Water Vapor Database
The FORCE Water Vapor Database (FORCE WVDB) component can be used to generate and maintain a water vapor database used for atmospheric correction of Landsat data in Level 2 Processing System.
The water vapor database is not necessary for Sentinel-2 because water vapor can be estimated from the images themselves.
The database holds water vapor values for the central coordinates of each WRS-2 frame. If available, day-specific values are used. If not, a monthly climatology is used instead.
The uncertainty of using the climatology was assessed in this paper: Frantz, D., Stellmes, M., & Hostert, P. (2019). A Global MODIS Water Vapor Database for the Operational Atmospheric Correction of Historic and Recent Landsat Imagery. Remote Sensing, 11, 257. https://doi.org/10.3390/rs11030257
A glimpse of what you get:
Figure Global, and monthly water vapor climatology for each land-intersecting Landsat WRS-2 scene.