Critical Humidity Posted on 6th November 201523rd July 2021 by ThePD (The Project Definition) Critical Humidity is the relative humidity of system above that increases sharply the atmospheric corrosion rate of metals. Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window)Click to share on LinkedIn (Opens in new window) Related Posted in Engineering and tagged humidity, property. ThePD (The Project Definition) ThePD has been developing the Preferred Project Definitions based on the actual project execution and operation experiences and knowledge with the Project Language, and sharing with you daily basis.