Rusty Shackleford

A major heatwave will begin next week across the central and eastern U.S. High temperatures in the 90s and 100s are likely as far north as Southeastern Pennsylvania. So, summer basically lol. Undoubtedly, there’ll be plenty climate hysteria to go around. But neither the number of hot days nor heatwaves have increased in the U.S. since 1895. The chart on the left shows the average annual number of days reaching 95°F (35°C), 100°F (37.8°C) and 105°F (40.6°C) per station at 657 United States NOAA GHCNd stations (area weighted) with at least 100 years of daily data and 90% daily completeness from 1895 to 2025. The long-term trend is down. ⬇️ 𝐓𝐨𝐩 𝟏𝟎 𝐲𝐞𝐚𝐫𝐬 𝐰/ 𝐦𝐨𝐬𝐭 𝐝𝐚𝐲𝐬 𝐓𝐦𝐚𝐱 ≥𝟗𝟓°𝐅: 1️⃣ 1936 2️⃣ 1934 3️⃣ 1954 4️⃣ 1931 5️⃣ 1933 6️⃣ 1913 7️⃣ 1925 8️⃣ 1980 (most recent) 9️⃣ 1930 🔟 1911 Interestingly, only one of top 15 has been in the last 70 years, and only two in the 21st century made the top 20 (2011 and 2012, which rank 17th and 19th place, respectively). 𝐓𝐨𝐩 𝟏𝟎 𝐲𝐞𝐚𝐫𝐬 𝐰/ 𝐦𝐨𝐬𝐭 𝐝𝐚𝐲𝐬 𝐓𝐦𝐚𝐱 ≥𝟏𝟎𝟎°𝐅: 1️⃣ 1936 2️⃣ 1934 3️⃣ 1954 4️⃣ 1930 5️⃣ 1901 6️⃣ 1913 7️⃣ 1980 (most recent) 8️⃣ 1931 9️⃣ 1925 🔟 1918 𝐓𝐨𝐩 𝟏𝟎 𝐲𝐞𝐚𝐫𝐬 𝐰/ 𝐦𝐨𝐬𝐭 𝐝𝐚𝐲𝐬 𝐓𝐦𝐚𝐱 ≥𝟏𝟎𝟓°𝐅: 1️⃣ 1936 2️⃣ 1934 3️⃣ 1954 4️⃣ 1930 5️⃣ 1901 6️⃣ 1980 7️⃣ 1913 8️⃣ 2023 (most recent) 9️⃣ 1918 🔟 1933 The heatwave in Texas and Oklahoma in 2023 bumped it up to 8th place. But a more robust metric to assess time-dependent changes in extreme heat events would be to look at the area-weighted average number of heatwaves per station per year. The chart on the right shows that. Heatwaves are defined here as a ≥3 consecutive day period where the daily maximum temperature is ≥90th percentile (against 1991-2020 averages) for that date for that station and for the months May-September. Once again, the trend is down. ⬇️ 𝐓𝐨𝐩 𝟏𝟎 𝐲𝐞𝐚𝐫𝐬 𝐰/ 𝐦𝐨𝐬𝐭 𝐡𝐞𝐚𝐭𝐰𝐚𝐯𝐞𝐬: 1️⃣ 1936: 5.77 2️⃣ 1934: 5.39 3️⃣ 1933: 5.15 4️⃣ 1931: 4.90 5️⃣ 1911: 4.85 6️⃣ 1954: 4.71 7️⃣ 1925: 4.59 8️⃣ 1930: 4.41 9️⃣ 1913: 4.35 🔟 1952 / 1939: 4.34 None of the top 20 have been in the 21st century. The most recent summer with the greatest number of heatwaves per U.S. station occurred in 1988. All other 19 occurred 70+ years ago. It is also worth mentioning that whether or not you consider the 1930s to be an outlier, 2012 is the most recent entry at #30. Most of the warming that we measure is in overnight low temperatures, not daytime highs, and a significant amount of that could be due to urbanization. So, if it reaches 100°F at your house next week, it’s just summer. Carry on! 🏖️ 🌴