A necessary step to develop an integrated national network of networks of surface observations is to assess the impact of existing mesonets that have different siting, equipment, and siting standards. As one measure of observation impact, the adjoint of a two-dimensional variational analysis system of surface weather parameters covering the entire continental United States has been developed to assess objectively the sensitivity of the resulting analyses to the source of the observations used in the analysis. The analysis system uses the 5 km resolution background fields used by the Real Time Mesoscale Analysis (RTMA) developed by the National Centers for Environmental Prediction. Roughly 12000 observations available each hour from many different mesonets are then used to modify the background grids and obtain hourly analyses. The sensitivity of the differences in weather parameters between the resulting analyses and the background fields are examined as a function of the various data assets, e.g., FAA/NWS stations; Remote Automated Weather Stations (RAWS), and stations installed and maintained by federal, state, and local agencies, commercial firms, and the public. Statistics for individual stations as well as for entire networks are obtained from a representative sample of analyses selected on the basis of the occurrence of significant weather.