountain you’re on slopes away from the sun, the air directly above its surface can become relatively cold and drain into the valley below, cooling it off as well. Also, low-lying clouds can block the sun from reaching the valley bottom and heating it up, leaving it cooler than the mountain top.