SciencePod — City Limits

roads and railways series #1

This week’s scientific

discovery that grabbed my attention has to do with cities, and how exactly their heat directly affects the local–and distant–environments surrounding them. A mea culpa before you go ahead and listen: I totally forgot to mention who the researchers were in this podcast! To be totally clear, the paper that this research was presented in was co-authored by Aixue Hu and Guang Zhang, and involved the Scripps Institution of Oceanography, University of California in San Diego, Florida State University, and the National Centre for Atmospheric Research. I first accessed the article on Eurekalert.


A story on how switching to white roofs might reduce urban heat island symptoms.

An interesting document experimenting and showing the difference colour makes to cooling.

The case for sprawling versus compact cities and their effect on heat.


Hello and welcome to another episode of the Tooth Soup Science Pod. My name is Phill English and today I’m going to be taking you away from the calming, breezy fronds of the willows from last episode and into the urban chaos of cities.

If there’s one thing we all associate with cities it is their bustle: some cities have more, some cities have less, but the entire point of a city is to be a hub of activity. They are where commerce is conducted, government held, food eaten, and nightlife enjoyed. There are cities that never sleep, cities of lights, sin cities, and cities of motors. Put simply, cities are where a whole bunch of people end up living their lives in or near.

But it’s that stereotype of the busy city that has sent researchers from the National Centre for Atmospheric Research (the NCAR) hurtling down the freeway towards understanding exactly what effect all this busyness, and business, has on the environment. Because while each city has a different vibe, all cities share a common property that has piqued the interest of climate scientists. This property is the amount of heat they generate, and what effect this has on the local environment.

Before we dive into the results of the study, I just want to quickly outline some things we already know about cities and their climates. We already know that cities tend to have a greater average of hot days compared to the rural areas surrounding them. This is usually put down to the fact that urban materials such as concrete and asphalt retain heat a lot more effectively than natural greenery; an effect that earns metropolitan areas the somewhat-tropical-sounding ‘urban heat island’ tag. This is the same effect that dictates that cities sprawl tend towards hotter days than cities that are more compact, since there is less vegetation in a larger area, and more of those materials that grab onto heat and keep the environment warmer.

So we kind of know already that cities contribute to a significant increase in the heat of the areas they occupy. But what is new and surprising in the study by the NCAR is it shows that this increase in heat can affect locations over 1,500 kilometres away. So how does it work?

To start with, picture a standard city, our ‘urban heat island’. A huge amount of what scientists call ‘waste heat’ is generated within cities by mechanical means such as cars, heating and air-conditioning units, and industrial production. And you’ll recall that this heat is then retained by urban materials, like concrete. Now this is where it gets interesting, and it’s important to remind you that this heat isn’t the same as that produced by greenhouse gases and aerosols. This is merely heat generated from people living and working and consuming energy in the dense urban environment of a city.

Anyway, that waste heat rises up into the atmosphere, and it just so happens that the major population centres and cities in North American and Eurasian continents are located right below some of the main jet streams in the upper atmosphere. So the heat from the city rises up and interferes with the way that the wind and the atmosphere circulates, resulting in changes in temperature far beyond any limits we previously thought a city’s influence may have. You can think of it somewhat like a river that gets diverted by a dam, leading the water to change its course. These columns of hot air rising from a city divert the normal path of the jet streams, leading to changes in temperatures in places that would ordinarily be cooled or heated by those jet streams.

It should be noted that, like a lot of climate science, the estimates that the researchers gave of their being up to a one degree Celsius warming of parts of Asia and North America are based off modelling data. The researchers have stated that they need to do more studies and samples to determine exactly how much heat a city produces. But so far it does seem to be another piece in the puzzle towards explaining why some areas are experiencing warmer temperatures than those predicted solely based on global warming.

Anyway, thanks for listening, as always you can find links to articles that I’ve accessed at the blog, as well as a transcript of the show. I hope you’ll join me next week for another new scientific discovery.

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