Can Anything be Done to Make Cities Safer During Heat Waves?

Three actions that landscape architects can take to reduce the danger of overheated cities

Google Earth image of a typical area in Paris, France

From August 4 to 18, 2003 France sweated through their hottest weather in more than 500 years.  Almost 15,000 people died as a result of the heat wave, many of them due to dehydration, hyperthermia, and heat stroke (Poumadère et al 2005). Areas of Europe that have long been considered to have a temperate climate are now threatened by an increasing frequency and intensity of heat waves (Meehl and Tebaldi 2004) and every segment of the population is at risk (Fouillet et al 2006).

All of Canada and much of northern USA are also currently considered temperate climates and in general we don’t worry too much about heat waves, but that seems to be changing.  Not only are heat waves becoming more common, they’re also increasing in severity (e.g. Meehl and Tebaldi 2004).

But do all parts of Canada experience the same number and intensity of heat waves?  Some regions of Canada are more affected by heat waves than others.  For example the Prairies, Southern Ontario, and the St. Lawrence River Valley typically have the most frequent and intense heat waves, and the effects are “profound and far-reaching” (Smoyer-Tomic et al 2003). 

Not all areas within a region are equally affected by heat waves.  Urban heat islands intensify heat waves in cities compared to the surrounding countryside increasing the health risks for urban dwellers. Considering that two thirds of Canadians live in urban areas it’s important to understand the characteristics of urban landscapes that have the biggest impact on human health during heat waves. 

One of my MLA advisees, Drew Graham, explored the relationship between physical characteristics of Toronto neighbourhoods and the incidence of heat-related morbidity during extreme heat events (Graham 2012).  If you attended the 2014 CSLA Congress in Ottawa you will have heard Drew talk about his research.  But in case you missed it I’ll review his results here.

He looked at four extreme heat events from three years for the city of Toronto.  He acquired emergency medical response (EMR) call data for the periods leading up to, during, and after each of the heat waves.  Heat-related ambulance calls were 12.3% higher during the heat events than in the preceding or the following week. This result was similar to other studies that have found that there are more medical emergencies during heat waves and confirms that heat waves are dangerous to human health. 

He then compared the number of EMR calls per Census Tract.  He found something potentially more interesting – something that will provide terrific evidence for landscape architects who are working in urban areas.

The number of heat-related ambulance calls was negatively correlated to canopy cover. That means, the more canopy cover, in general, the fewer emergency medical response calls during heat waves.  Have a look at this graph:


Toronto Census Tracts with less than 10% canopy cover (on the left-hand side of the graph) had approximately twice as many heat-related calls during heat waves as those with more tree cover, and nearly four times as many heat-related calls as Census Tracts with a high canopy cover (>70%).  These results have important implications for human health during heat events, particularly in the context of global climate change and urban heat island intensification, both of which are trending toward hotter urban environments in future.

Action #1 – Take every opportunity to increase the canopy cover in urban areas, particularly areas that currently have few trees.

A Google Earth image of an area in Toronto, Canada.

Drew took his study a step further in an attempt to quantify the human health value of increased canopy cover.  He selected two Census Tracts that were in the lowest canopy cover category and estimated the effect of increasing the canopy cover to 12% and 25% respectively.  The simulations indicated that the increased canopy cover would reduce the number of heat-related EMS call frequencies by 40 to 50% as compared to existing conditions.

He then looked at the micro-scale and investigated whether it mattered where the trees were planted within an area.  He used a human energy budget model called COMFA (Brown and Gillespie 1986) to estimate the effects that various landscape treatments had on the heat input to individual people.  The simulations suggested two actions that had beneficial effect in cooling people during heat waves.

Action #2 - Plant deciduous trees to the south and west of areas where people are likely to be.

Providing shade for people in outdoor areas reduces the risk of them becoming overheated from a large input of solar radiation.


Action #3 - Plant deciduous trees to the south and west of buildings, parking lots, and streets.

Unshaded hard dry surfaces heat up when they absorb solar radiation.  Hot surfaces emit large amounts of terrestrial radiation that will be absorbed by people in the landscape, heating them up.  Reducing the solar radiation falling on hard surfaces will keep them cooler so they will emit less terrestrial radiation onto people. 

