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Is early summer in Chicago getting cooler?
June 25, 2009

Dear Cecil:

Recently the Tribune's weather page pointed out that the first 12 days of June had been the coolest in 50 years, averaging just 59.5 degrees.  Earlier weatherman Tom Skilling pointed out that many Chicagoans believe the summers here are getting cooler, but the statistics he cited were inconclusive — in fact summers overall have grown slightly warmer. Can you take another look? My recollection as a kid 50 years ago is that spring could be cold, but once past Memorial Day you could count on warm and often scorchingly hot weather. In the early '80s, it seems to me, that began to change. Some years, like this one, the chill persisted into mid-June, and I've occasionally had to wear a sweatshirt on the 4th of July. I don't recall that happening when I was little. Am I imagining things? One difference is that I grew up on the far west side, but today live not far from the lake. 

— Ned R., Lake View

Cecil replies:

I'm going to be slinging a lot of statistics this week, which may be a trial for you numberphobes. However, no avoiding it. There's definitely something going on. 

The first thing I did was collect historical Chicago weather data from the Midwestern Regional Climate Center in Champaign. (Thanks to climatologist Shelley Schindler for her patient assistance.) Want to know the dew point and wet-bulb temperature at O'Hare on August 12, 1961? Got 'em right here. But all I really wanted to know was the daily high and low temperatures.

I loaded all 21,000+ lines of data into a spreadsheet. Even at first glance, Pops, one thing was obvious: your memories of the summers of your youth are a bit selective. The days weren't always warm. Here are the high temperatures for the 4th of July at Midway Airport from 1950 till now:

Sure, the 4th of July is usually hot — but not always. In 1967, for example, the high was just 66 degrees. But cool 4ths have gotten rarer over time. Maybe you wore a sweatshirt on July 4, 1997, when the high was just 72. Overall, though, the temperature trend is up.

What about earlier in the year, though? I computed the average high temperature from May 15 to June 15 for every year from 1950 till now, as measured at Midway Airport. Here's what I got:

Different story. Two things jump out at you:

  • Average late spring temperatures were remarkably stable, by Chicago standards, from 1950 through 1968. After that they began to fluctuate occasionally, with a low in 1969 and a high in 1977. From the latter year on, though, things really began to scatter, with the annual averages swinging through a much wider range.

  • Overall, the May 15-June 15 temperature trend is down. Quite a few years — not just this one — it's been damn cold.

In short, Ned, your recollection that late spring/early summer in Chicago was reliably warm when you were a kid is on the money. Today things are different. The temperature has gotten less predictable in the last 30 years, but on the whole the beginning of summer is cooler than it was.

The fact that you're living closer to the lake now has undoubtedly exacerbated matters. Historical temperature readings for downtown Chicago weren't readily available, so I substituted the temps for two other lakeside towns, Waukegan and Gary. (The available Gary numbers started in 1982, those from Waukegan in 1990):

Surprise! It's cooler by the lake — although a lot depends on where on the lakefront you are. Gary is cooler than Midway some years, but more often it's about the same. Waukegan on the other hand is almost always much cooler in late spring and early summer — some years as much as five degrees.

That got me thinking. Supposedly we live in an era of global warming. But the lake is still cold this time of year, and persistent easterly breezes have clearly been a factor in keeping the lakefront chilly over the past few weeks. I wondered if the early summer cooling trend was the result of a change in the predominant wind direction. The more I thought about it, the more certain I became that lake winds were keeping things colder here.

There was no easy way to analyze wind direction statistics, but I had an idea. I asked for temperature data for DeKalb, 66 miles due west of Chicago. The only obvious environmental difference was that Chicago was next to Lake Michigan and DeKalb wasn't.  (OK, big cities tend to be warmer than small ones, but let's not worry about that.) Surely if I plotted the temperatures side by side, I thought, I'd find late spring and early summer were warmer in DeKalb:

They weren't — at least not consistently. Some years DeKalb was warmer than Midway, some years it was the other way around. On the whole, though, the average temperatures were pretty close — and they both showed the same sharp fluctuations over the years, especially from 1977 on. DeKalb numbers earlier than 1966 weren't available, but still I wondered about the long-term trend. So I did another chart:

In short — hey, the scientific method in action — my hypothesis was completely wrong. Temperatures in DeKalb in late spring/early summer not only weren't consistently warmer than in Chicago, they were dropping faster — judging from the trend line, a good five degrees since 1966. Far from chilling the city in the spring, the lake arguably stabilized the temperature.

So what explains the wacky, and generally cooler, early-summer weather? No idea. I don't doubt temperatures are rising globally, and for the most part in the Midwest — the winters are definitely warmer than they were 30 years ago. But there's no getting around the fact that, during some parts of the year, things have been headed the opposite way.

"It's a scientific puzzle," said the state climatologist, Jim Angel of the Illinois State Water Survey, when I asked him about the cooling trend. He'd noticed something similar when studying the number of summer days in the 90s. "With global warming, you'd think we should be seeing more hot days, but you look around Illinois and you're just not seeing that," he said. Whatever may be said for the 4th of July, the number of very hot days has held steady or decreased slightly over the past couple decades.

At first I thought I'd leave it at that, but it was depressing to think the weather was getting steadily worse. So I looked for another way to parse the data — admittedly a fool's project, since once you start cheesing up the numbers you can achieve any desired result. But I came up with a rationale: The average late May/early June temperature dropped steeply between 1977 and 1983 — almost 15 degrees. Let's suppose that was a one-time event. Here's the trend in the 26 years since:


DeKalb is still on a steep slide — one wonders what impact that's had on the growing season out in corn country. For us, on the other hand, the trend since 1983 has been essentially flat. I've spent too many years listening to Cubs fans to believe better days are coming. But you can make the case things aren't getting any worse.

— Cecil Adams
Photo by Pat O'Neil

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