Surprise: Cold wave reveals potential benefits of urban heat islands

From the PRINCETON UNIVERSITY, ENGINEERING SCHOOL and the “UHI is good for something” department. Still, it’s not a good place to measure climate change.

Cities like Atlanta, shown here, are heat islands whose temperatures are significantly higher than in surrounding regions. (NASA graphic)

The concrete and asphalt that make city summers brutally hot might not be a bad thing during winter’s deep freeze.

Researchers from Princeton University have found that the urban heat island effect — cities are hotter in the summer than their surrounding areas — also helps keep cities warmer during extreme cold. The findings have implications for urban planners in areas such as New York City or Chicago, which experience marked seasonal temperature swings.

Jiachuan Yang, a post-doctoral researcher, and Elie Bou-Zeid, a professor of civil and environmental engineering, analyzed urban temperatures in 12 U.S. cities in the Northeast and Midwest during a 2014 cold wave. They found that urban areas stayed warmer than the surrounding suburbs and country. The difference in temperature was greatest during the cold wave, which set more than 49 low-temperature records. The temperatures differences were also more pronounced at night than during the day.

The findings, reported in the June issue of the Journal of Applied Meteorology and Climatology, could inform policies for northern cities seeking to mitigate extreme temperatures and curb year-round energy use.

Urban heat islands have been extensively studied during heat waves, with recent analyses showing city temperature boosts can be as high as 8 degrees Fahrenheit for large cities like New York City or Washington. Yang and Bou-Zeid’s study is among the first to examine the phenomenon during cold waves. Yang said that for some areas, cold extremes will continue to pose a challenge even as the climate warms. “More weather-related mortality worldwide is attributed to exposure to excessive cold than to excessive heat, and global warming is unlikely to change this reality,” he said.

The researchers combined temperature and land use data with high-resolution weather simulations to investigate the 2014 cold wave in Chicago. The results suggested that heat released from buildings was a key contributor to the stronger heat island effect observed during the cold wave.

“When you heat buildings, the heat is going to be released from the buildings and go out into the urban environment,” Yang said. In the city, skyscrapers create street canyons that trap heat, while in rural areas heat from buildings dissipates more rapidly into the surroundings. During cold waves, street canyons help cities reduce heating demand and make being outdoors more tolerable, Yang said.

During the cold wave, Yang and Bou-Zeid also found an enhanced difference in the amount of heat released at night in the city compared to rural areas. Their simulations showed this was due to the city’s thermal battery function — concrete and other engineered materials’ ability to store more heat than soils, and to discharge this heat when temperatures drop. Colder nighttime temperatures appeared to stimulate heat exchange.

Many cities are working on ways to mitigate summer heat. These include installing cool roofs, which are covered with light-reflecting materials; and green roofs, which are covered with plants that reduce temperatures by evaporative cooling when moisture is released into the air. The researchers modified their simulation to test the effects of cool roofs and green roofs on the urban heat island during a cold wave. They found that cool roofs reduced daytime temperatures, while green roofs led to a small increase in daytime temperatures and a somewhat larger reduction in nighttime temperatures, due to the lower heat storage capacity of soils compared to traditional building materials.

Dan Li, an assistant professor at Boston University, said the work offers lessons for planners. “This work highlights the fact that we need to consider the seasonal variability of urban heat islands and assess the associated hazards and benefits in a comprehensive way,” said Li, who conducted doctoral and post-doctoral research with Bou-Zeid but was not involved in the present study.

Both Chicago and New York City are pursuing ambitious climate action plans, which include the promotion of cool roofs and green roofs to reduce the urban heat island effect. Yang said it remains important to combat summer heat, but cities should not ignore the risks of extreme cold when planning mitigation efforts.

As a potential solution, Bou-Zeid’s group is collaborating with materials expert Anna Laura Pisello and her lab at the University of Perugia in Italy on studies of new color-changing building materials that could ease summer heat while keeping cities warmer in the winter. Yang is also applying the models developed in this study to explore the most effective spatial arrangements for heat island mitigation strategies within a city.


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Wim Röst
July 25, 2018 6:00 am

The Urban Heat Island effect during wintertime has been observed before: in Oslo in 2007


July 25, 2018 6:09 am

Wow, where can I get a research grant to STATE THE BLEEDIN’ OBVIOUS??!

Thomas Homer
Reply to  Adrian
July 25, 2018 7:09 am

Exactly … and now that we’ve established that the UHI effect tends to raise temperatures both in summer and winter, can we revisit the point several recent WUWT articles have posited that weather stations’ thermometers are impacted by UHI and skewing their readings in only one direction?

Reply to  Adrian
July 25, 2018 8:47 am

I thought it was just me. I thought, Surely no one would publish a paper with such trivial “findings.” There must be some deeper discovery, here, one too subtle for the uninitiated to grasp, or one which was accidentally erased. But no.

