Global Level of Connected Moisture (GLCM) and total moisture content (TMC) in porous over-layers on metals play a vital role in atmospheric corrosion. Using a model based on Kelvin condensation and Extended Hoshen-Kopelman algorithm these are computed as a function of relative humidity (RH) for 17 different over-layers with varying degrees of affinity to water. GLCM exhibits a critical RH (RH*) but TMC does not. The computed RH* agrees with the experimental result for iron for selected model parameters. RH* values are computed for several metal oxide and polymer over-layers on metals in chloride-bearing and non-chloride-bearing atmospheres. RH* of chloride-bearing atmospheres is found to be less than that of non-chloride-bearing atmospheres. A new formula is advanced for time of wetness using GLCM. Using RH and temperature data obtained from India Meteorological Department for three cities, time of wetness (TOW) is computed for the 17 systems. These results are compared with the predictions of a simplistic formula presently available. (C) 2016 The Electrochemical Society. All rights reserved.