Stuck in the 70s
August 2018 Weather Recap: Unusually Wet, With Unusually Warm Nights

Lackluster Heat Waves




This post was inspired by the rather tepid four-day heat wave of Aug. 5-8, 2018, which had an average high of 90.8° (with highs of 91°, 92°, 90° and 90°).  Of the close to 70 four-day heat waves that have occurred since 1870 (thru June 2023) only one, in July 1896, had a lower average high, 90.3°.  (2018's low-grade heat wave matched one in August 2009 and would be matched again in June 2021.)  On average, four-day heat waves have had an average high close to 94°.  (The hottest four-day heat wave on record took place in the summer of 2010, when the high temperature from July 4-7 was 99.5°.)


Chart- most tepid 4-day heat waves

However, the story changes when low temperatures are included in the analysis.  For example, August 2018's heat wave had an average low of 75.0°, which was 1.3 degrees warmer than the average four-day streak and warmer than two-thirds of the four-day heat waves examined.  When combined with the average high, the mean temperature ranked as 27th coolest - quite a difference in ranking compared to its average high alone.  (This follows the weather storyline of this century, whereby nighttime temperatures in New York are warming more than daytime temperatures.)


Chart - coolest 4-day heat waves based on mean temp


Finally, while the typical four-day heat wave had a 20-degree difference between its high and low, this August's was 15.8 degrees apart, which was the third smallest diurnal variation of the heat waves studied (a heat wave in July 1995 had the smallest, 14.7 degrees, while the second smallest was in July 1870).  


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does this analysis include heat waves that have lasted for more than four days or just four days exactly? also, did 2018 set some kind of record for one of the warmest beginnings to August since the first ten days all had highs above 80° and lows above 70°?


This analysis was limited to heat waves of four days in length. Regarding your second question, the hottest beginning to August was in 1955 when the first seven days were all in the 90s and averaged a high/low of 96/75. And the first ten days of August 2001 had an average high/low of 93/74, with eight days in the 90s.


Since this post concerns four day heat waves: The heat wave of June 6-9, 2021, (highs of 92, 91, 90, 90; average = 90.8) tied the heat waves in August 2018 and August 2009 for the heat wave with the second lowest average high temperature. Even though the high humidity in June 2021 pushed the heat index up to 100 degrees at one point and made it feel uncomfortable, the high temperatures were not too hot as to challenge record highs in 1925 and 1933.


I'm glad you found this post. (I'll have to update the chart.) Yes, hot temperatures, at least in CPK, seem to have been suppressed during the past 10 years compared to hot days of the past. But, despite "cooler" hot readings, the mean temperatures are often higher, because daily lows are noticeably warmer; this seems to be how climate change is having its impact (at least in CPK).

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