Reply to  jorgekafkazar
July 25, 2018 9:03 am

If you read the paper you’ll find that their main objective was to determine what mitigation techniques would do to the beneficial effects during the winter.
“It is therefore imperative to examine the evolution of UHIs during critical cold-wave periods and how this evolution is potentially altered by the implementation of mitigation measures, but such studies remain missing from the literature. This study bridges this gap by addressing the following questions. To what extent will the UHI intensify or weaken under anomalously low regional temperatures? Will commonly used mitigation measures weaken wintertime UHI and limit its potential benefits during cold waves? The findings allow us to assess the interaction between cold-wave hazards and the urban environment as well as to provide more comprehensive guidance to policy makers on the need for appropriate mitigation measures that limit summertime UHI with negligible change to the wintertime UHI.”

Cold in Wisconsin
July 25, 2018 6:13 am

My dogs must be climate savants. In the middle of winter on the coldest nights, they will always gravitate to the asphalt surface, avoiding the grass until it is absolutely necessary, and then moving briskly back to the asphalt. They prefer the asphalt to all other surfaces, whether snow, ice, gravel, grass, etc. This is most noticeable on the very coldest nights. I have observed that they must be able to sense a slight temperature difference between the dark asphalt and the other lighter surfaces. A little “ouch” dance occurs on the colder surfaces. Perhaps I could get a grant to do further study?

Farmer Ch E retired
Reply to  Cold in Wisconsin
July 25, 2018 7:40 am

I understand Bill Gates has a place at the Yellowstone Club in Big Sky, MT with a heated driveway. No wonder the elk hang out there in the winter.

Reply to  Cold in Wisconsin
July 25, 2018 8:43 am

Even snakes know this, gets them run over, as in the desert where it is often cool at night. Long ago paper on this. Small amount of heat attracts cold-blooded animals like fish to warm water outfalls. Once saw an alligator crawl out of one after a hard freeze to get a little sun. They are black. Something about evolution.

Reply to  Cold in Wisconsin
July 30, 2018 6:21 am

Having measured both surfaces in the dead of winter with an IR thermometer, there can be a 20 or 30 F difference between asphalt and grass, and about half that for concrete.
Grass, acts like an air gap insulator and isolates the warm earth from the cold sky.

Bruce Cobb
July 25, 2018 6:23 am

Also, in another study, scientists are surprised to learn that water is wet. Actually, they knew that it was, but not how wet. Further study is required to see if climate change is in fact making water wetter, which could have surprisingly disasterous effects.

Reply to  Bruce Cobb
July 25, 2018 7:46 am


You’re clearly an ignorant skeptic. Don’t you know that water isn’t wet?

Sheesh! Next you’ll try to claim that the earth isn’t a real greenhouse…



Reply to  ripshin
July 25, 2018 9:01 am

Whether water is wet depends on what is is.

Ed Reid
Reply to  eyesonu
July 26, 2018 2:28 pm

One positive vote is insufficient.

Reply to  ripshin
July 25, 2018 10:50 am

Where did they get the idea that “Wetness is the ability of a liquid to adhere to the surface of a solid?”

The Oxford Dictionary defines wet as “Covered or saturated with water or another liquid.” and various less relevant things.

So basically the authors of the article have devised their own definition of “wet” so that they can claim that water is not wet. A bit like the way con artists have their own definition of “climate change.”

Reply to  BillP
July 25, 2018 11:52 am

If wetness is the state of being covered in or saturated with water, that raises the question of whether or not something can be covering itself. We never say that a dollop of jam is covered with jam so I say that water can’t be covered with water because it is water. Therefore water isn’t wet- it can only make other things wet.

old construction worker
Reply to  BCBill
July 25, 2018 6:47 pm

“water isn’t wet”: Interesting so water in it’s natural state is a liquid, if it’s cold it is called ice and if it’s to warm it’s called water vapor. Any liquid that hit a surface the surface will become wet. The next time I have a Crown Royal and water on the rocks I’ll think about it. Is the water making the Crown Royal wet?

Gary Ashe
Reply to  Bruce Cobb
July 26, 2018 9:04 am

Yeah back-water.

Making water more watery

Or the luke waterers.

Makes watery water lose its watery-ness slower due to splash-back.

July 25, 2018 6:28 am

A lot of ice in Hudson Bay, in the Beaufort Sea and Baffin Bay.

July 25, 2018 6:38 am

But don’t they adjust a 1/2 degree for this…..then everything is fine

…fit’s right in with global warming theory….night temps will get warmer….convenient

July 25, 2018 6:44 am

Anthony … Your ENSO Meter seems to be missing on all posts ??

July 25, 2018 6:47 am

Researchers from Princeton University have found that the urban heat island effect — cities are hotter in the summer than their surrounding areas — also helps keep cities warmer during extreme cold.

