Most people are aware of the fact that air temperature drops as elevation increases.
Anyone who has taken a trip from the lowlands up into a mountainous region has experienced this. A pleasant, mild day in the lowlands might turn a bit chilly as you ascend several thousand feet up a mountain. But why or how does this happen?
After all, the higher up in the mountains you travel, the closer you are to the sun, right? So, logically, air temperature should increase with increasing elevation since you are closer to the sun's powerful rays.
Well, when it comes to air temperature, things are not that simple.
Since I am a bit of a meteorology/weather buff, I learned long ago (in my weather and climate 101 class) an important truth in regards to air temperature: that air is heated not from above -- but from below!
The sun's rays pass through the air and strike the ground. The ground, in turn, heats up, and by way of a process called convection, the heat from the ground radiates into the air above, heating it.
As the sun rises higher and higher into the daytime sky, the sun's ultraviolet rays intensify and increase surface (ground) temperatures. As the ground warms up, so does the air directly above it.
This process continues, with each layer of air gradually warming up the layer directly above it. This process occurs for thousands of feet into the atmosphere.
Also, and this is important, the effects of gravity pull the densest parts of the atmosphere close to the ground.
What this means is, close to the ground, air molecules are numerous and they are tightly packed together. Heat from the ground is transmitted easily to this dense layer of atmosphere, and the tightly packed air molecules transfer heat to neighboring air molecules.
However, as elevation increases, the effects of gravity are lessened; the atmosphere literally "thins out" and air molecules spread farther apart, reducing their capacity to transfer heat to neighboring molecules.
So, in high-elevation areas, air molecules are less numerous and less tightly packed. As a result, there is less atmosphere in which to hold and transfer heat.
This is why the temperature at sea level may be 85-degrees, while a mountain location nearby at 6,000 feet may register 65-70-degrees. The mountain area is located in a less dense part of the atmosphere, where heat transfer and heat storage are minimized.
A general rule of thumb is that air temperature decreases by about 3.5 degrees for every 1,000 feet of elevation gain. This number is called the lapse rate, which is just an estimate.
The lapse rate can vary depending on weather conditions and wind patterns, but the 3.5 degrees drop is a good estimate under normal, calm conditions.
So, there you have it. Mountain tops and high elevation areas are cooler because they are located in less dense areas of atmosphere that cannot retain and transfer heat as effectively as low elevation areas located in denser parts of the atmosphere!