From Longman Dictionary of Contemporary Englishmerciless heat/cold/wind etcmerciless heat/cold/wind etcheat, cold etc that is very great or strong and unpleasant, and does not stop It brings some relief from the merciless summer heat. → merciless
Examples from the Corpus
merciless heat/cold/wind etc• During the merciless heat of noon one of the frailer females collapsed, far from any possible shelter. 