Well, thank goodness we have such genius researchers around to tell us these things. If you go INSIDE the buildings in winter, it’s even warmer!

Reply to  beng135
July 25, 2018 7:57 am

I suspect that a good bit of the extra warmth is from exfiltration (leakage) from buildings and exhausts. Oddly most buildings don’t run their furnaces in the summer, so much of that “lost heat” will not be present to lose during summer months.
Additionally higher latitude (both N & S) obviously receive far less solar radiation during the winters (its why its cold and why we call it winter) so contribution from solar heating will be also far less.
The temperature differences are more due to the fact we heat cities in the winter and lose heat, and we cannot cool them quickly enough during summer when the sun is beating down on them.
Black roofs during winter and white roofs during summer. Just flip the tarps. 🙂

Reply to  rocketscientist
July 25, 2018 8:09 am

Oddly most buildings don’t run their furnaces in the summer, so much of that “lost heat” will not be present to lose during summer months.

But as Steve Fraser says below, the warmth inside the buildings in summer is transferred outside by the A/C exhausts. So heat is indeed “leaking” to the outside in summer too.

Reply to  beng135
July 25, 2018 8:18 am

There’s a bit of difference between escaping and being actively dumped. One infers that you want to retain it, the other not so much.
AC is actively dumping the heat that is “leaking in”. Furnaces are trying to replace the heat that leaks out. Its the difference between a leaky boat and a leaky bucket. One you are attempting to empty as it fills, the other you attempting to fill as it empties.

Reply to  rocketscientist
July 25, 2018 10:54 am

True but irrelevant. The only important thing is that buildings emit heat summer and winter.

Reply to  BillP
July 26, 2018 12:23 am

Not irrelevant at all. If you are trying to change something you need to know what is happening. If my building is too cold, is it because the AC is still on or my furnace isn’t working or I’ve left the windows open?

Gary Ashe
Reply to  BillP
July 26, 2018 3:48 pm

The buildings generate heat summer and winter.

If the building wasn’t there only the foot-print would be absorbing Solar IR.
The building becomes an up-right element.

Steven Fraser
July 25, 2018 6:55 am

Last winter, Joe Bastardi put up a temp map that happened to show these warmer areas around cities as different tones of color. It was very interesting to see.

One thing that the UHI mavens don’t discuss is that some of the heat has been relocated from the interiors of buildings in Summer. The 2-meter temperature and humidity of the interior of a block of buildings is nothing like that found only a few feet away (over the sidewalk) or at the locations where the AC vents the heat.

July 25, 2018 7:07 am

I obviously shouldn’t have a difficulty in getting grant money for my upcoming research project, The Subtlety of the Comma in Milton’s “Paradise Lost”.

David Chappell
Reply to  Art
July 25, 2018 7:27 am

Only if you can blame it on climate change (to state the bleeding obvious).

July 25, 2018 7:10 am

Ride a motorcycle at night through small towns and you don’t need any thermometers or sensors to tell you about UHI effects. Ride through rolling hills and you learn that cold air is heavier than warm air. That’s if the deer don’t get you on that ride. Grant money available for my rides?

John Garrett
July 25, 2018 7:14 am

Princeton ???
You’ve got to be kidding me. One would think there have to be more important things for them to be doing.

Thanks a lot for the blinding glimpse of the obvious.

I do not count this waste of time as a contribution to the advance of knowledge.

July 25, 2018 7:41 am

So … concrete, bricks, mortar and skyscrapers increase city temps. No sh*t Sherlock@PU.

I thought the thermometers showed that it was all because of CO2. Thanks so much for updating me with the latest findings. I can now relax.

Roy Spencer
July 25, 2018 7:55 am
Jim Clarke
July 25, 2018 7:56 am

Publish or perish, baby! They gotta right something! At least this wasn’t worse than we thought. It was already what we thought.

Myron Mesecke
July 25, 2018 8:44 am

They needed a study to figure this out?
All it took for me to learn this was some simple observation.
In central Texas as fall arrives flocks of Grackles (black birds) move into the city to roost. They congregate in trees and on power lines above asphalt parking lots.
The birds have figured out UHI and that nights in the city are warmer than nights in the country.

July 25, 2018 9:02 am

If they could figure out how to pump some of that summer heat underground and return it during the winter …

Reply to  Gary
July 25, 2018 9:16 am

Well long ago they developed a method to save some of the cold from winter to be used during the summer. They were called “ice houses”.

Non Nomen
Reply to  rocketscientist
July 26, 2018 11:19 am

The Little Ice House Age.

July 25, 2018 9:04 am

In all fairness, stating the obvious is necessary to educate the ivory tower elitists who can’t see the obvious.

July 25, 2018 9:28 am

Warmer in winter… are the temperatures adjusted?

