Can trees help cool our cities down?

Two men and a dog shelter from last year's Mumbai heatwave in the shade of a tree. Image: Getty.

In cities around the world, trees are often planted to help control temperatures and mitigate the effects of the “urban heat island”. But while trees have been called “nature’s air conditioners”, in practice, scientists often have difficulty demonstrating their cooling properties.

The most obvious way to measure the cooling effect of trees would be to compare the air temperature in parks with that in nearby streets. But this method often comes up with disappointing results: even in large, leafy parks, the daytime air temperature is usually less than 1°C cooler than in the stuffy streets – and at night the temperature in parks can actually be higher.

To explain this contradiction, we need to think more clearly about the physics of heat flows in our cities, and the scale of the measurements we are taking.

Shady days

Theoretically, trees can help provide cooling in two ways: by providing shade, and through a process known as evapotranspiration.

Locally, trees provide most of their cooling effect by shading. How warm we feel actually depends less on local air temperature, and more on how much electromagnetic radiation we emit to, and absorb from, our surroundings. A tree’s canopy acts like a parasol, blocking out up to 90 per cent of the sun’s radiation, and increasing the amount of heat that we lose to our surroundings by cooling the ground beneath us.

Shade cools the ground. Image: Author provided.

All up, the shade provided by trees can reduce our physiologically equivalent temperature (that is, how warm we feel our surroundings to be) by between seven and 15°C, depending on our latitude. So it’s no surprise that, in the height of summer, people throng to the delicious coolness of the shade provided by London parks, Parisian boulevards, and Mediterranean plazas.


Trees can also cool down buildings – especially when planted to the east or west – as their shade prevents solar radiation from penetrating windows, or heating up external walls. Experimental investigations and modelling studies in the USA have shown that shade from trees can reduce the air conditioning costs of detached houses by 20 to 30 per cent.

But air conditioning is more common in some places than in others: for example, while three out of four Australian households have an air conditioner, they’re much less common in Northern Europe, leaving the population there more vulnerable to the harms of urban heat. During the 2003 European heatwave, there were 70,000 more deaths recorded, compared with equivalent cool periods. We urgently need more research to find out how much shade from trees could cool down the terraced houses and apartment blocks, where so many less well-off people live.

Beating the heat

Trees can also be used to tackle a bigger problem: the urban heat island. During periods of calm, sunny weather, the air temperature of cities can be raised above that of the surrounding countryside by up to 7°C, especially at night. In cities, the hard, dark asphalt and brick surfaces absorb almost all the incoming short-wave radiation from the sun, heating up to between 40°C and 60°C, and storing energy which is then released into the air during the still of night, when it can be trapped in the narrow street canyons.

Evapotranspiration in action. Image: Author provided.

Urban trees can counter this process by intercepting the radiation before it reaches the ground, and using the energy for evapotranspiration. Evapotranspiration occurs when the sun’s rays hit the trees' canopy, causing water to evaporate from the leaves. This cools them down – just as sweating cools our skin – thereby reducing the amount of energy left to warm the air.

The effects of evapotranspiration can be quantified in two ways. First, you can measure the temperature of the tree canopy, which is typically much cooler than built surfaces – only 2°C to 3°C above air temperature. Unfortunately, we can’t really claim that this temperature difference is evidence of cooling capacity; leaves would be cooler than built surfaces even if they weren’t losing water, because they are cooled more effectively by convection.

A better method is to calculate the cooling effect of a tree directly, by measuring how much water it is losing. You can do this by measuring the sap flow up its trunk, or the water loss from single leaves. These methods show that tree canopies can divert over 60% of the incoming radiation to evapotranspiration. Even a small (4m high) Callery pear tree – a commonly planted species in Northern Europe – can provide around 6kW of cooling: the equivalent of two small air-conditioning units.


