In weather discussions, low temperatures are like Cinderella - largely an afterthought. Case in point, it was reported that July 2013 was New York's eighth warmest on record(now ranked ninth), but what was ignored was the fact that July's average low temperature of 73.4° was the warmest of any July in 100 years (and second warmest of all time, behind July 1908). It beat the previous Julys with warmest lows, July 1999 and 2010, by a wide margin - 0.8 degrees. It was only because of these warm nights that July 2013 ranked as high as it did, since July's average high of 86.3° was just the twenty-fifth warmest.
What also stood out was July's small diurnal temperature variation of 12.9 degrees (i.e., the difference between the month's average high/low of 86.3/73.4). In the past 100 years only July 2004's variation was smaller, and just by 0.1 degree (1996 had the same variation as this July). Meanwhile, other Julys with warmer high temperatures than this year's had diurnal variations averaging 18.1 degrees.
A final observation - Although July 1966 is in the record books for having the highest average high temperature of any month, 90.3°, its average mean temperature of 79.7° is 0.1 degree below July 2013 because its average low was 4.3 degrees cooler than 2013's (69.1 vs. 73.4). That July had a diurnal variation of 21.2 degrees.