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Warmest Days of the Year for the U.S.

By: Christopher C. Burt, 5:35 PM GMT on July 09, 2014

Warmest Days of the Year for the U.S.

NOAA recently produced an interesting map showing when the hottest day of the year is likely to occur in the contiguous U.S. Complimenting this map is one produced by Brian Brettschneider of Borealis Scientific, LLC, which illustrates the date of summer’s midpoint (peak of summer average temperatures) which was reproduced in my blog posted last August. Brian has also produced maps of such for the Fall, Winter and Spring seasons. There is also some other great material from Brian herein.

Below is the map that NOAA recently published:

The above map shows in what date range the HOTTEST DAY of the year falls on average (that is the daily maximum temperature, not daily average temperature). Map from NOAA.

And a similar map that Brian Brettschneider produced last summer (2013):

Brian’s map above shows what date range the maximum DAILY AVERAGE temperature normally falls, not the hottest daily temperature as in the NOAA map. This helps explain the differences between the two maps. Map produced by Brian Brettschneider.

Note the sharp difference in dates from Texas to the Southwest. The early peak of heat in the Southwest is because in late June or early July the summer monsoon begins to ramp up in the Southwest, and thus the associated clouds and rainfall keep afternoon temperatures lower for most of July and August versus June. Also note how the coastal areas of California, Oregon, and Washington normally experience their warmest temperatures during September, or even later, due to the summer coastal fog that dominates these regions between June and August.

Brian adds the following comments concerning the map he created:

Many people were surprised to learn that the annual march of temperatures begins to fall in portions of the desert southwest and the Ohio River valley several weeks before they reach their seasonal peak along the Gulf coast. Both maps use the same data set: NCDC 1981-2010 climate normal temperatures for over 7,000 stations, but have some subtle differences. My [Brian’s] map shows the peak date of the daily normal temperature (normal high plus normal low divided by two) while NOAA’s map shows the peak date of the highest temperature to be expected. In many cases, there is a 3-4 day difference between those two dates. Also, the NOAA map uses 15 color categories while my [Brian’s] map uses 8 color categories. It is worth mentioning that if a series of days all have the same daily temperature, the middle of the dates was assigned as the peak temperature date. For example, the normal daily average temperature for Minneapolis is 74.1° on July 11th-17th. Therefore, the middle of those dates, July 14th, was assigned as the peak summer temperature for the summer midpoint map.

Below is Brian’s map version for the ‘Winter’ months:

Map above produced by Brian Brettschneider.

The same methodology as used for the summer map can also be used to determine the date when winter temperatures reach their seasonal minimum; i.e., look for the date(s) where the daily normal temperature is lower than surrounding dates (see map above). Computationally this is a little challenging due to some dates falling before the New Year.

Factors determining midpoint dates

Brian explains: Summer midpoints are a little more straightforward to explain than winter midpoints since there is no snow variability to account for. Snow is never an issue in summer, it is always snow-free. What we find is that the annual temperature range is a strong predictor of when temperatures reach their summer peak. Annual temperature range is largely driven by the proximity to large bodies of water. Since the Gulf of Mexico water reaches its annual peak temperature in early August, most Gulf coastal stations reach their annual temperature peak at that time as well. In the middle latitudes, the influence of maritime air from the Gulf is greatly reduced. Farther east, the prevailing westerlies limit the influence of warm, maritime air from the Atlantic Ocean.

In most cases there is a strong relationship between temperature range and the summer midpoint date. Stations with large temperature ranges usually have an earlier summer midpoint and stations with small temperature ranges have a later summer midpoint. Stations along the immediate west coast and in Hawai’i do not follow this trend as they experience very low annual average temperature ranges. In addition to the annual temperature range, as an explanatory variable, seasonal variations in cloud cover and precipitation also have an impact on the date of the peak (and minimum) temperature dates (as seen in the Southwest during the summer).

Fall and Spring maps

For the Fall and Spring maps Brian has the following comments.

Deciding what constitutes the middle of fall and spring from a climatological perspective is not as straightforward as that for summer and winter. For starters, when exactly does spring or fall start and end? In the absence of a good definition, I [Brian] have arbitrarily decided that the middle of spring can be defined as the point exactly half way between the middle of winter and the middle of summer. Similarly, the middle of fall was defined as the point exactly half way between the middle of summer and the middle of winter. The above maps show the middle point for fall and spring respectively. Here is a selection of stations representing different climatological (temperature) figures for all seasons:

Dates of the summer, winter, spring, and fall midpoints for several stations across the U.S. The difference (°F) between the summer peak temperature and the winter minimum temperature is also shown.

Video Time-lapse Visualizations of Temperature Changes over the Course of a Year

Here is something really fantastic that Brian has created. It is a visualization of the progression of normal temperatures throughout the year as a cycle through all 365 daily normal temperatures for both the winter minimum and summer maximum temperatures. Brian has also generated a side-by-side mosaic of annual average temperatures using the same NCDC normal temperature data that the previous figures utilized. The link is found here. Be sure to check these out! They are awesome!

KUDOS: Brian Brettschneider of Borealis Scientific, LLC for his maps and commentary.

I will be away for vacation July 10-July 25. My next blog, ‘June Global Weather Extremes’ will be posted on July 26th. Please excuse my absence and the tardy posting of the ‘June Global Weather Extremes’ summary that I would normally post around the 15th of each month.

