The performance of three time of wetness (TOW) determination methods during outdoor exposure was examined. Two impedance-based TOW sensor types, RH, and temperature sensors were exposed to coastal marine and laboratory environments for up to three months. The sensor and ISO 9223 methods examined provided incomparable estimates. Inaccuracy of the ISO method originated from a humidity threshold well above the drying point of deposited aerosols and differences between surface and ambient RH. TOW sensor accuracy was linked to electrode configuration. Small electrode spacing was demonstrated to be required to capture the range of wetting events of importance to atmospheric corrosion.