Wednesday, June 6, 2018

Warmest Day of the Year

"Just wait until the dog days of August" - Anonymous

June is upon us and many locations have already experienced triple-digit heat. Some locations in the Upper Midwest topped the century mark in May! Will these be the warmest temperatures of the year? Of course no one has a crystal ball, but climatology allows us to peek into the record books and figure out what time of the year is the hottest.

The thing about climatology is that no two places are exactly alike. It surprises no one that the climate of Texas is different than the climate of California. What most people do not realize is that patterns of hot/cold and wet/dry are wildly variable. For example, summer is the wet time of year in the central U.S., but summer is the dry time of year along the West Coast. Temperatures share a similar characteristic of varying across relatively short distances.

Back to the hottest day of the year. When I was growing up in eastern Texas, everyone knew that the hottest time of the year was August. The map below shows the average date (actually the median date) for the occurrence of the single hottest day of the year. It turns out that this aphorism is accurate for only a small slice of the U.S. centered on eastern Texas and adjacent areas.

People that live in western Texas have a much different view of the summer. Residents of El Paso and Midland, Texas, know that late June is the hottest time of the year. Why such variability? The energy output from the sun does not change along lines of latitude. The reasons for the variability of the annual maximum temperature dates are complex. 

In eastern Texas, July and August are the two driest months of the year. Without much soil moisture, the sun's energy is fully devoted to heating the ground – not evaporating water. In western Texas and southern Arizona/New Mexico, June and early July is the sunniest and driest time of the year. Later in July, the Southwest Monsoon sets in. To a large degree, cloud and precipitation climatology drive the variations in the map. More clouds and precipitation mean cooler temperatures during the warm time of the year. Fewer clouds and precipitation mean higher temperatures.

How often is June the month where the highest temperature of the year is observed? What about July? August? The three maps below show the relative frequency for the the hottest day of the year to occur in each of the three core summer months (June, July, and August). Only a small area in the southwestern U.S. is likely to see the hottest temperature of the year occur in June. The same is true for August (more so than June though). The big "winner" is July. For 78% of the U.S. and Canada, July is the most common month to observe the highest temperature of the year.

Perception of weather and climate does not usually match what the data tells us. In the case of the hottest time of year, the Venn Diagram of perception and reality has significant overlap, but far less than most people realize.

If we revisit this article at the end of July and ask if the hottest temperature of the year is behind us, for the vast majority of you, the answer will be yes.

Bonus maps:

© Brian Brettschneider

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