These actions might already be quite familiar to you, but the results from Drew’s study provide strong evidence that should convince even the most skeptical client. There are many ways to make cities safer during heat waves.  These three actions are a good start.


Brown, R.D. and T.J. Gillespie. 1986. Estimating outdoor thermal comfort using a cylindrical radiation thermometer and an energy budget model. International Journal of Biometeorology. 30:43-52.

Fouillet, A., G. Rey, F. Laurent, G. Pavillon, S. Bellec, C. Guihenneuc-Jouyaux, J. Clavel, E. Jougla, Denis Hémon. 2006. Excess mortality related to the August 2003 heat wave in France.  International Archives of Occupational and Environmental Health. October 2006, Volume 80, Issue 1, pp 16-24

Graham, Drew, A. 2012. Census Tract-Level Outdoor Human Thermal Comfort Modelling and Heat-Related Morbidity Analysis During Extreme Heat Events in Toronto: The Impact of Design Modifications to the Urban Landscape.  Master of Landscape Architecture thesis, University of Guelph, Canada. (You can download a PDF of Drew’s thesis from the Atrium of the University of Guelph Library.)

Poumadère, M., Mays, C., Le Mer, S. and Blong, R. 2005. The 2003 Heat Wave in France: Dangerous Climate Change Here and Now. Risk Analysis, 25: 1483–1494. doi: 10.1111/j.1539-6924.2005.00694.x   

Meehl, G.A., and C. Tebaldi. 2004.  More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st Century. Science 13 August 2004:Vol. 305 no. 5686 pp. 994-997. DOI: 10.1126/science.1098704

Smoyer-Tomic K.E., R. Kuhn, A. Hudson. 2003. Heat Wave Hazards: An Overview of Heat Wave Impacts in Canada. Natural Hazards. March 2003, Volume 28, Issue 2-3, pp 465-486

Urban hot spot? Here's a creative solution

When Kassel University and Dr. Sanda Lenzholzer mapped the urban climate of Arnhem, Netherlands, they identified several 'hot spots'.  The red splotches on the map indicate places where people would be overheated on summer days.

Urban climate map of Arnhem, Netherlands

Urban climate map of Arnhem, Netherlands

Sanda was not satisfied with simply identifying hot spots - she wanted to do something to make them cooler.  An analysis of the microclimate of these locations identified some typical urban heat island (UHI) problems: too many hard surfaces and too few trees.

The red area outlined with a pink ring on the map is a densely populated area centred on the Graaf Ottoplein square (below) that is almost devoid of trees. Sanda wondered how she could get more trees growing in that neighbourhood.  She also wondered if there was some way to do it while educating the public as to the microclimatic value of trees in urban areas so that they might also take action around their own houses.

Google Earth image of Graaf Ottoplein, Arnhem

Google Earth image of Graaf Ottoplein, Arnhem

It is well known that trees have many microclimate-modifying characteristics.  They intercept solar radiation before it reaches the ground which means cooler surface temperatures in the shade of trees and consequently less terrestrial radiation emitted by these surfaces. Evapotranspiration from their leaves humidifies and cools the air. The cumulative effect of many trees means that urban areas with a larger amount of tree cover are cooler and more thermally comfortable than areas with few trees and a large amount of hard surfaces. Furthermore trees can filter fine dust from the air and remove carbon dioxide from the air. Trees have many benefits for urban climate!

Sanda describes their thinking.  "We wanted to raise more awareness of people for these benefits of trees on urban climate. We think that is important that people know the role that trees play in providing more pleasant microclimatic environments in cities. And because nobody in the Netherlands or elsewhere has raised awareness for the important role that trees play in urban climate before in a very explicit way, we wanted to be the first to do so."

So she collaborated with some creative friends and designers, and they came up with a unique idea.