July 25, 2018 10:09 am

Heat Island concept. Sometimes it just doesn’t matter.
Decades ago, the company I was with transferred me to Winnipeg, Manitoba. A cold big city.
When it got really cold, an ice fog would form. When landing at the airport, you could see the fog begin at the verge of town and increase until you could barely see the buildings downtown.
And thick. When stopped at a red light fog would be coming out of the exhaust pipe of the car in front. Such that it was impossible to see the light change. Had to wait for some clearing before the signal light could be seen.
That was around 35 below and it was so grim that we had our lunches in the dreadful cafeteria. It went on for most of a week before it changed.
It was a Friday morning and George said: “Let’s go to Champs for a decent lunch”. Not being a local, George had complained all winter about the cold.
As we were driving, the weather report was: “It is bright and sunny, the sky is clear, there is no wind, the ice-fog has gone and it is 18 below”.
George started to unwind his window and I said: “What are you doing?”. Or words to that effect.
George said: “Bob if it is this hot now, what’s it going to be like when summer comes?”
The low that winter was 49 below F. No wind-chill BS.
And if the wind was blowing, it was -49 downtown and out on the bald prairie as well. And it would be 49 below, downwind from the city.
Bob Hoye

Reply to  Bob Hoye
July 25, 2018 10:13 am

Can I get a research grant for this report?

July 25, 2018 11:02 am

Heat spontaneously always flows towards cold. Why spend money verifying this ?

son of mulder
July 25, 2018 11:44 am

I’ve just discovered that water flows downhill, Can I have a Phd please?

July 25, 2018 12:21 pm

Where’s Mosher to tell us that the UHI has no effect on the Climate?


Dr. S. Jeevananda Reddy
July 25, 2018 5:23 pm

Urban-heat-island effect has no capacity to stop cold waves effect in winter and heat waves effect in summer as both these are related to regional circulation .

Dr. S. Jeevananda Reddy

old construction worker
July 25, 2018 6:34 pm

“might not be a bad thing during winter’s deep freeze.” That may be true but when the temperature is around freezing and it rains instead of snowing it’s down right cold to work out side. Darn water vapor.

Roger Knights
July 25, 2018 10:06 pm

Because of UHI, fewer record-cold events are reported than they should be, and studies that argue that fewer cold than hot record days implies a warming trend are faulty.

July 26, 2018 12:28 am

I think most are missing the point. This isn’t about the discovery of UHI but how focusing on just one aspect – higher temperatures in summer – ignores the possibly beneficial aspects of higher temperatures in winter.

It’s actually sensible stuff and admits that winter temperatures are a real problem.

Reply to  Phoenix44
July 26, 2018 4:43 am

Baby steps, baby! Up to this report, it seemed the only investigation of UHI was how to use it to further cook the books I mean adjust temperature records I mean homogenize… This looks like someone is finally investigating the WHY there’s UHI, what HAPPENS when UHI happens, HOW does UHI change when we do THIS, etc. There is still way too much use of “…simulations…” for my taste, but I think it’s progress.

Reply to  Phoenix44
July 26, 2018 9:26 am

This isn’t about the discovery of UHI but how focusing on just one aspect – higher temperatures in summer – ignores the possibly beneficial aspects of higher temperatures in winter.

And are there methods for mitigating in the summer which won’t effect the benefits in the winter. What you might expect from an engineering department.

I pointed that out yesterday but no response. I guess most comment without taking the trouble to read the paper.

July 26, 2018 11:27 am

Warm places are warm.

Johann Wundersamer
July 26, 2018 6:49 pm

“When you heat buildings, the heat is coming to the buildings and going out into the urban environment,”

But above all, warm wastewater from kitchens and toilets warm the streets from the sewers below.

Johann Wundersamer
July 26, 2018 7:06 pm

This is the reason why snow and ice prevail in winter in rural areas.

But in the streets of the cities prevail in the winter especially slush and runoff water.

NOT during the holidays between Christmas and New Year:

then the city dwellers are visiting parents and family.

The apartments are not heated, kitchens and toilets are not used.

Dudley Horscroft
July 29, 2018 6:16 am

“while green roofs led to a small increase in daytime temperatures and a somewhat larger reduction in nighttime temperatures, due to the lower heat storage capacity of soils compared to traditional building materials.”

the ‘green roofs’ I have come across have a very substantial mass, being composed of some sort of free draining material – gravel or polystyrene granules – plus suitable version of ‘earth’ to grow the plants in. And the surface of the gravel (etc) is normally wet. This has a far higher mass than simple tiles or corrugated iron sheeting, even corrugated aluminium or fibreglass sheeting. So should have a far higher heat storage capability.

Or are they referring to a ‘green roof’ comprising corrugated iron painted green, like the ACT football pitch?

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