Warmest Days of the Year for the U.S.

Published: 17:35 GMT le 09 juillet 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

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About The Author
Christopher C. Burt is the author of 'Extreme Weather; A Guide and Record Book'. He studied meteorology at the Univ. of Wisconsin-Madison.

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