Visualizing the Urban Heat Island
I found myself explaining the Urban Heat Island to a cop a few nights ago after a lady in the neighborhood called the cops on me for planting trees on city property next to her house. This is the second time this has happened to me in the town I live in, a region inhabited by a populace that I (only half-jokingly) frequently refer to as “hating trees”. For many, this is hard to understand : why anybody would hate trees. Shit, it was a hard concept for me to understand until I experienced it.
South Texas is known for its high rate of illiteracy and obesity, among other things. People can be incredibly kind here, and the general risk of having your shit stolen by crackheads (not at all uncommon where I used to live) and then shamed for being upset about it by a 25-year-old “anarchist” with rich parents (also not at all uncommon where I used to live) is relatively low. Those are the pros. I mention them because I am about to get to the cons, which are an abundance of automobile-dependent infrastructure (which itself encourages physical health decline and discourages exercise) & a dominant culture (certainly upwards of 50% of the populace) that for the most part hates trees and native plants, mostly because they are supposedly “messy”. In this cultural climate (remember, culture is mostly subconsciously spread by humans due to our social propensity to run with our herd, ie. to conform to whatever happens to be the dominant paradigm in a given area), concrete and asphalt are seen as “clean” whereas plants are seen as “messy” and “inviting snakes”. This kind of cultural zeitgeist is by no means endemic to South Texas. It occurs in many places throughout the United States in regions where the human infrastructure is bleak enough and most of the native habitat has been eradicated. This kind of mindset is neither partial to the rich or to the poor, as I’ve heard it espoused by both classes. It does, however, represent a complete disconnection from - and usually a fear of - “nature” and the biosphere of planet Earth itself. It is a mindset that is both corny as hell and nauseating, and I focus on its pervasiveness in South Texas because this is the first place I have lived where I have seen it to such a dominant extent.
I can understand why people dislike native plants if they have no experience with them. Often when people tell me that they “like plants”, I come to find out (to no surprise) that they generally mean horticultural garbage (normally cultivars bred for aesthetic) and houseplants, things that generally make me yawn and couldn’t hold my attention anymore than a golf convention. Liking “native plants” means something different : it means enjoying plants that have a context for the region where one lives, and being educated enough to understand that both evolution via natural selection as well as ecology tie those plants to the history of that place.
Yet even if a person doesn’t know (or doesn’t care) about all that, I would generally like to be able to assume that most people know the benefits of plants, whether they are native or not. Trees for instance, provide shade. Roots break up soil and help water infiltrate during high rainfall events. Leaves “catch pollution” and prevent particulate exhaust from settling on windowsills and cars (something I had extensive experience with in my old neighborhood in oakland, where you could spit on your finger and run it along the windowsill and have it come up black).
Since moving to South Texas, among the many ecological services provided by trees and perennial shrubs, the offer of shade came to be the most important to me. South Texas is the hottest climate anywhere in the United States. Phoenix may get individually hotter days in the summer, but Phoenix also gets substantially cooler than South Texas in the winter. Just the other day here in South Texas, it was 91 degrees fahrenheit on January 14th. We get maybe one month a year where temperatures may even reach down to the low 40s at night. When you understand this, it is no wonder that plants like Peyote and Astrophytum grow native and thrive down here.
The reason that shade is so important is that concrete and asphalt are basically “heat batteries”. They are heat batteries that are “charged” by the sun, and to reduce their effectiveness as heat batteries, one needs to prevent the sun from “charging” them up by shading them. This is where trees come in. Did I stand any chance of explaining this to the cop, or to the lady who called the police on me for planting street trees? From the way his eyes began to wander when I was talking to him, I could tell it was not going to happen. He had the look of a bored high school student in science class who had long since given up trying to grasp any of the material. Let’s look at how the urban heat island works and why it’s such a bad problem in areas like commercial and residential areas in South Texas that have a large amount of land surface area covered in concrete and asphalt and a low amount of urban tree canopy cover :
THE URBAN HEAT ISLAND : CONCRETE AND ASPHALT AS “SPACE HEATERS”…
For a 6-inch thick asphalt slab starting at 141°F (60.6°C) on a calm, clear summer night with a low of 75°F (24°C), it would take approximately 12-14 hours for the surface to cool to within about 10°F of the ambient air temperature.
Asphalt doesn't cool like a cup of coffee. It's a solid slab losing heat from its surface. The interior heat must conduct to the surface before it can radiate away. This internal resistance is why cooling is slow.
This calculation reveals the severity of the problem:
The Night is Not Long Enough: With only ~10-12 hours of darkness in summer, the asphalt never reaches equilibrium with the night air. It's still actively radiating significant heat at sunrise.
Cumulative Heat Build-Up: On multi-day heat waves, each day starts with the pavement already pre-heated from the previous night, leading to even higher peak temperatures.
The "Space Heater" Metaphor : This 12-14 hour cooling time means the asphalt is indeed a vast, low-temperature radiator from sunset until just before dawn, directly causing the elevated nighttime urban temperatures you described.
Every new parking lot, road, and stretch of asphalt that is not somehow blocked from the sun, whether by trees or by sheet metal purgalo structures and carports, must be seen for what it is : a layer of material that will effectively become a night-time space heater during the summer.
