June 21, 2017

Hottest Day of the Year Map

Here is an interesting map showing the average day with the warmest temperatures around the U.S.   Amazing differences, with some places having their warmest temperatures in June (southern Arizona and New Mexico), while others having their typical high temperatures in September (coastal CA).  

A three month range!

Why do different locations have such radically different dates of warm temperatures?   Let's think about it.

On a superficial level, one might suppose that the hottest day of the year would be the day when the sun is strongest and most overhead:  the summer solstice (June 21st).

But thinking about it a bit more carefully, it is apparent that warming occurs as long as the amount coming in, i.e., the solar radiation, exceeds what is going out (infrared radiation to space).  And that might be  later than the maximum solar radiation.

Consider the daily (diurnal) temperature variation--the warmest temperatures are not at solar noon (roughly 1 PM PDT) but several hours later (around 5 PM) in summer.    Furthermore, there is thermal inertia of the atmosphere and the surface that further delays the warming (takes a while for the soils, water, and air to warm up).

So one might conclude that having the warmest day of the year in July or early August might be expected.

But why are things so different around the country, with the warmest day ranging from near the solstice (June 21st) to the equinox (Sept 21)?

I think I can offer an explanation.

First, consider the very early dates from western Texas to Arizona.  I think those are due to the North American or Southwest Monsoon, a period of clouds and thunderstorms over the interior SW U.S. that develop in late June and extends to early September.  With all the clouds and moisture moving in during late June, the high temperatures are lowered during mid to late summer, pushing the highs into June.

But why are the high temperatures so late in the summer in steamy eastern Texas, Louisiana and Arkansas?    Because they have a lot of thunderstorms and clouds during late spring and early summer, which delays the onset of high temperatures.  This precipitation characteristic is illustrated by the monthly precipitation in Houston, Texas (see below), with June being their wettest month.

Moving back to the western U.S., most of the interior has a late July maximum, which makes sense from the radiation viewpoint discussed earlier.   But what about the craziness along the thin coastal strip, where many locations don't hit their high temperatures until late August or September?

The reason has to do with the cool Pacific Ocean and the eastern Pacific high that dominates the region from late spring into mid-summer.  The Pacific Ocean is cool year around (roughly 50F near the West Coast).   During spring and most of the summer, high pressure exists offshore, with lower pressure over Arizona (see surface pressure map for June), which creates a large onshore pressure gradient along the West Coast that pushes the cool air and low clouds into

the coastal zone, keeping things cool (see satellite image for Monday afternoon).

By September, the situation changes (see below), with pressure building over the Pacific Northwest and the coastal pressure gradient weakening.
Occasionally, high pressure builds further over the inland western U.S., as cold air starts to move southward, producing offshore flow that is much warmer than the cold onshore flow off the Pacific.  Furthermore, the offshore flow can be warmed by compression as it sinks along the West Coast mountains, producing high temperatures along the coastal strip.  These situation produce the high temperatures during late summer near the coast.

In short, the variations in the dates of the high temperature days actually make sense with a little meteorological sleuthing.


  1. Remember where Earth is in its (our) elliptical orbit around the Sun - perhelion and aphelion:

    Year Perihelion Distance Aphelion Distance
    2017 January 4, 2017 6:17 am 91,404,322 mi July 3, 2017 1:11 pm 94,505,901 mi

    Minor note on this year's temps, at least here in Glacier - we're still having cool nights. Last night's low was 39.7 F. Mt Baker ski area, due to altitude, is almost always 10 degrees colder than Glacier. Just sharing.

  2. Really interesting! Clarified a lot of questions I had about this particular issue but had never bothered to track down. Thanks Cliff!

  3. A very interesting map. I am a native Midwesterner but came to the San Francisco area in 2006 and then up to Seattle in 2012. It was very interesting to me that some of the hottest days in San Francisco were in October and even early November.

    Curiously, though, the reverse seems to happen with respect to the coldest day of the year. It seems to come earlier on the west coast than in most of the country. It looks like the lowest normal high of the year in Seattle is 45 degrees, which occurs from December 17 through December 31; take the midpoint of these days and Christmas Day is the coldest day of the year. So, with a late summer maximum and an early winter minimum, our climatology seems to be one of a long, slow warmup and a rather rapid autumn cooling. That seems counterintuitive for a maritime location where you would think that the heat capacity of all that Pacific water would cause a delay in when we hit our winter minimum.

  4. I don't mind the heat. I just hope it won't be like last time where we still got hot with onshore flow. I remember in June 5 last year, it was 93 with a 55ish dew point. To my knowledge, thermal troughs drive dew points down to upper 30s/low 40s. That's what I'm wishing for this weekend.

  5. Interesting observation Andrew. I would agree it seem the coldest tempetures for me at the Admiralty inlet is mid November to mid December.

  6. Yes, Andrew, it's not a sine curve like some places. I remarked in this before once. Cliff, care to comment?

  7. Somewhere I read that the hottest day in the US is on average July 26, but this clears up the details very nicely, thanks!

  8. Do you have the same info for the coldest day of the year?


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