While viewing cloud cover from space may be an example of a remote sensing enterprise which is intuitive, the "sounding" of, say, the vertical variations of temperature, moisture, or ozone through the atmosphere is not obvious or intuitive at all. The signals (in very narrow wavelength intervals of the carbon dioxide, water vapor, and ozone absorptions bands in the infrared) which relate to vertical profile variability must be carefully sorted out, and then subjected to a class of mathematical problems called "inverse integral problems". Unfortunately, these mathematical problems are not at all well behaved, and not very easy to solve.