This part of IEC 61300 details a procedure for determining the suitability of a fibre optic device or closure to withstand variations in humidity and temperature that may occur during operation, storage and/or transport. The test is intended to indicate the performance of such devices when exposed to heat and humidity followed by short-term freezing. In general terms, this test provides a high temperature to induce potential failures due to softening and expansion, a high humidity to encourage moisture absorption and swelling and a low temperature to facilitate ice formation, embrittlement and contraction. This test differs from other cyclic environmental tests, notably the damp heat cyclic test of IEC 61300-2-46 and the composite temperature-humidity cyclic test of 61300-2-21, by incorporating alternative levels of severity. This is achieved through a) a greater number of cycles; b) a greater cyclic temperature range; c) a decreased cyclic period.