Reviewing New York's weather statistics over the past 145 years reveals that the years before 1930 were noticeably cooler than the years that followed (especially after 1980), which have seen some of the warmest years on record. But, curiously, a number of the summers during the first decade of the 20th century had uncharacteristically warm nights, in particular, 1908. That year stands out for a seven-week period with unusually warm nighttime temperatures unlike any that have been experienced - even in recent years. Although highs during those weeks were three or four degrees above average, the average low of 74.5° was eight degrees above average. Between June 30 and Aug. 17, fifteen days had lows of 77° or warmer, eight of them of 80°+. No other summer has had that many sultry low temperatures. It was these lows that made July the warmest on record until 1952.
Sultry low temperatures became more common after 1990, but not during the first half of the 20th century. In addition, the diurnal variation in the summer of 1908 was very narrow, with high and lows on many days ten degrees apart or less (typical is about sixteen degrees). What was it about the air masses or jet stream in July 1908 that prevented temperatures from cooling down after dark? Was it a matter of air pollution/particles of soot that prevented the temperature from falling at night? This holding onto daytime heat is a characteristic of global warming experienced during the 21st century, but one hundred years ago it was unheard of.
The chart below compares July 1908's average high/low temperature to those of eighteen other hot Julys. What it shows is that the average highs of these hot months were two or three degrees warmer than July 1908, but July 1908's average low was warmer than every hot July by that same margin, with the exception of July 2013, which had the same sultry low.