Cool mountain air, thunderstorms, and the atmospheric sound channel.

Most of you have experienced the fact that if you drive to the mountains, the air is cooler. The reason for this is simple: when wind blows air to a higher altitude, the air expands, and when air expands, it gets cooler. So any air blown to higher altitude is cooler. Likewise, when air is blown from higher altitude to a lower altitude, it gets compressed. When air is compressed, it gets warmer. As long as wind continually mixes the atmosphere, the air at the ground will usually be warmer than the air several thousand feet higher. Typically the air gets cooler by abouat 1 C (about 9/5 Farenheit) for every 500 feet increase in altitude. If you fly in an airplane at 50,000 feet, you may be surprised to hear the pilot say that the outside air temperature is -100 degrees F. But that is not surprising. That is 100 times greater than 500 feet, so you expect 100 times as big a temperature drop. 100 x 9/5 F is 180 degrees cooler. 180 degrees cooler than 80 is -100 F.

However, once you get to about 60,000 feet, we get to the ozone layer. This layer absorbs ultraviolet radiation (called UV radiation) from the sun, protecting us from these cancer causing rays. But because the ozone absorbs the light, it causes this layer to be warmer. So the coldest layer is actually about 50,000 ft. high.

A thunderstorm is created when air near the ground is heated. Because it is hot, it expands, and is less dense than the surrounding air. This causes it to rise. As it rises, it cools -- but it still is hotter than the surrounding air (since the surrounding air at high altitude is even cooler). The air continues to rise until it reaches the ozone layer. At this altitude, the surrounding air is hotter (and so is less dense), so the thunderstorm air no longer rises. This creates the anvil-head shape of a large thunderstorm. The thunderstorm rises to the altitude of the ozone layer, and then spreads out. When you look at the top of an anvil-head thunderstorm, you are seeing the beginning of the ozone layer. There is serious concern that man-made chemicals (especially Freon) are destroying this layer. There is good evidence that this is already happening over Antarctica, but it is not yet happening over North America.

The velocity of sound in air depends on the velocity of the molecules, which depends on temperature. So the velocity of sound is lowest when the air is the coolest. That means that the velocity of sound is slowest at about 50,000 feet. This minimum in the velocity of sound creates a sound channel in the atmosphere, similar to the sound channel in the oceans. The project Mogul of Maurice Ewing was designed to detect the sound from a Russian nuclear explosion that would travel around the world in this sound channel. And project Mogul led to the Roswell incident, and the worldwide belief (mistaken) in flying saucers.