Infrared- And Millimeter-Wavelength Absorption In The Atmospheric Windows By Water Vapor And Nitrogen: Measurements And Models

Abstract

Attenuation of electromagnetic waves by atmospheric gases is an im-portant consideration in a variety of radar and electro-optical applica-tions. Molecular absorption by strong bands of H 2 O and CO 2 defines the atmospheric window regions. The window regions are not totally transparent and feature weak line absorption and continuum absorption. The experimental character of continuum absorption by water vapor in atmospheric window regions at millimeter wavelengths and 10, 4, and 2.2 Am and nitrogen at 4 μm is surveyed. This includes recent measurements at The Johns Hopkins University Applied Physics Laboratory on H 2 O at 2.2 μm and the nitrogen collisioninduced band at 4.3 μm. Also, some of the concepts and models used to characterize these phenomena will be reviewed. The search for a unified theory on the water vapor continuum has been elusive, yet the frequency and pressure dependence is consistent with current far-wing theories. The temperature dependence is not totally understood. A model describing the temperature dependence of the nitrogen continuum is emphasized.