radio waves whose wavelength is 1–10 mm. Because of the substantial absorption of such waves by water vapor and gases contained in the atmosphere, their use for terrestrial radio communication is restricted to “radio windows,” which are narrow wavelength bands in which absorption is minimal. Rain, fog, and snow cause virtually complete absorption. Millimeter waves can be transmitted along wave guides and quasi-optical lines. They are used in space communications out-side the tropospheres of the earth and other planets.