A map of the warmest day of the year in the lower 48, based on average temperatures from 1981-2010. (NOAA)

As we push toward the end of July, many in the United States have seen their average warmest day of the year. But for some, the heat will peak well into August or even September.

Many people wrongly assume that the hottest month of the year in Washington, D.C., is August. After all, that’s when everyone vacates the capital and Congress goes into recess, and it makes sense that people would want to clear out when the mercury spikes.

But for those in D.C., the average warmest day of the year passed nearly two weeks ago, on July 15. This month’s average high temperature is 88.4 degrees, and August is a bit cooler at 86.5 degrees. Troughs carrying cooler air have a harder time punching this far south in July, but by August, a lower sun angle takes the toasty edge off.

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August does register the average warmest day of the year for many other locations in the U.S., though, including parts of New England, the northern Plains and Rockies and a vast swath of the South. Parts of coastal Louisiana and Texas still have a long way to go to see their hottest day of the year, which won’t come until the third week of August.

(NOAA) (NOAA)

That’s quite a difference from southwest Texas, which sees its warmest day in the first two weeks of June — the earliest in the lower 48. Much of the Southwest is seeing peak heating in June, with a sharp line of demarcation through West Texas.

Intuitively this makes little sense, until you consider the effects of the Southwest monsoon, or the North American monsoon, which really gets going starting in July. The persistent, monsoon-y cloud cover tends to keep the heat at bay through the rest of summer, which means the warmest days come right before its onset. It’s actually the rapid heating in June and early July that drives the monsoon itself, creating a vast area of low pressure which draws moist, tropical Pacific air northward.

What’s up with the West Coast? Up and down the coast from southern California to Washington, the warmest day of the year comes in August and even as late as September.

The culprit there is similar to the Southwest monsoon — during the months when the sun is highest in the sky, inland areas heat up quickly, which draws a cool layer of marine air on shore. Ever heard of “June gloom?” Instead of sunshine and blue sky, the early summer months on the West Coast tend to feature low clouds and blowing fog, which often catches visitors hoping for a warm retreat off guard.

(NOAA) (NOAA)

But as autumn sets in the pattern reverses, quite literally, and the flow turns from onshore to offshore — which spreads warm, dry air across the coast. The Santa Ana winds in Southern California are an example of this reversal, and some of the hottest days of the year can come from this phenomenon.

Hawaii sees some of the latest peak heating in the U.S., but it has little to do with the sun. Surrounded by the vast Pacific Ocean, Hawaii’s weather is dominated by water. The ocean helps to regulate the climate, preventing big spikes in temperature. Thus, Hawaii’s warmest days come when ocean temperature peaks for the year — something that happens in the fall rather than the summer.

Water takes more energy to heat up than air does, and it also takes longer to cool. All through the summer months, there’s more energy coming into the oceans than is being emitted, so the temperature continues to crank up until the fall, when the sun angle finally gets low enough for the process to reverse and for oceans to cool down. Since Hawaii follows the temperature of the surrounding ocean so closely, its peak is also very late in the year.