merciless heat/cold/wind etc
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. → mercilessExamples from the Corpusmerciless heat/cold/wind etc• During the merciless heat of noon one of the frailer females collapsed, far from any possible shelter.