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You can find the shownotes through this link.
Are you interested in how to mitigate hyper-local urban heat exposure?
Our debate today works with the article titled More than surface temperature: Mitigating exposure in hyper-local land system from 2022, by V. Kelly Turner, Morgan L. Rogers, Yujia Zhang, Ariane Middel, Florian A. Schneider, Jonathan P. Ocón, Megs Seeley, and John Dialesandro, published in the Journal of Land Use Science.
This is a great preparation to our next interview with V. Kelly Turner in episode 472 talking about how urban heat is more than just a surface temperature measured from a distance.
Since we are investigating the future of cities, I thought it would be interesting to see how useful remote sensing of land surface temperature really is for urban planning at hyper-local scales. This article argues that conventional remote sensing is inadequate for guiding heat adaptation because it overlooks the critical role of solar radiation and local geometry.
[intro music]
Welcome to today’s What is The Future For Cities podcast and its Research episode; my name is Fanni, and today we will introduce a research by summarising it. The episode really is just a short summary of the original investigation, and, in case it is interesting enough, I would encourage everyone to check out the whole documentation on the WTF4Cities website where you can find the shownotes and more information on supporting the podcast. This conversation was produced and generated with Gemini LM as two hosts dissecting the whole research.
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Speaker 1: What if the very strategy your city is using to cool down your neighbourhood is actually making you hotter? Today, we are looking at why painting streets white might satisfy a satellite in space, but completely bake a pedestrian on the ground.
Speaker 2: It is a massive, highly counterintuitive problem because when planners try to mitigate extreme urban heat They instinctively reach for a map.
Speaker 1: They look for the glowing red spots.
Speaker 2: Exactly, and they try to make them blue. But we are finding out that the dimensions of those maps might be entirely wrong for the actual human beings living in those spaces.
Speaker 1: And our discussion today is grounded in a 2022 study by V. Kelly Turner and her colleagues. It is called More Than Surface Temperature: Mitigating Thermal Exposure in Hyperlocal Land System. They basically went to the hot semi-arid desert of Tucson, Arizona, and examined a new urbanist community called Savano.
Speaker 2: And Savano is the perfect laboratory for this. New urbanism is all about moving away from those sprawling uniform suburban grids. So Savano was intentionally designed with clustered buildings, varied shade, native vegetation, high albedo roofs. It creates a very heterogeneous environment with lots of microclimates packed together.
Speaker 1: It really is a brilliant testing ground. I will be arguing today that despite their limitations, the satellite-derived maps that cities use, specifically land surface temperature maps, or LST maps, remain the most scalable necessary baseline we have. If we want to reduce overall heat retention, we absolutely need these top-down tools.
Speaker 2: And I will argue that municipal planning based primarily on land surface temperature is actively misleading. It completely ignores mean radiant temperature, which is the true measure of the heat burden on the human body.
Speaker 1: Let me start by defending the utility of that regional LST map. When cities do heat planning, they typically use 30-meter resolution data from Landsat satellites.
Speaker 2: Which is a huge area.
Speaker 1: It is about the size of two basketball courts, but that is highly effective at diagnosing macro level urban heat island effects across a whole metropolitan area.
Speaker 2: But it misses the finer details.
Speaker 1: But look at the study. The satellite data confirmed exactly what we expect physically. Impervious surfaces like asphalt roads are the absolute hottest features in the environment.
Speaker 2: Sure, dark materials absorb solar radiation.
Speaker 1: Exactly, and they retain that heat late into the evening. The Savano data shows asphalt is a primary driver of the regional heat island at 8:00 PM.
Speaker 2: Yes, at night.
Speaker 1: So for municipalities lacking the funding or computing power for hyperlocal microclimate modelling, these satellite maps are invaluable. They tell cities to target surface albedo and add vegetation to lower surface temperatures
Speaker 2: I understand the appeal of scale, I really do, but I come at it from a completely different angle. The data coming out of Savano essentially proves that surface temperature is a remarkably poor proxy for how humans actually experience heat. What we feel physically is much better measured by mean radiant temperature, or MRT.
