This value is the absolute vertical water vapor column in atm-cm. Figure 1 contains the density profiles for H₂O for each model atmosphere.

Figure 1. MODTRAN^® Model Atmosphere Profiles for H₂O. Note there is a break in the altitude vertical scale, at 10 km.

When changed, the model water number density values are scaled from the model atmosphere to match the specified integrated path content. Note, the water number density at each profile altitude is not increased above 100% relative humidity or by more than 5 times the original model atmosphere value. When the relative humidity limit is reached, the excess water is distributed to other levels to the extent possible to achieve the input water column.