Global warming at work: July was the hottest month of nights in US history

In Galveston, 24 out of 31 nights set record low temperatures last month. The average daily high in Texas was 100.2, making July the second-hottest month on record for the state by this measure.

The Frazier Fire rages across the prairies of Wichita County, Texas on July 22. Texas had its hottest and fifth driest July on record. Texas A&M Forest Service

Bob Henson, editor of Texas Climate News, wrote this article for Yale Climate Connectionswhere he originally appeared.

As it shatters records in the Northern Hemisphere, the scorching summer of 2022 has ousted the hottest month of nights in US history. Sweltering nights are a source of problems during major heat waves, as they give human bodies in non-air-conditioned places less chance to recover from the more intense heat of the day.

In his July 2022 national climate roundup, NOAA reported Monday, August 8 that the monthly average temperature of 76.42 degrees Fahrenheit (24.68 degrees Celsius) across the contiguous United States was the third warmest on record, just behind July 1936 and July 2012.

It was one of the ten warmest Julys for 20 U.S. states in an arc stretching from the Pacific Northwest to the southern plains, across the south-central, and into central Atlantic and southern New England (see Fig. 2 below).

A preliminary total of 30 U.S. sites recorded all-time highs in July, and 18 more tied their previous all-time highs, according to NOAA. Daily weather records website.

Cozy by day and still warm by night

Even more remarkably, the national average daily minimum temperatures (63.57°F) were the warmest of any month in the 128 years of record keeping in the United States, slightly exceeding 63.55°F. of July 2012.

The 10 months with the warmest average daily lows have all been Julys, and nine of those 10 months have occurred in the past 25 years. Since 1970, daily July lows have warmed more than 2.5°F and daily July highs have climbed about 1.8°F.

Daily minimum temperatures for July since 1895, averages over the month and over the contiguous United States. The linear trend since 1970 is also shown, capturing the era of increased warming over the past half century. NOAA/NCEI

Theory and observations indicate that a planet warmed by human-produced greenhouse gases will tend to get more of that warming there and when it is normally cool: at higher latitudes, in winter and at night. . A landmark 1993 study found that daily lows were warming three times faster than the daily maxima over the northern hemisphere. This conclusion does not mean that summer days are immune to human-induced heating, just that the temperature rise at night tends to be even greater.

Two US states recorded their hottest average daily lows for any month on record in July 2022 by substantial margins:

  • Colorado: 15°C (old record of 15°C from July 1954)
  • New Mexico: 18.0°C (old high of 18.0°C from July 2011)

The trend isn’t limited to the United States, of course. So-called “tropical nights” (those with lows of 68°F or higher) are increasingly common in the UK, as Stephen Burt (University of Reading) underlined in an essay on July 15, just before the peak of a UK record heat wave. Burt noted that half of all tropical nights since 1814 in Oxford up to that point had occurred in the past 25 years.

According to a global analysis published in 2020, places with the most skewed response to warming (i.e. places where nights warm up fastest compared to days) tend to become cloudier, wetter, and wetter.

Daily minimum temperatures for July since 1895, averages over the month and over the contiguous United States. The linear trend since 1970 is also shown, capturing the era of increased warming over the past half century. NOAA/NCEI

Of the U.S. cities that had their warmest July on record in 2022, based on average temperature (daily lows plus daily highs):

  • Salt Lake City: 30°C (old record 30°C since 2021)
  • San Antonio, TX: 89.8°F (old high of 88.7°F from 2009)
  • Tampa, Florida: 86.3°F (old high of 85.15°F from 2020)

The statewide average for daily highs in Texas in July 2022 was 100.2°F, which topped the July record of 99.8°F from the infamous 1980 heat wave. The single hottest month for daily maximums in Texas was the 101.6°F recorded in August 2011 during another brutal heat wave.

As for the hot nights, it’s hard to overstate what happened in July 2022 in Galveston, Texas. 24 out of 31 nights set record low temperatures. During the last 13 days of this month, the temperature never dipped below 84°F, and there was a tie at eight for the hottest night on record in July (86°F).

A tale of two humidity anomalies

Rainfall was very mixed in the United States in July. The drought spread to a large part of the country (51%) at the end of the month, according to the US Drought Watch. The severe drought intensified in several areas, particularly in the southern plains.

Statewide ranking of average rainfall for July 22, compared to each July since records began in 1895. Darker shades of green indicate higher rankings for humidity, with 1 indicating driest month on record and 128 wettest. NOAA/NCEI

During this same period, massive downpours snuffed out records and led to destructive flash floods in other places. A targeted area of ​​flooding stretched from the Midwest to Appalachia, as record rains fell in both St. Louis and eastern Kentucky. Catastrophic flooding followed in the latter, killing at least 37 people.

Last month was the second driest July on record in Rhode Island, the fifth driest in Texas and the tenth driest in Connecticut. The only state where it ranked in the top ten wettest months was Kentucky, which had its fourth wettest July on record.

Bob Henson is editor of Texas Climate News. Colorado-based meteorologist and science writer, he is the author of “The Thinking Person’s Guide to Climate Change.”

Teresa H. Sadler