Summer's in full swing across the USA. Actually...wait. What if we told you some parts of America have already seen their hottest day, on average? It's true! Before getting into July, when school is out and the beaches pack up, a few places in the United States are already on their way down the temperature chart. For the vast majority of us, though, we're still climbing into peak heat, so hop on our WeatherWorks tour bus (yes, groupies are welcome) as we ride on through and explain when and why places see their climatologically hottest day of the year.
Our first stop is where the hottest days of the year already occurred - in the Desert Southwest: West Texas, New Mexico and Arizona. With the blistering summer sun, how could the heat peak so early? We can thank the North American Monsoon, which bubbles up from Mexico into the area, and makes it harder for the sun to heat things up. So while the temperatures may be "colder", it's not a dry heat anymore, it's humid! (Image: Monthly average temperatures and precipitation for El Paso, TX. Note how temperatures peak in June, before the clouds and rain increase in July and August)
As we continue our road trip into early July, we have a lot of ground to cover, as a large chunk of the country sees their hottest day of the year, on average. This peak summer heat runs from Phoenix to Santa Fe on up into Omaha and even up to our Storm Alert clients in Chicago, Indy and Columbus.
So what is the reason for the peak here? In the Southwest, the North American Monsoon expands into this area, bringing in a more muggy heat, as opposed to a dry, but "hotter" temperatures. Meanwhile, here's our theory for an early July peak in the Midwest: With vast areas of farmland producing crops, evapotranspiration, or the process in which water is transferred from the land to the atmosphere, becomes very high starting in mid-July. During this time, the air becomes increasingly humid, as water from the crops and soil is picked up into the sky, making it harder for air temperatures to rise quickly. Thus, the air cannot heat as quickly in mid-July as it would in early July. (Image: Average daily high, low and average temperature for Indy during July. Note how temperatures drop after the 10th)
Mid to Late July (and the first few days of August)
Heading into the midway point of summer, much of the Southwest, Western U.S. and the East Coast see their warmest days of the year. Although the highest daily net radiation gain is seen on the Summer Solstice, mid to late July is when much of the Northern Hemisphere reaches their hottest time of the year. This is mostly due to a seasonal lag in peak temperatures. In other words, it takes time for these areas to recover from the heating deficit built up during the winter months. (Figure: Monthly average temperatures and precipitation for Newark, NJ.)
After a many oil changes for our bus along the way, we arrive into August, making stops in the western portion of the Deep South, much of Texas and South Florida. We can thank the soothing shore breezes for the peak heat here. In the Deep South and Texas, the wind direction is most frequently from the south or southeast during the summer, drawing in sultry air from the Gulf of Mexico, whose water temperature peaks in August. Therefore, we mix a still positive net radiation balance with the warmest waters of the year to give the region the hottest temperatures of the year, during this time.
South Florida operates much in the same way. With most wind directions a sea-breeze they too peak in August, during the early to middle portion of the month. By late August, temperatures begin a (very slow) climb down. (Graphic: Average monthly high temperature, precipitation and Gulf of Mexico water temperature for Houston, TX. Note how August has the highest air and water temperature, all with lower rainfall than the surrounding months)
Finally, as the sun gets lower on the horizon, the nights become longer and even the leaves begin to change colors for the year well to the north. However, we keep our bags packed and head to the West Coast. For much of the year, the prevailing westerly winds keep an inversion and fog over Los Angeles, Coos Bay, Oregon, as well as the coast of Washington. As a result, the sun cannot break through the clouds much, making it damp and only mild. However, during September, Santa Ana winds become common, blowing in dry air from the deserts, with temperatures sometimes soaring into the 90s and 100s!
While not every September and October day feels like summer, there are enough of them during this time to bring the monthly average up. Therefore, this becomes the hottest part of the year. In fact, the all-time record highs in San Diego, Los Angeles, San Francisco and Astoria, Oregon (to name a few) all ocurred during September. (Image: Monthly average temperatures and precipitation for San Francisco, CA. Note the peak in temperatures during September.
(Day of warmest high temperatures across the United States. Courtesy of NOAA.)
As We End...
With tens of thousands of miles on the bus, plenty of Storm Alerts sent to our clients and more than a few rounds of golf, we finally park the bus. While we couldn't get out there at the time, we didn't forget about Alaska, who sees its peak heat in July. Meanwhile, Hawaii's hottest month falls in August.