The Universe sends us light at all wavelengths of the electromagnetic spectrum. However, most of this light does not reach us at ground level here on Earth. Why? Because we have an atmosphere which blocks out many types of radiation while letting other types through. Fortunately for life on Earth, our atmosphere blocks out harmful, high energy radiation like X-rays, gamma rays and most of the ultraviolet rays. It also block out most infrared radiation, as well as very low energy radio waves. On the other hand, our atmosphere lets visible light, most radio waves, and small wavelength ranges of infrared light through, allowing astronomers to view the Universe at these wavelengths.

Most of the infrared light coming to us from the Universe is absorbed by water vapor and carbon dioxide in the Earth's atmosphere. Only in a few narrow wavelength ranges, can infrared light make it through (at least partially) to a ground based infrared telescope.

The Earth's atmosphere causes another problem for infrared astronomers. The atmosphere itself radiates strongly in the infrared, often putting out more infrared light than the object in space being observed. This atmospheric infrared emission peaks at a wavelength of about 10 microns (micron is short for a micrometer or one millionth of a meter).

So the best view of the infrared universe, from ground based telescopes, are at infrared wavelengths which can pass through the Earth's atmosphere and at which the atmosphere is dim in the infrared. Ground based infrared observatories are usually placed near the summit of high, dry mountains to get above as much of the atmosphere as possible. Even so, most infrared wavelengths are completely absorbed by the atmosphere and never make it to the ground.

From the table below, you can see that only a few of the infrared "windows" have both high sky transparency and low sky emission. These infrared windows are mainly at infrared wavelengths below 4 microns.