Tag Archives: urban heat island

Even a single urban tree can make a big difference during the summer heatwaves

Even a single tree planted on a street can help urban dwellers cope with urban heat, a new study reports. Researchers found that temperatures were 1.4ºC cooler around 6-7 pm in neighborhoods where at least half the area was covered by a canopy from trees, and even a single tree can make a significant difference for a bloc.

Image credit: Flickr / Steven Penton

Trees provide a remarkable number of environmental services. From filtering the air to preventing erosion and regulating stormwater runoff, there’s no denying trees don’t get enough credit for all the help they provide. Trees also play a role in regulating urban temperatures: they reduce the amount of sunlight striking buildings and pavement, which lowers the amount of energy absorbed and re-radiated into the air.

They also cool the air through a process known as transpiration cooling. As they release water into the atmosphere from the leaves, the surrounding air is cooled as water goes from liquid to vapor. 

But these benefits can be lost as cities expand and get filled with concrete instead of parks and forests. Dark surfaces absorb more heat from the sun, and concrete and asphalt (think roads and buildings) are excellent heat magnets, producing the so-called urban heat island effect, where cities are hotter than their surroundings. Paved surfaces also mean that plants and trees can’t breathe properly, reducing the important evapotranspiration

Urban heat can cause a range of human health problems, and we know that green areas can help reduce these issues. But the exact positive impact of trees has not been quantified. Now, a new study finds that even a couple of trees can make a difference.

“There are plenty of good reasons to plant trees, but our study shows we shouldn’t underestimate the role that individual trees can play in mitigating heat in urban areas,” Michael Alonzo, lead author of the new study, said in a statement. “City planners can take advantage of the small spaces that abound in urban areas to plant individual trees.” 

In the study, Alonzo and colleagues looked at air temperature readings from one hot summer day in 2018 across different areas in Washington DC. They collected 70,000 air temperature readings from multiple days throughout the day. In their analysis, they examined tree canopy over paved and unpaved surfaces as well as distributed trees, such as those planted in the back yard. 

While urban parks provided mid-day cooling, individual trees helped tackle heat especially in the evening, the findings showed. The researchers recorded lower temperatures in neighborhoods that were covered by canopy from distributed trees (trees that stand “by themselves”, not in a forest or a park. Even places with 20% of canopy cover were cooler than those with no trees, showing that the evening cooling effect lasts well into the night. 

In urban areas, people are more likely to live near distributed trees rather than parks. In Washington DC, where the study was carried out, there are many places where individual trees could be planted, such as streets with family homes and streets with rowhouses. This could increase the racial and socio-economic equity of tree planting, the researchers wrote. 

“Evenings are not quite the respite from heat that we once had,” Alonzo said. “These distributed trees do help the city cool off in the evening and that’s important for human health.”

The study was published in the journal Environmental Research Letters. 

Spray-on treatment could keep roads strong for longer while also making cities cooler

Is the heat getting you down? Most people can empathize. Enough of them, in fact, that one company is piloting a new asphalt treatment meant to reduce temperatures and eliminate pollutant particles, all while helping to keep roadways in good condition.

Image credits Maxx Girr.

The compound’s exact makeup is, as you’d expect, still a company secret. But we do know that it is based on titanium dioxide and meant to be sprayed over asphalt surfaces in cities struggling with the urban heat island effect. Although it does help reduce overall temperatures by making built surfaces absorb less heat, the treatment — named A.R.A.-1 TI — is marketed as a “road rejuvenator” and a seal for roadways.

Spray the heat away

The company behind this treatment, Pavement Technology Inc., is collaborating with Texas A&M University to measure its efficiency. This process involves sending road cores (samples retrieved from treated roadways) and air quality measurements to the university’s labs in order to determine what effect the treatment has in real-life situations.

But if the theory translates to practice, it should definitely help cool cities down. The source of the urban heat island effect is sunlight, which carries energy in the form of heat to asphalt and concrete surfaces, such as roads and buildings. These are very good at heating up, which makes everything that much more unbearable during the day (because you’re now standing, on a hot day, in a mile-wide hot surface). At night, these surfaces cede heat back into the environment, keeping the night’s air from cooling down. The more buildings there are in the city, the taller they are, and the more densely-packed, the more heat will be captured, and cities can be between 1 to 7 degrees F (0.6 to 3.4 degrees C) warmer than the areas around them.

All in all, a terrible experience for everyone involved.

Titanium dioxide is more commonly known as titanium white. Chances are that most white things you’ve ever encountered in your life, apart from foods, were painted using titanium white as a pigment. The plan is for this substance, which reflects incoming sunlight, to have a cooling effect on the dark surface of asphalt, which absorbs a lot of heat during the day. We’ve seen previously how green spaces can help reduce the intensity of the urban heat island effect by blocking sunlight; this treatment can be seen as a complementary to greenery, in that it helps reflect part of the sunlight that isn’t blocked by plants such as trees.