But there’s a catch: trees only provide this cooling effect when they are growing well. By measuring water loss from individual leaves, we showed that sparser, slower-growing plum and crab apple trees provided only a quarter of the cooling effect of the Callery pears.

What’s more, the effectiveness of trees can be greatly reduced if the growing conditions are poor. We found that the transpiration of Callery pears could be reduced by a factor of five, if the roots were growing through compacted or poorly aerated soil. Much more research is needed on the relative performance of large and small trees, whether they’re planted on streets or in parks.

One final difficulty in working out the cooling power of trees is to determine how much a given tree’s evapotranspiration will actually reduce the air temperature. As so often in science, a modelling approach is needed, with physicists, engineers and biologists working together. We need to put realistic trees into detailed regional climate models, which can mimic the complex daily movements of air and energy through the city. Only then can we determine the regional benefits of the urban forest, and work out how to use trees to make our cities cooler and more pleasant places to live in.

Roland Ennos is professor of biomechanics at the University of Hull. The Conversation

This article was originally published on The Conversation. Read the original article.

 
 
 
 

“You don’t look like a train buff”: on sexism in the trainspotting community

A female guard on London’s former Metropolitan Railway. Image: Getty.

I am a railway enthusiast. I like looking at trains, I like travelling by train and I like the quirks of the vast number of different train units, transit maps and train operating companies.

I get goosebumps standing on a platform watching my train approach, eyeballing the names of the destinations on the dot matrix display over and over again, straining to hear the tinny departure announcements on the tannoy.  I’m fortunate enough to work on the site of a former railway station that not only houses beautiful old goods sheds, but still has an active railway line running alongside it. You can imagine my colleagues’ elation as I exclaim: “Wow! Look at that one!” for the sixth time that day, as another brilliantly gaudy freight train trundles past.

I am also a woman in my twenties. A few weeks my request to join a railway-related Facebook group was declined because I – and I quote here – “don’t look like a train buff”.

After posting about this exchange on Twitter, my outrage was widely shared. “They should be thrilled to have you!” said one. “What does a train buff look like?!” many others asked.

The answer, of course, is a middle-aged white man with an anorak and notebook. Supposedly, anyway. That’s the ancient stereotype of a “trainspotter”, which sadly shows no sign of waning.

I’m not alone in feeling marginalised in the railway community. Sarah, a railway enthusiast from Bournemouth, says she is used to funny looks when she tells people that she is not only into trains, but an engineer.

She speaks of her annoyance at seeing a poster bearing the phrase: “Beware Rail Enthusiasts Disease: Highly Infectious To Males Of All Ages”. “That did bug me,” she says, “because women can enjoy trains just as much as men.”


Vicki Pipe is best known as being one half of the YouTube sensation All The Stations, which saw her and her partner Geoff Marshall spend 2017 visiting every railway station in Great Britain.

“During our 2017 adventure I was often asked ‘How did your boyfriend persuade you to come along?’” she says. “I think some found it unusual that a woman might be independently interested or excited enough about the railways to spend sixteen weeks travelling to every station on the network.”

Pipe, who earlier this year travelled to all the stations in Ireland and Northern Ireland, is passionate about changing the way in which people think of the railways, including the perception of women in the industry.

“For me it’s the people that make the railways such an exciting place to explore – and many of these are women,” she explains. “Women have historically and continue to play an important part in the railway industry – throughout our journey we met female train drivers, conductors, station staff, signallers and engineers. I feel it is important that more female voices are heard so that women of the future recognise the railways as a place they too can be part of.”

Despite the progress being made, it’s clear there is still a long way to go in challenging stereotypes and proving that girls can like trains, too.

I’m appalled that in 2019 our life choices are still subjected to critique. This is why I want to encourage women to embrace their interests and aspirations – however “nerdy”, or unusual, or untraditionally “female” they may be – and to speak up for things that I was worried to speak about for so long.

We might not change the world by doing so but, one by one, we’ll let others know that we’ll do what we want – because we can.