July Is the Warmest Time of the Year for Much of the U.S.

Jon Erdman
Published: July 11, 2017

All across the country, temperatures are rising as we reach the middle of climatological summer's second month.

You may have an idea when the warmest time of the year is where you live. A map from NOAA's National Climatic Data Center (NCDC), shown below, lays out how the average warmest day varies across the Lower 48 states.

(MORE: July 2017 Temperature Outlook)

Of course, these are just long-term averages. In any given year, bulging areas of high pressure aloft can deliver the year's hottest weather a few weeks on either side of the average dates.

This map shows the average warmest day of the year across the Lower 48 states.
(NCDC)

There are several interesting features on this map that caught our eye, and here's a closer look at each.

After the Summer Solstice, Mainly

With the exception of a swath from the Big Bend of Texas to southeastern Arizona, the hottest time of the year over much of the U.S. arrives days, or in some cases weeks, after the summer solstice. This year's summer solstice occurred June 21 at 12:24 a.m. EDT.

(MORE: Here's a Fascinating New Way to Define the Seasons)

While the sun is highest in the sky, delivering its most direct radiation over the Northern Hemisphere, at the summer solstice, it takes multiple days or weeks to warm up the Earth's surface in order to make appreciable changes, and then for the air above it to warm. There exists, therefore, a seasonal lag between the solstice and the typical hottest day of the year.

Speaking of the Desert Southwest

That Big-Bend-to-southeastern-Arizona swath comes relatively early compared to other locations, thanks to the North American monsoon – a seasonal shift in wind direction bringing increased moisture fueling more frequent showers and thunderstorms as we progress through July.

(MORE: Southwest Monsoon 2017 Forecast)

These are the average monthly temperatures for Phoenix, based on 1981-2010 averages.
(NOAA/NWS)

June 13, the first U.S. climate station (out of 7,454) reached its warmest time of the year, according to climatologist Dr. Brian Brettschneider of the University of Alaska-Fairbanks and the Western Regional Climate Center (WRCC).

July for Many

For a swath from the central Rockies to the central Plains, upper Midwest, Great Lakes, Ohio Valley and East, July is the warmest month.

These are the average monthly temperatures for Washington D.C., based on 1981-2010 averages.
(NOAA/NWS)

(MORE: Here's When You Can Expect Your First 80- and 90-Degree Temperatures)

August For the Deep South

Shaded in green and blue, early to mid-August tends to be the hottest time for a swath of the South from central and eastern Texas into the Ozarks, lower Mississippi Valley and Tennessee Valley.

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.

These are the average monthly temperatures for Houston, based on 1981-2010 averages.
(NOAA/NWS)

Early August is also typically the hottest time along the northern tier of states from North Dakota to interior Washington.

West Coast Lag

As you can see, parts of the West Coast aren't typically their warmest until September.

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, 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.

These are the average monthly temperatures for downtown San Francisco, based on 1981-2010 averages.
(NOAA/NWS)

What About Alaska and Hawaii?

July is generally the warmest time of the year for much of Alaska, but along the southern coastal areas, peak warmth doesn't occur until the last week of July into August.

This map shows the average warmest day of the year across Alaska.
(NCDC)

In Hawaii, most areas don't see their warmest time of the year until August or September. Some high-mountain locations see peak summer temperatures much earlier, however, in July or even June.

This map shows the average warmest day of the year across Hawaii.
(NCDC)

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