The titanium dioxide in the spray scatters and absorbs both visible light and ultraviolet rays — which makes it a popular component in sunscreens — but it also starts a chemical reaction in the presence of light which oxidizes and breaks down pollutants. Although it’s still in the pilot phase so the figures aren’t final yet, Pavement Technologies says its treatment so far has reduced levels of nitrogen oxide (NOx) by 30% to 40% in areas where it’s being trialed. One mile of roadway sprayed with this treatment has the same pollution-eating effect as 20 acres of trees, the company further claims.

The compound is being tested in three regions in Charleston County as of April 2021.

Still, its main intended role is to keep roads working for longer. The spray works by replacing compounds known as maltenes in old asphalt. Maltenes are found in bitumen, the black, oily fraction of asphalt, and they’re what gives fresh asphalt its bouncy, flexible nature. Over time, however, they degrade, and the material becomes brittle, cracking under strain.

This is what the urban heat island looks like

As the world struggles to cope with ever-more-common heatwaves, people living in cities are suffering the most.

Urban hotspots mapped in the summer of 2018. Image credits: National Oceanic and Atmospheric Administration.

The urban heat island is a phenomenon that affects over 1 billion people worldwide. There are several reasons for this effect — it’s mainly owed to land modification. Dark surfaces absorb more heat from the sun, and concrete and asphalt (think roads and buildings) are excellent heat magnets. Not only do these surfaces attract more heat, but they also capture it for a longer time. Paved surfaces also mean that plants and trees can’t breathe properly, and reduced evapotranspiration contributes to the issue. The overall lower number of trees affects urban areas in more than one way, as cities lose not only the evaporative effect but also the shade.

Pollution and remnant heat from energy production are also important causes.

Cities are hotter than their surroundings. We feel this intuitively when we go to the countryside, and we also know it scientifically. But these maps help put in perspective just how much hotter cities can get. At times, Washington can get 17 degrees hotter than the spots just outside of it. The hottest areas are in the northern residential and commercial areas, with the only the Armed Forces Retirement Home serving as a cool oasis.

Elsewhere, in Baltimore, a similar heat island emerged, but with a different configuration: the hottest area is downtown (as is the case for many cities).

Image credits: NOAA.

Urban heat islands can be represented in different ways, but they always tell the same story. Urbanization doesn’t just transform (or eliminate) ecosystems, it also alters fundamental parameters of weather and climate. Surface temperature, evaporation, and the overall energy balance lie at the very core of every natural system, and the changes we are producing generate cascading effects. For instance, the urban heat island affects not only the city but also its surroundings by changing how much it rains in the vicinity.

Wind patterns, fog formation, and humidity are also affected by urban heat islands.

Atlanta, Georgia, US. Degrees are in Celsius. Image credits: NASA.

This is also not a new phenomenon. The pair of images above depict Atlanta on September 28, 2000 — almost two decades ago. The bottom image is a land surface map showing the heat temperature differences, painting a striking difference. Where development is densest, the land surface temperature increases.

However, as global warming comes into power, heatwaves are becoming more and more common, and overall, temperatures are rising steadily. This means that baring cities’ scorching temperatures during the summer is becoming more and more difficult.

Barcelona shows a striking example of an urban heat island. Image credits: Barcelona development agency.

Ideally, city planners and policymakers would use this data to make sure that cities are more resilient to the threat of climate change and develop more suitable infrastructure. Over a third of the temperature increase is due to the prevalence of dark roofs, and most of the remainder is due to dark-colored pavement and the declining of vegetation.

In the long-run, increasing tree canopy cover and implementing new types of pavement can yield excellent results, but using whiter or reflective materials to build houses, roofs, and roads can offer more immediate results. This increases the overall albedo of the city and reduces the urban heat island effect. Another option is to increase the amount of well-watered vegetation. These two options can be combined with the implementation of green roofs.

Images of Salt Lake City, Utah, show a positive correlation between white reflective roofs and cooler temperatures. The image on the left is an aerial view of Salt Lake City, while the image on the right is a thermal infrared image of the same area, showing hot (red and yellow) and cool (green and blue) spots.

So far, the response has been rather inconsistent in most parts of the world despite the severe effects we are already seeing. Within the United States alone, extreme heat kills over 1,000 people every year, and in Europe, killer heatwaves have become the norm. Other parts of the world, like China, Australia, or Japan, are also not spared.

Needless to say, humans are not the only ones affected by this phenomenon. For urban biodiversity, these increased temperatures can be devastating.

Paris just experienced its hottest temperature ever. This heatmap is from 2003. Image credits: ESA.

These ‘heat islands’ are particularly noticeable at night. During the day, cities accumulate solar radiation and release the energy after the Sun sets. However, they can still be dangerous during the day.

If you are out in the city, make sure to stay hydrated and avoid being in the sun for too long. Use plenty of sunscreen, a hat, and take advantage of the shade whenever possible. These heat island maps may be pretty — but their effects can be very ugly.

Portland heat island. Image credits: Portland State University.

Last but not least, if you are concerned about the heat island effect, or climate change in general, send a message to your elected leaders and policymakers.

We can all make a difference in our day to day life, but without healthy policy, transitioning to a sustainable future becomes exponentially more difficult.

Chicago City Hall Green Roof. Image credits: Tony the Tiger.