ABSTRACT
Hydrometeors-atmospheric water droplets and ice particles in all of their different forms-absorb and scatter microwaves at all wavelengths. The absorption coefficients of water droplets and ice particles are very small at low frequencies, but increase with increasing frequency. The amount of water in the atmosphere and the form it is in varies greatly with time and place, so we can use microwaves to study the properties and concentrations of water in the atmosphere. Water vapor has prominent absorption lines at 22 GHz and 183 GHz (see Figure 2-2). At frequencies between 1 and 10 GHz or so, clouds have little effect on microwave radiation, so microwave instruments can be used to look through clouds at the earth’s surface at these wavelengths. This is often an important consideration, as we cannot see through clouds with visible or infrared radiation. At higher frequencies, microwave radiometers are used to sense the state of the atmosphere.
