Warm weather has arrived, but is it a heat wave? In general terms a heat wave is described as a period of abnormally and uncomfortably hot weather, usually accompanied by higher than normal humidity. Typically, this means warmer than normal temperatures for at least a couple of days, with some heat waves lasting for weeks. In the 1900s, A.T. Burrow defined a “hot wave” as a period of at least 3 days where the daily maximum shade temperature exceeded 90 degrees Fahrenheit. More realistically, the threshold varies from region to region, depending upon what is normal in that area. And some researchers argue for using the low temperature to set the threshold. This is because some studies have shown an increase occurrence of heat related illnesses and deaths where low temperature remain above 75 °F, regardless of what the high temperatures reach.
Mary Knapp, Weather Data Library