Imagine you’re out walking in your local big city on a bright and very hot day. Before you set out from your suburban home, you noticed a light breeze on the air but here, upon exiting the underground station, the air is heavy and still. You have a look around you. Where did that nice breeze go? A nearby tall building captures your attention, you look up and there on the roof is a pair of small wind turbines, furiously turning in the wind that has been deflected above your head.
You frown, why is the wind all the way up there, it’s down here on street level that you need to feel its benefit. Because it is a nice day, you decide to walk to the big city park. A short distance later, you turn off of the wide road and into a side street. Suddenly, the breeze hits you but it is much stronger than you remembered, whipping around your clothes and your hair. Looking around, you realise that this street is much more narrow than the road with the train station on it. Because it is facing a different way, the wind is channeled directly into it and is being buffeted by the tall buildings on either side.
You’re thankful when you reach the end of this street. You turn again into a street running parallel to the first. It isn’t quite as wide as the first street. Once you turn the corner, the wind has gone again and you walk a little while before you have to cross over. When you do, you can feel the wind again, pulling at your clothes and the stack of papers you were taking to the park to read. Better not lose them.
A few turns later, through narrow streets with hardly any wind at all and a hot, humid feel to them, you reach the park. It feels cooler here and the light breeze has returned. You settle yourself in the shade of a tree and begin to read.
The route of this hypothetical journey through the city is shown in the below drawing. Take a moment to look at the airflow that is depicted on each stage of the trip.
The street with the train station on (1) is wide and runs perpendicular to the direction of the wind. The wind hits the tall buildings and is deflected upwards, into the flow of the air turbine which is positioned to capture the most consistent wind direction. The narrow street which is parallel to wind direction (2) funnels any wind that has not been deflected by tall buildings, decreasing flow at the edges but increasing it down the centre of the road. The next road perpendicular to wind direction (3) is not quite as wide as the first and the buildings are not quite so tall. The walker was first in the ‘wake’ of the building (the area in which the building has interrupted the air flow) but when they crossed over, they exited the wake and were once more affected by the air stream. The narrow streets that are perpendicular to wind direction (4) do not receive much replenishment of air on this day. The air feels warm here and a bit difficult to breathe thanks to the traffic. If the air is not getting replaced due to the flow simply ‘skipping’ over the top of this street – what might be the consequence for air quality? It’s a relief for the walker to reach the park (5), where the trees are having a cooling effect on the warm air and that breeze has returned as a result of the wide open space without building interference.
The structure of the city (i.e. where are all the tall buildings and how are they oriented) has a large influence on the city’s microclimate. The effect of the city buildings on the meteorology of the region has lots of important conseqences for city planning, building design, placement of energy services such as wind turbines and structures such as tall masts and for air quality and greenhouse gas dispersion. The climate of the city and the breathability of the air is vastly important for the quality of life of city residents and workers and the experience of visitors.
In my next blog article, I will take a step outside of the city and look at things on a much larger scale, talking about the many methods in which a country can estimate its greenhouse gas emissions and the current science on which of these methods is closest to giving us a good answer.