Christopher C. Burt
Weather Historian

Extreme Weather Temperature

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

Reader Comments

Thanks for sharing this Chris. If anyone is interested, here are the YouTube links for the high and low temperature visualizations as stand-alone videos:

  • Side-by-Side Panel

  • Low Temperature Progression

  • Low Temperature Progression

  • Precipitation Progression

  • Thanks for the chart! I'm surprised that some places well inland such as northeast TX, Arkansas, eastern Oklahoma have their hottest average temps in August, later than many places nearer the Atlantic and Gulf coasts.
    Thanks for your efforts Chris, greatly appreciated.
    Yeah...except for that cold weather thing hitting the Great Lakes and northeast next week.
    This disabused me of my notion that August was the hottest month in NY upstate - driest perhaps? Anyway, a lot of data merged into some cool maps. Thanks.
    Novosibirsk, Russia seems to have hit an all time record high on July 12th.

    98.6F / 37.0C according to wunderground history

    Wiki has the record as 97.9F / 36.6C.

    Novosibirsk is not on the Hererra list, is it possible to look into this?
    Quoting 6. DonnieBwkGA:

    Novosibirsk, Russia seems to have hit an all time record high on July 12th.

    98.6F / 37.0C according to wunderground history

    Wiki has the record as 97.9F / 36.6C.

    Novosibirsk is not on the Hererra list, is it possible to look into this?

    I think that there's often a delay to appearing on that list. I'm watching for the Kamloops, Canada airport to appear on that list too.
    I would expect a delay, sure, records have to be confirmed. It's just that Novosibirsk isn't on the list at all.
    Thank You for the research & hard work.

    my crappy 22cents.
    Other midpoints THOUGH NOT ACCEPTED as to pure physics...yet. Try finding the midpoint by using the actual start of passover (sidereal) hitting our Sun & the same, but as to "fall" (sidereal) where that "dark" area of space (dark nebulae and dust in Sagittarius ) creates a center focus onto the Sun. These might seem as a waste of time yet its what nature and our internal clocks use to switch ON/OFF as to summer & winter. i use them as to my Galacsic (did not misspelled Galactic, its my own word) calendar & my weather influencing device.
    How does this fit into this blogbit?
    Simple if one can find something that shows Earth is going off center as to its internal clock then one can prove that something man made threw Earth off centers as before even with much more CO2 (though natural ie Volcanic activities) Earth kept "in tune" (not just the 8Hz) with its center pinning(s) as to "Galacsic Spring & Fall"
    Hope i didn't go too far but since i'm on a neighbors compu'r (my compu'r "died" a week ago) if i'm banned again for leaving clues as to discoveries at least i'm banned for posting science/weather related things not cursing, trolling or personal arguments & my neighbors wanted me to post it,peace
    The new record all-time hottest temperature at the Kamloops airport is 40.7 C / 105.3 F, which was set on July 13. The previous record was almost as hot, at 40.6 C / 105.1 F.

    EDIT: See the update in comment 12.

    Hello how it come's Valkenswaard, Netherlands is not in the records
    We got the highest temperature of the Netherlands 31`degrees celcius
    Higher then any other city in The Netherlands.

    Greetings Angel Wynton

    http://www.wunderground.com/cgi-bin/findweather/g etForecast?query=pws:INOORDBR101

    Quoting 10. DCSwithunderscores:

    The new record all-time hottest temperature at the Kamloops airport is 40.7 C / 105.3 F, which was set on July 13. The previous record was almost as hot, at 40.6 C / 105.1 F.

    UPDATE: I was led to believe that the temperature of 40.7 C / 105.3 F was a record all-time hottest temperature at this location, but this might not have been an all-time record.
    Thanks Chris for the interesting post..
    Great use of charts to substantiate..
    I've bookmarked the vids and charts..
    Will mull them over..
    Thanks again for what you do.. :)
    Hi Dr. Burt, I would be interested in just what is going on with the cold blast we have seen hitting the midwest and east coast this summer... will they continue into August/September and what are the chances of another extreme winter?
    'The Northern Hemisphere Is on Fire'

    "Rarely has the continent been covered so extensively with smoke."

    Typically, those aren't the sort of opening words you want to read on a website tasked with monitoring the nation's air quality. But it's how Dr. Raymond Hoff, the physicist and professor emeritus at the University of Maryland who helps run the US Air Quality blog, framed his weekend edition.

    "Canadian wildfires and fires in the Pacific Northwest have conspired to make much of the northern half of the US a hazy, smoky mess," Hoff writes.

    As of the end of last week, here's how much of the continent was covered in smoke plumes—the light grey—and fires themselves, indicated with the red dots.

    Thank you for this enlightening visual. As a one time annual traveler to the San Diego/SoCal area, I've learned what one area calls a heat wave and too much humidity left me feeling so great about the summer weather I mowed the lawn as the June Gloom didn't show up that visit (the foggy time June-August that keeps their temperatures cooler and holds the heat out until later in their summer), while the jokes of a snowflake hitting the ground here and TX shuts down might make more sense to others.

    TX, most of it but not all, simply does not have the basic equipment to combat a serious ice and snow weekend, let alone icing on overpasses. No standing salting/sanding trucks that have only that duty, having enough sand in one town to do more than the base basics without equipment designed for the work, drivers with very little skill on ice or snow due to that. If we get a serious snowfall, into the half foot up to a foot range, which I've lived in twice, I don't think we could find a snowplow within 200 miles to get us out. We do regularly get bitter cold winters further from the Gulf Coast, nights below freezing and days that don't get far above that point. It's the ice from the sky that shuts the state down.

    Many here (line) dance, but very few drive on ice without terror involved.
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