Temperatures soared across the eastern U.S. to kick off the workweek, the mercury some 25 degrees or more above normal in spots for the fourth of five days in a row. For many, the warm weather marked a genuine “second summer,” a stretch of unseasonable warmth coming in the middle of fall. But how unusual is this mildness?

In the Mid-Atlantic and Northeast, second summers occur once every two to four years, making them the exception rather than the rule in a particular year, but not all that rare. However, having one with temperatures as high as this November warm spell is quite rare.

Over the Plains and Intermountain West, battling air masses wage meteorological war during the fall, the seasons sloshing back and forth. In some parts of the northern High Plains, wild temperature swings characteristic of autumn make second summers almost routine.

Looking back at a decade’s worth of data from 10 cities across the country, it’s clear how rare these bursts of warmth are depending upon the location.

What is a second summer?

While there is no strict technical definition as to what constitutes a second summer, writers in magazines, novels and other works often use a sudden revitalization of summer weather to evoke feelings of nostalgia or allude to events in the past.

The National Weather Service has loosely defined a second summer as “any spell of warm, quiet, hazy weather that may occur in October or even early November."

For the purposes of this investigation, a second summer is defined as a three-day period during which high temperatures average 70 degrees or higher after Oct. 1, and after a season’s first 40-degree reading.

This definition proves to be a good indicator of second summers on the East Coast and across the northern tier of the Lower 48, but it doesn’t capture the phenomenon as well on the Plains, where temperature swings are usually more significant.

Washington, D.C.

The nation’s capital, at the tail end of a second summer currently, also saw such events during 2018 and 2015.

In addition, D.C. experienced a very unusual late December warm-up in 2013, during which two consecutive days peaked at 72 degrees long after subfreezing temperatures and the first snows had arrived.

This year’s warmth is perhaps the most dramatic in recent memory, with highs in the low to mid-70s for at least five days in a row.

New York City

Bona fide second summers are rarer in the Big Apple, but they can still happen. This year’s spate of warmth brought a pronounced second summer, with temperatures reaching or exceeding 70 for multiple days in a row.

In 2013, the same winter warm-up that gave a two-day shot of warmth to D.C. in 2013 also brought a 71-degree high to New York City on Dec. 22, but it was brief enough that it wasn’t a true second summer.

The 2015 fall season featured exceptional temperature swings with multiple sharp warm-ups during the fall. There were two periods of impressive mildness in December, too, that came close but didn’t reach the 70-degree threshold.

Boston

This year brought an extreme example of a second summer to Boston. The city picked up 4.3 inches of snow the day before Halloween, establishing an all-time record for the snowiest October. Its temperature then dropped into the 20s the next day, yet when the weather pattern shifted, the city managed to hit 70 degrees five times in the past week. Sidewalk dining was booming in the city and its suburbs as people sought to take advantage of the warmth before winter truly settled in.

On average, every second or third year features a prominent second summer in Boston; 2017 did, as did 2015 — like in New York City — and 2010. The second summer of 2010 was especially noteworthy, since the temperature at Logan Airport peaked at 78 degrees late in October.

Even more remarkable is an episode we didn’t quite know how to handle — three days of extreme February warmth in 2017 that met our criteria for a second summer but occurred in the heart of New England winter.

Great Lakes and Upper Midwest

In the Midwest, Chicago and Minneapolis have second summers that occur on average once every two years. While both cities snagged a second summer this year, they appeared to take turns hosting late-season summerlike warmth — Chicago had second summers in 2015, 2014 and 2012, while Minneapolis did so during 2017, 2016 and 2011.

In 2011, Minneapolis’s second summer was more the result of an unusually early start to fall. After dropping into the 30s to kick off October, the Twin Cities spent the next eight days with highs above 80 — some 15 to 20 degrees above average. They even hit 88 degrees on Oct. 5, 2011.

Chicago and Minneapolis also saw a significant second summer in October 2010, when lows in the 30s gave way to mid- to upper 80s in barely a week’s time.

The Great Lakes and Upper Midwest region can expect a second summer every two or three years.

Pacific Northwest

Seattle is an indicator of how frequently the Pacific Northwest may see second summers. The verdict? Seattle didn’t experience a single one in the past decade.

The reason is simple: Seattle is on the water, which provides a stabilizing influence to temperatures. It’s also in an area that’s relatively shielded from early season Arctic outbreaks, which tend to occur well to the east. Overall, Seattle takes a lot of criticism for its gray winters, but it’s less likely to experience the hefty temperature swings commonplace across the Plains.

Moreover, it’s tricky to get dramatic spells of late-season heat; a warm air mass would have to come from the south or southeast, and ordinarily a breeze from those directions in Seattle is fleeting this time of year.

It appears second summers are uncommon in the Pacific Northwest overall.

Southwest

In the Southwest, large temperature swings are common because the air is so dry. Whether that yields a second summer is largely dependent on local influences — like terrain and altitude, etc.

In Phoenix, for example, it would be foolish to hunt for second summers, since the average low never dips much below 45 degrees, even in the heart of winter.

Instead, let’s take a look at Albuquerque, where one or two second summers a year are routine — the city saw one of them in 2011, 2012, 2013, 2018 and 2019, and three each in 2014 and 2017.

This year has been unusual, in that Albuquerque hit 40 degrees for a low in early September, and then spent most of the next month in the 80s.

The definition used in this analysis fell flat in late October, when Albuquerque started reporting sub-40-degree lows and highs in the upper 70s on the same day for multiple days, a symptom of dry air fostering big-time temperature swings. This is because moist air holds more heat than dry air, so a drier atmosphere can heat up or cool down more quickly and is, therefore, more susceptible to dramatic temperature changes.

The Plains

If you’re looking for crisp, neat data points associated with second summers on the Plains, good luck. Out there, all four seasons can happen in a week.

The cities of Denver, Oklahoma City and Rapid City, S.D., routinely have enormous temperature seesaws, and summer can reappear pretty much whenever the atmosphere feels like it.

Denver, for instance, went from temperatures near 100 degrees to snow over Labor Day weekend this year; now it is experiencing record warmth again. It has seen dozens of second summers in the past decade, the fluctuations so routine that they lose their meaning.

Things tend to be a bit less erratic away from the Rockies. Oklahoma City averaged two or three second summers each year during the past decade. It’s not unusual to have a 40-degree or greater separation between a day’s high and low temperature during the fall in Oklahoma City.

And in Rapid City, S.D., where the nearby Black Hills are known for some of the world’s most extreme temperature swings, there really is no such thing as autumn — just a boxing match between summer and winter.

This year, Rapid City hit a low of minus-7 at the end of October, only to spend the start of November near 80 degrees.

In 2014, the weather was equally wild — eight consecutive days of subfreezing lows in November followed by highs in the 70s in mid-December.

So if you live on the Plains, you’re used to second summers. After all, what would fall be without a hint of winter, spring and summer?