You often refer to the amount of evaporated water of precipitable water in the atmosphere. How is this determined?
The term precipitable water is what the depth of all of moisture in the troposphere would be if it were condensed and deposited on the earth’s surface. It is mathematically determined by integrating the moisture content in the atmosphere from the surface to more than 30,000 feet. These moisture values are determined from, surface observations, upper air balloon radiosonde ascents, satellite measurements and computer interpolation. Computer models extrapolate the moisture values from the initial conditions and provide forecast values that extend out to many days. Precipitable moisture values that approach or exceed two inches are considered to be very high and are a red flag to forecasters that heavy rainfall is likely to occur.