Speaker 1: Break that down for us a bit.
Speaker 2: Think about standing in a totally unshaded black asphalt parking lot in the middle of July.
Speaker 1: Oh, brutal.
Speaker 2: That oppressive oven-like feeling. The air temperature might only say 90 degrees Fahrenheit, but it feels infinitely hotter.
Speaker 1: Because of the pavement?
Speaker 2: Yes. Heat is not just beaming down from the sun. It is bouncing up from the asphalt into your legs. It is radiating sideways from the brick buildings. MRT captures all of that. It is a 360-degree sphere of heat.
Speaker 1: And your argument is that the satellite looking straight down cannot capture that 360-degree experience.
Speaker 2: Not even close. And worse, look at the timing. The study highlights that satellite LST data is often derived from a single 10:00 AM pass.
Speaker 1: Because that is just when the satellite happens to orbit overhead.
Speaker 2: Way before the peak heat of the day. Relying on a 10:00 AM pass is judging rush hour traffic based on a photograph taken at 4:00 AM.
Speaker 1: That is a bit of an exaggeration.
Speaker 2: It really is not. Turner’s team found the satellite vastly underestimated daytime temperature peaks. It reduces urban heat to a flat 2D surface problem to be painted over.
Speaker 1: But let us look at how that data is used in practice. Yes, 10:00 AM is not the peak of human thermal stress, but it still provides a vital comparative baseline across an entire city. It shows you where heat is accumulating faster than other areas.
Speaker 2: A baseline that misses the extremes entirely. When researchers actually walked the streets, they used biometeorological instruments called MARDI carts.
Speaker 1: The modified golf push carts.
Speaker 2: Yes, loaded with sensors pointing up, down, north, south, east, west. They paired that physical field data with 3D simulations. The results were staggering.
Speaker 1: How staggering?
Speaker 2: They recorded a 54.4 degree Celsius swing in mean radiant temperature over the course of a single day in these microclimates.
Speaker 1: Wait, a 54-degree swing?
Speaker 2: Yes.
Speaker 1: Just to be clear, we are talking about Celsius, so that is a massive shift. Over what time period?
Speaker 2: It is a diurnal swing compounded by the spatial differences. You can walk just a few meters from an unshaded street into a densely shaded courtyard in Savano and experience a massive drop in thermal burden.
Speaker 1: Sure, shade is cooler.
Speaker 2: But satellites looking at 30-meter pixels average all of that out. If you have a blistering hot street and a perfectly shaded sidewalk next to it The satellite just blends them together.
Speaker 1: I see the limitation, but I am not convinced by that line of reasoning for abandoning the top-down approach.
Speaker 2: Why not? It is highly inaccurate.
Speaker 1: Because throwing away a regional satellite heat map just because it averages out the micro level is throwing away a global weather map because it does not tell you if it is raining on your specific street.
Speaker 2: That is not a fair comparison.
Speaker 1: It absolutely is. Cities must address the regional urban heat island effect. You cannot manage a regional climate crisis exclusively one shaded sidewalk at a time.
Speaker 2: But if you only use that regional map, you end up with maladaptation. You deploy solutions that literally make things worse for people on the ground.
Speaker 1: So if satellites are missing the human level, how is that actually harming city planning?
Speaker 2: Let us talk about cool streets. This is one of the most critical warnings from the Solano analysis.
Speaker 1: Regarding targeting surface albedo.
Speaker 2: When planners rely strictly on LST, their logical move is to increase reflectivity. They paint the streets white.
Speaker 1: Which the physics show successfully lowers the surface temperature of the asphalt.
Speaker 2: It lowers the surface temperature, yes, but where does that incoming solar radiation go? It does not just disappear.
Speaker 1: Oh, I see where you were going with this.
Speaker 2: It hits that highly reflective street and bounces directly upward, right onto the pedestrians walking on it.
Speaker 1: The shortwave radiation is reflected into the human space.
Speaker 2: Exactly. By increasing the albedo of the street, you dramatically increase the mean radiant temperature for the pedestrian during the midday hours. You solved the 2D pixel on the satellite map, but you actively increased the thermal burden on the human being.
