At night, if your eyes were sensitive enough you would see a faint glow in the sky. Because the airglow brightness is affected by atmospheric motions, we can use it to get a picture of what is happening at that height. One can often see buoyancy waves rippling through the airglow layer like waves on a pond surface. These waves travel throughout the atmosphere and help to move energy and momentum from one place to the other. Because of this, our understanding of buoyancy waves is extremely important to our understanding of the atmosphere as a whole.
Some clouds are too thin to be seen with the naked eye.
A lidar can detect such clouds. It operates like a radar but uses visible or infrared waves in place of radio waves, and can observe things that a radar can not. Arecibo scientists also study the atmosphere using optical radars known as lidars. Lidars can measure atmospheric density, temperature, winds, as well as dust and pollutants. The extraterrestrial metal atoms deposited in the upper atmosphere by meteors can also be observed by the Arecibo lidars.