"We inaugurated the world’s first ‘urban climate tree’. The ‘urban climate tree’ is a young Sophora japonica tree that was planted on Graaf Ottoplein square. This square lies in the middle of the neighbourhood that has heat problems and thus was an ideal location to demonstrate the positive effects of a tree. Apart from that, the square itself also has too many stony surfaces that add to the heat problems, and thus needs more trees. The ‘urban climate tree’ itself is surrounded by a ring shaped piece of art made from Cor Ten steel that explains the role of a tree on urban climate. Translated from Dutch it says: ‘tree=coolness= fresh air= wind protection=urban climate=’ The text on the ring can thus be read in a loop emphasizing that all these benefits are always connected with each other."

And they made it into a celebration. Dr. Lenzholzer continues "The inauguration ceremony was opened by the inventor of the ‘urban climate tree’ idea, Frans Tak, followed by a brief introduction that I gave on the role of trees on urban climate. Then the alderman Henk Kok unveiled the ring around the tree and highlighted the role of trees for urban climate in the neighbourhood where it is placed. And last but not least, he emphasized how proud he is of having the world’s first urban climate tree in the city of Arnhem."

Photo credit: Han Koppers

Photo credit: Han Koppers

"After the inauguration ceremony we held a mini-symposium in the neighbourhood centre where local speakers gave presentations about the importance of trees and greenery on urban climate. They showed what can be done in such a densely built- up area as the neighbourhood that is now home to the world’s first urban climate tree. "

Sanda Lenzholzer (left) with the urban climate tree.  Photo Credit: Han Koppers

It's very fitting that one of the common names for the Sophora japonica is Scholar tree.  Clearly scholars are doing something very good for the urban microclimate of Arnhem and hopefully the initiative will spread into an urban climate forest around the world.

Not all Shade Trees are Created Equal

2 things to consider when selecting the most appropriate species


When it comes to producing shade not all trees are created equal. But no single species is the best to use in all situations. You need to match the most appropriate species with the intended use of a place. A school playground, for example, is intensively used during spring and fall but is often underused during summer months. Trees that provide shade early in the spring and hold their leaves late into the fall will provide shade when school is in session and the kind of shade that they provide in the middle of summer is far less important. In contrast, many urban parks with playgrounds are used more in the summer when kids are on break from school. Trees that provide a heavy shade in mid-summer might be preferred in these locations, and whether or not they leaf early or late is not as important.

There are two main characteristics of trees that will have the greatest impact on your decision on how to provide the best shade for any given situation:

  • leafing period; and
  • density of the canopy.

There’s a table of this information that you can refer to in “Microclimatic Landscape Design”, a book that Terry Gillespie and I published with John Wiley & Sons, New York. Many trees are not particularly distinctive in either leafing period or density of shade, but a few are noteworthy.

Trees that tend to leaf later in the spring and drop their leaves earlier in the fall include:

  • Catalpa speciosa
  • Juglans nigra
  • Tilia cordata

Trees that leaf earlier in the spring and hold them later into the fall include:

  • Acer platanoides
  • Aesculus hippocastanum

Trees that produce particularly heavy shade in summer include:

  • Acer platanoides
  • Fagus sylvatica
  • Juglans nigra
  • Tilia cordata. 

Trees that have a particularly light shade in summer include:

  • Gleditsia triacanthos inermis
  • Carya ovata.

The list of trees in the book is far from complete. More studies need to be done to fill in the empty spaces in the existing list and test other trees. And of course there can be a lot of variation among individual trees of the same species depending on health and size. But by the very nature of evidence-based landscape architecture we have to use the best information available, even if it’s incomplete.

You'll notice that there are several non-native species on these lists.  I encourage people to use native or non-invasive species whenever possible, especially when planting in or near a natural area. Acer platanoides is known to be an invasive species in southern Ontario but has one of the densest canopies and provides heavy shade. I would encourage its use only in situations such as highly-urbanized locations where it can provide important cooling of the urban heat island.

Maple blue Birch green
Maple blue Birch green

I walked around my neighbourhood and took thermal images of the shade created by different species of trees growing side by side. In the image above the coolest temperatures (dark blue) were in the shade of an Acer platanoides while right beside it the temperature of the asphalt in the shade of a Betula papyrifera (yellow/green) was warmer. The warmest surface temperatures (red, orange and white) were of asphalt in the full sun.

