John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Mapping when temperatures hit the max all over the U.S.
As the sun beats down ever harder, turning 'pits and brows into squelchy geysers for sweat, it's time to ask: Is this the hottest day of the year?
If you happen to live in the Southwest, the answer is "possibly yes." Temperatures typically spike in southern New Mexico and Arizona and western Texas during the latter half of June, right before the monsoon season rolls in with its majestic haboobs. Residents of Phoenix and Albuquerque can usually look forward to the rest of the summer cooling off, thanks to the appearance of more clouds and rain (though with the ongoing drought, who knows if that will happen in 2014).
As for peak heat elsewhere, take a look at this map showing the average date when temperatures hit their max throughout the nation:
Put together by the National Climatic Data Center using data from 1981 to 2010, the feverish cartography shows how the hottest weather typically moves from the Southwest to the East and Midwest to the West and South before finally spreading to the edge of the Pacific.
California's late-blooming coast is in a class all by itself, experiencing its most T-shirt-moistening warmth in August and September. As to why that is, the NCDC explains that the "persistence of the marine layer along the Pacific Coast leads to cool temperatures in early summer with the warmest days on average later in the season."
Shaded in green and blue, early-mid August tends to be the hottest time for a swath of the South from central and east Texas into the Ozarks, Lower Mississippi and Tennessee Valleys.
By that time, the somewhat daily, slow-moving thunderstorms of early summer will have given way to more persistent high pressure aloft over the South, suppressing clouds and drying soil, helping temperatures to soar.
And here's a more detailed explanation for the West Coast's late-season heat:
In the heart of summer, the combination of the hot interior deserts and cool air over the ocean drives winds onshore along the coast, keeping low clouds and fog and cool air firmly in place.
By September, the upper-level wind pattern can set up to drive hot, dry winds from the deserts to the coast and offshore, known as Santa Ana winds in Southern California, and Diablo Winds in the Bay Area.
This map will likely look much different in the future as the atmosphere heats up. Globally, this past month was the warmest May on record dating back to 1880, and April likewise was the hottest on record, with abnormal torridness in much of the American West and Alaska.