Speaker 1: I will completely acknowledge that cool streets can create a daytime radiant heat trap for humans. It is a profound paradox.
Speaker 2: It is a major blind spot in current municipal planning.
Speaker 1: It is. But let me push back here because we cannot look at the daytime in isolation, because pedestrians running errands at 2:00 PM are not the only concern in a heatwave. If we do not reflect that heat during the day, the entire neighbourhood bakes at night.
Speaker 2: But at what cost during the day?
Speaker 1: Hear me out. Dark asphalt acts like a massive thermal battery. It absorbs the sun’s energy all day long.
Speaker 2: I do not disagree with the thermal battery mechanism.
Speaker 1: And when the sun goes down, it releases all that stored heat back into the environment as longwave radiation.
Speaker 2: Yes, driving the nighttime heat island.
Speaker 1: Exactly. And nighttime heat is historically when the most vulnerable populations suffer the most. Without high albedo surfaces, the asphalt charges that thermal battery. We have to reflect that energy back out to space.
Speaker 2: That is a compelling argument for nighttime relief. But you can achieve daytime cooling and prevent the thermal battery from charging without blinding pedestrians at noon.
Speaker 1: Which brings us to shade.
Speaker 2: Yes, three-dimensional urban design. In Savano, they clustered buildings tightly together to cast shadows. They used extensive tree canopies.
Speaker 1: And shade blocks the shortwave radiation from reaching the pedestrian.
Speaker 2: And it prevents the radiation from hitting the asphalt, which stops the thermal battery from charging. You get daytime comfort, and you prevent heat absorption.
Speaker 1: True But what is the issue?
Speaker 2: The issue is satellites looking straight down cannot even see the shaded areas beneath thick trees. The Landsat just sees the top of the canopy. It is completely blind to our most effective dual-purpose thermal comfort intervention.
Speaker 1: I will concede that shade is paramount for daytime pedestrian comfort, but shade is not a magic bullet, and there is a fascinating trade-off in the data regarding trees.
Speaker 2: A trade-off with trees?
Speaker 1: Yes. The study points out that while trees cool the street level area during the day, those exact same tree canopies were actually warmer spots after sunset.
Speaker 2: Warmer compared to what?
Speaker 1: Warmer compared to exposed bare soil.
Speaker 2: Wait, really, explain the mechanism there.
Speaker 1: A dense tree canopy acts physically like a blanket. At night, whatever residual heat was absorbed by the ground is trying to escape back into the atmosphere. With bare soil, it radiates freely into the cold night sky.
Speaker 2: But the canopy traps it.
Speaker 1: Precisely. The canopy traps that outgoing long wave radiation. So if your only metric is 3D daytime shade, you might accidentally design a neighbourhood that traps heat at night. That is why we need the regional surface level understanding that satellites provide.
Speaker 2: Which brings us to the core issue. Protecting human health requires optimizing for mean radiant temperature. If we treat urban heat strictly as a surface problem, as a 2D map, we risk deploying interventions like the albedo pavement that actively harm pedestrians.
Speaker 1: And while 3D shade is unquestionably critical for midday comfort, I maintain that regional LST maps remain an indispensable tool. They are often the only tool resource strapped cities have to combat the broader urban heat island effect at a macroscopic scale.
Speaker 2: It seems we are converging on the ultimate conclusion of Turner and her colleagues, which is the desperate need for urban heat literacy.
Speaker 1: I think that is the perfect term for it. Planners and citizens must clarify exactly which heat problem they are trying to solve.
Speaker 2: Exactly right. A cool street might not be cool for the person walking on it, and a shade tree might act like a blanket at midnight.
Speaker 1: Are you trying to lower surface temperature, air temperature, or radiant heat? You have to define the problem first.
Speaker 2: There is so much more to explore in this material, but it absolutely challenges us to ask a fundamental question.
Speaker 1: Are we measuring the street or the people standing on it?
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Episode and transcript generated with Descript assistance (affiliate link).
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