The dangers of heat waves engulfing the Northern Hemisphere are not over yet, the UN World Meteorological Organization (WMO) warned on Tuesday. The agency stressed in the warning that heat waves are among the deadliest natural hazards, and WMO's chief heat advisor John Nairn told reporters that extreme temperatures will grow in frequency, duration, and intensity "Repeated high nighttime temperatures are particularly dangerous to human health because the body is unable to recover from the sustained heat," he said. "This leads to increased heart attack and death rates."