The total precipitable water (TPW or total column water vapor) is one of the most accurate of the microwave products, one of the first to be developed (during the 1970s), and is widely used by the meteorological community. Numerous studies comparing radiosonde and microwave TPW show RMS errors for the microwave TPW product near 10 percent (~1 mm).

Retrievals of TPW rely upon brightness temperature measurements near the center of the weak water vapor absorption region at 22 GHz (see figure above). For SSM/I TPW, these measurements are obtained with the 22-GHz channel and for AMSU TPW, the 23-GHz channel is used. Contamination caused by clouds, rain, and over-the-ocean surface winds is minimized by combining these measurements with window channels at other frequencies. The TPW algorithm is empirical, fitting brightness temperature observations with simulated brightness temperatures via coefficients determined through regression. See the products summary chart on the first page of the Products and Applications section for details on all the channels used for both the AMSU and SSM/I TPW algorithms.

TPW cannot be derived over land within the 22-GHz region due to the large and variable surface emissivities. The large surface emissivities over land result in higher brightness temperatures that dominate the smaller water vapor brightness temperature signal. This masking effect makes detection of the smaller signal and retrieval of TPW impractical.