Cyclic moisture testing is performed for the purpose of evaluating, in an accelerated manner, the resistance of component parts and materials to the deteriorative effects of the heat conditions and high-humidity typical of tropical environments. Most tropical degradation results indirectly or directly from the absorption of moisture vapor and films by vulnerable insulating materials, and from the surface wetting of metals and insulation. These phenomena produce many types of deterioration, including corrosion of metals, constituents of materials and detrimental changes in electrical properties.

This test, as performed per MIL-STD-883, test method 1004, differs from the steady-state humidity test and derives its added effectiveness in its employment of temperature cycling, which provides alternate periods of condensation and drying essential to the development of the corrosion processes. In addition, it produces a breathing action of moisture into partially sealed containers. Increased effectiveness is also obtained by use of a higher temperature, which intensifies the effects of the humidity.

Cyclic moisture resistance testing includes a low-temperature subcycle that acts as an accelerant to reveal otherwise indiscernible evidences of deterioration as stresses caused by freezing moisture tend to widen cracks and fissures. As a result, the deterioration can be detected by the measurement of electrical characteristics, e.g. voltage breakdown or insulation resistance testing, or by performance of a test for sealing. TM 1004 also provides for the electrical loading of certain components, if desired, in order to determine the resistance of current-carrying components to electrochemical corrosion.