Maple blue Honeylocust yellow
Maple blue Honeylocust yellow

This image shows again the coolest surface temperatures (dark blue) in the shade of an Acer platanoides, warmer surfaces (green and yellow) in the shade of a Gleditisa triacanthos inermis in the foreground, and the warmest surface temperatures (red and white) were unshaded from the sun.

If you were designing an outdoor café that the owner wants to open early in the spring and stay open late into the fall you might want to select trees that leaf late and drop their leaves early so that the solar radiation can warm customers on cool days. Alternatively in designing a caféwhere the owner wants to focus on the summertime lunch crowd you might select trees that provide a heavy shade when in full leaf.

There are many things that need to be considered when selecting which tree to plant but make sure that you think about the leafing period and the density of the canopy, and how these characteristics will affect the long term use of the place.


Brown, Robert D. and Terry J. Gillespie. 1995. Microclimatic Landscape Design: Creating Thermal Comfort and Energy Efficiency. John Wiley & Sons, New York.

Brown, Robert D. 2010. Design with Microclimate: The Secret to Comfortable Outdoor Space. Island Press, Washington D.C.

Artificial turf look cool? Think again!

Newly installed artificial turf looks magnificent - like a well maintained lawn.  But as is so often the case there's a big difference between form and function.  It might look like healthy grass, but it functions more like an asphalt parking lot when it comes to microclimate. Everything on earth emits terrestrial radiation based on its temperature.  The hotter an object is, the more energy it emits. A fireplace emits a lot of terrestrial radiation and will warm you on a cool day.  In contrast, a cold window will not be emitting nearly as much radiation and standing near it will chill you.

A thermal camera takes pictures of objects' temperatures.  In the thermal image below, the truck and the asphalt parking lot are about 46C (pink) while the trees in the background are closer to 25C (yellow and green).   A person standing in the parking lot would receive a large amount of terrestrial radiation from the asphalt and truck and would, over time, become overheated.  Someone standing on the grass near the trees would be receiving a lot less radiation and would feel much cooler.

Asphalt parking lot jpeg
Asphalt parking lot jpeg

Now let's look at a thermal image of an artificial turf playing field taken on the same day at almost exactly the same time.  The temperature of the artificial turf is even hotter than the asphalt parking lot.  It's almost 48C!  That's hot enough to cook an egg.  And look at the real grass in the foreground.  It's about 25C. A person standing on the playing field would be receiving a lot of terrestrial radiation, while a person standing or sitting on the sidelines would be receiving a lot less.

Artificial turf playing field jpeg
Artificial turf playing field jpeg

Think about the implications for people using the field.  There are times when the heat from the artificial turf would be welcome - say late in the fall when it's cool or cold outside.  But during mid-summer when the air temperature and humidity are high and the sun is shining, the last thing you want is to receive additional heat.  You certainly wouldn't choose to play sports on a black asphalt parking lot on a hot summer day.

Spectators standing or sitting on the sidelines on real grass and in the shade of a tree would likely feel quite comfortably cool, even on a hot day.  But the athletes playing on the artificial turf would be generating internal energy, and this in addition to the hot, humid, sunny conditions would be enough to make them feel very warm or even hot.  The additional heat they receive from the artificial turf could put them over the top.

Many of the components of microclimate are totally invisible to the human eye.  We can't see the air temperature, the humidity, or the wind.  Fortunately we now have cameras that make the invisible terrestrial radiation visible.  Now there's no excuse for designing playing fields that are as hot as asphalt parking lots.

Asphalt parking lot
Asphalt parking lot

Hot Weather Ahead: Learn how to design outdoor spaces to improve thermal comfort.

Bari street 3 We've had a long cold winter and and cool spring, but it's bound to be getting hot soon.  And when it does everyone will be looking for outdoor places that feel cool.  If you can spare 30 minutes I'd encourage you to watch the webinar that I gave for Health Canada last summer.  It'll give you ideas about how you can design outdoor places so that they'll feel cool on those hot, sunny summer days.

Listen to the webinar and watch the slides at the following link:

Scroll down to the August 20 entry, and select "webinar recording".  The first half was presented by a colleague.  My presentation is in the second half starting at 32:20.