The official definition of a heat wave, established by the National Weather Service, is three or more days in a row with a high of 90° or hotter. On average, a year has two hot spells like this, and the average heat wave lasts for four days. The last year without one was in 2014. Since records began in 1870 there have been 22 years without any, or about once every six or seven years. (The most in one year was six, in 2010 and 1995). Some other quick facts:
- The longest gap between years with no heat waves is 28, between 1951 and 1979. The only back-to-back years with no heat waves was in 1874 and 1875. And there were one-year gaps in 1996-1998, 1902-1904, 1886-1884 and 1875-1877.
- 1979 had the most 90-degree days without having a heat wave, 19. (Average number of days in the 90s per year is 17). Typically, a year with no heat waves averages seven 90-degree days.
- In the years with no heat waves the hottest reading was 97° in 1942.