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When and Where in the U.S. Does the Summer Heat Reach its Peak?

By: Christopher C. Burt, 8:12 PM GMT on August 06, 2013

When and Where in the U.S. Does the Summer Heat Reach its Peak?

Mike Seidel of The Weather Channel shared an interesting story with me concerning just which week of the summer is the hottest for various regions of the United States. Here’s a brief recap of that.

Environmental scientist Brian Brettschneider (of SWCA Environmental Studies in Anchorage, Alaska) produced a fascinating map of the U.S. illustrating which week of the summer normally proves to be the hottest.

Map of which week is normally the warmest for a typical summer in the U.S. Brian explains, “The map is based upon data from 7,800 stations across the country, using their 1981-2010 climate normals. If a number of days have the same peak value, the selected date is the middle of those dates. For Bush Intercontinental Airport, the official weather station of Houston, the dates of peak temperatures are August 7th – August 11th (85.1 F). The midpoint of those dates is August 9th.” Map courtesy of Brian Brettschneider.

Of special interest is the contrast across the state of Texas.

A close up of Texas where the peak of the heat ranges from June in the far western region to mid August in the southeastern portion of the state.

Brian explains the anomaly here,

“First, the water temperature in the southern Gulf of Mexico is generally warmer than the northern Gulf of Mexico, which means the air temperature near the water will retain its heat for a longer duration after the solar angle starts decreasing. Since the prevailing wind during summer is from the southeast, you will also notice a NE-SW orientation to the isolines. It is also possible that the northern Gulf experiences more sea breeze cloudiness later in the summer, which will bring the afternoon temperature down a little. I am not sure why the area along the Neches River is earlier than other areas nearby.”

I would propose that the reason for the peak of the heat occurring in June in western Texas is a result of maximum diurnal radiation occurring at that time, and also that June is usually a very dry month whereas in July monsoon moisture begins to flow into the Southwest (including extreme western Texas) thus inhibiting maximum afternoon temperatures due to increased cloud cover.

El Paso, Texas normally experiences its hottest temperatures of the year in June. The average daily maximum temperature in June is 96.5° versus 96.1° in July and 93.5° in August.

It is also interesting to see how late in the summer the ‘heat peak’ occurs in coastal California and Oregon. In fact, September is normally the warmest month of the year along the immediate West Coast when the prevailing wind begins to flow offshore inhibiting the cool marine layer that normally penetrates inland several miles during June through August.

How does all this correlate to extreme maximum temperature records?

Indeed, if we look at all-time record high temperatures we see that they have occurred in June for most locations in southern New Mexico, southern Arizona, and western Texas with some of these locations even experiencing their warmest single month on record in June as well:

El Paso, Texas: 114° on June 30, 1994. Warmest month on record: 89.0° June 1994.

Midland, Texas: 116° on June 27, 1994. Warmest month on record: 88.0° June 2011.

Roswell, New Mexico: 114° on June 27, 1994. Warmest month on record: 85.4° July 1980.

Tucson, Arizona: 117° on June 26, 1990. Warmest month on record: 90.6° July 2005.

Conversely, the hottest temperatures at many locations of eastern Texas have occurred in September (but no sites have had September as their warmest month on record):

Austin, Texas: 112° on September 5, 2000 (and August 28, 2011)

San Antonio, Texas: 111° on September 5, 2011

Corpus Christi, Texas: 109° on September 5, 2000

Houston, Texas: 109° on September 4, 2000 (and also on August 27, 2011).

Cool marine air and near perpetual fog from June through August keep much of coastal California and Oregon from measuring warm summer temperatures. September, when the prevailing winds begin to flow offshore, result in that being the warmest month of the year for these locations. Photo by Scott Sawyer.

Looking at the hottest temperatures at many coastal locations of California we see these also have occurred in September (or even October!):

San Diego, California: 111° on September 15, 1963. Warmest month on record: 78.9° September 1984.

Burbank, California 113° on September 12, 1971. Warmest month on record: 82.0° July 2006.

Eureka, California: 87° on October 26, 1993. Warmest month on record: 62.2° September 1979.

Bandon, Oregon: 100° on September 21, 1990. Warmest month on record: 62.3° August 1997.

San Francisco, California’s warmest month on record was September 1984 with a 69.4° average. Ditto for Los Angeles with an average of 81.3° that same month.

KUDOS: To Brian Brettschneider (of SWCA Environmental Studies, Anchorage, Alaska) for the maps and Mike Seidel at TWC for bringing this to my attention.

Christopher C. Burt
Weather Historian

Extreme Weather

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.