"Hell Week" refers to the time during the summer when New York City is most likely to have temperatures in the 90s. The week beginning July 16 encompasses the three most likely days: July 18, July 19 and July 21. Whereas the average day in July has experienced a 90-degree reading 29 times (since record-keeping began in 1869), July 18, 19 and 21 have reached 90° or hotter 45, 44 and 38 times, respectively (thru 2019). And July 21 and 22 have had the most occurrences of triple-digit heat: six times on July 21 and four on July 22. (And there's a mini-Hell Week between July 7-10, in which the average number of 90-degrees days is 33.)
Looking at the history of these dates reveals that July 18 didn't emerge as the date most likely to have a 90-degree reading until the late 1980s. Before that, July 9, July 19 and July 31 were the dates that vied for bragging rights. However, since 1980 60% of the years have had highs in the 90s on July 18 (including four years in a row from 2010-2013). By contrast, before 1980 (going back to 1872) just 23% of the years had 90-degree highs.
*Note that "Summer" refers to meteorological summer, June 1- August 31.
Many readers may be surprised to discover that the two days most likely to see 90° has only a 30% chance in any given year to see temperatures that hot (i.e., 45 out of 151 years). Perhaps it's because it's human nature to remember the days that are in the extreme. Actually, even days in mid-summer have experienced long stretches without 90-degree readings. For instance, there were no 90-degree readings on July 18 for twenty years in a row between 1923 and 1942. And in the most extreme case, July 1 had no 90-degree readings between 1975 and 2011, a span of thirty-seven years.
Of course, as most New Yorkers are well aware, getting through "Hell Week" doesn't put us in the clear. In fact, of twenty-two heat waves of seven days or longer, only a handful have occurred in mid-July. New York's longest heat wave on record, lasting twelve days, occurred between Aug. 24 and Sept. 4 in 1953. The next longest, 11 days, started in the second half of July (July 23) in 1999. And the third and fourth longest, both 10 days long, were in early August 1896 and early July 1993.
I want to thank my fellow weather "nut", Eugene DeMarco of Queens, for helping me by supplying and organizing some of the weather statistics that provided the background material for this and recent posts.