Connectivity of green and blue infrastructures: living veins for biodiverse and healthy cities

Heat islands

Its hot and I’m in a heat island.

Urban heat islands are urban areas that are hotter than surrounding rural areas. During a workshop at SIRTA (Instrumental Research Site for Atmospheric Remote Detection) ( at the Institut Pierre Simon Laplace outside Paris the other week, I got to be reminded all about the physics of weather. We talked a lot about clouds. They also showed us all their instruments, sitting neatly around on the lawn like sculptures from the 1970s. The instrument for measuring temperature was not very visually arresting and I forgot to take a picture of it (but you can sort of see it just on the left side of the photo of the rain guage, inside the chained-off square). It was a small mount with three small rods coming off it, just over a cement tile placed in the grass. Why not have it over the grass? we asked. They said the grass would complicate the temperature data. Evaporation from grass absorbs heat, so it would be cooler over the grass. Secondly, the fluxes between evaporation and traspiration are complicated.

Recall that evaporation is when water molecules (two hydrogens attached to an oxygen) change phase from liquid to gas. Molecules in a liquid move around less than those in a gas. So, to escape the liquid and become a gas (known as water vapor), water molecules need to absorb heat energy—that is, get hotter. Heat is measured as a change in temperature, via transfer of heat energy. Recall that this can occur by convection (movement of bodies of liquids and gases), by conduction (solid to solid by particle contact), or by radiation (movement of infrared waves). If you are hot, you radiate infrared waves—this is why snakes and people with infrared goggles can see you in the dark
So, when water evaporates, it gains energy from radiation or convection.

Transpiration is the movement of water through a plant, drawn out of the soil by its roots and drawn towards the evaporation of water from its leaves. The evaporation of the water molecules out of the stomata of the leaves pulls the water molecules behind them. Plants regulate the rate of transpiration through their physiology and behaviour, for example by opening and closing their stomata.

So: a plant layer is cooler, but in a fluctuating way, than a cement tile. The evaporation and transpiration from the plants aborbs their heat, which goes into the atmosphere with the water vapor.

Water vapor in the air can be measured as humidity. In the past, according to what I learned at the SIRTA workshop, humidity was measured by the tension of a horsehair. Horsehairs absorb water. Now, humidity is measured with capacitators. Cellulose paper between two electrical plates absorbs water, which changes the transfer of electricity from one plate to the other. Another way to measure water vapor is to measure its density in the air with a visibilimeter, which is like a pole with two arms wearing some kind of sausage-shaped gauntlets, one that emits white light and one with some kind of camera, staring at each other. Another instrument that had a ring set on a tripod, holding two metal tubes with slits facing one another, each fed by corrugated plastic tubing, measures the distribution of water droplets in the air. There is also a spider living in it. Water droplets are tiny spheres of liquid water suspended in the air—not the same as humidity which is gaseous water. Water droplets form fog and clouds. Apparently climate change has a big effect on fog events, on which many plants and animals (including humans) in arid regions depend for water.

Fog events can occur in cities, but an urban heat island effect will counteract this because fog (a cloud on the ground) happens when the air near the ground is colder than the air above it. Smog events can occur in cities that are very polluted as the particulate matter in the air can act as a nucleus for droplet formation, but these are clouds not fogs.

Cities are heat islands because they have little plant cover to draw water out of the soil and send heat into the atmosphere along with it, and a lot of materials that absorb heat, like cement and asphalt. Such materials then re-radiate heat at night, so that the urban heat island effect is often largest at night. The urban heat island effect can be reduced with gardens, roof gardens, and reflective (whiter) buildings. Vernacular architecture, meaning architecture based on local materials, traditions, and needs, can reduce urban heat islands. As we discussed at the SIRTA workshop, the traditional Moorish architecture in Spain consisting of a courtyard with narrow shallow pools and fountains facilitates evaporation in the courtyard, lowering the air temperature next to the water, and setting up the possibility for cool air convection. A paper [1] about urban heat islands and vernacular architecture from Iran points out that the narrow passages between blocks of buidlings in the old city center of Qeshm Island are of a particular wall-height-to-passage-width ratio and orientation so that most of them are usually in shade. They also argue that the zigzagging passages “would cause changes in air pressure and internal air flows of the alleys” or in other words a nice breeze. They also discuss how windows and screens are placed to create and capture windflow, and the placement of palm trees.

During the heat wave here in Paris, people in a local park were frolicking in several large kiddie pools that someone had installed. The park also has several poles from which mist comes out, which were installed when the park was redone last year, and children were playing around those. I guess these are the artificial trees of the future (remember that Parisian adults apparently don’t like trees because they attract pigeon feces). Many animals have a harder time cooling off. Lizards, for example, are ectothermic so the only way to avoid fatally overheating is to hide under a cool rock. Birds are endotherms and do regulate their internal temperature, but they neither sweat nor pant. Rather, they find ways to increase the radiation (through their beaks for example), and the convection of air around their bodies. Thus, in the park I saw the crows holding their wings slightly open in a droopy way, and walking around with their mouths open. Apparently by vibrating their neck muscles and keeping their mouths open they increase evaporation from their lungs [2]. Birds thus lose less water than mammals that sweat, but they still need to drink more when its hot and dry.

I end with the remark about how climate change is making all this more extreme. You know that. I would think of something clever to say here but all my metaphors seem to have denatured. Well, except that one.

[1] Mazraeh, H. M., & Pazhouhanfar, M. (2018). Effects of vernacular architecture structure on urban sustainability case study: Qeshm Island, Iran. Frontiers of Architectural Research, 7(1), 11-24.

--Meredith Root-Bernstein, 27/06/2019