2. 8 UHI & Skyviews
Figure 2.8: My photograph of a closed-in street in downtown Durham depicts a small skyview factor — the small patch of viewable sky versus all the walls blocking a larger skyview. This skyview factor (0 means no viewable sky and 1 means a fully open sky) resolves the North American–European difference seen in Figure 2.7. Plotting a city’s urban heat island effect against its skyview factor, instead of its population, results in a single curve for all three regions plotted here (after Oke 1982). Evidently, large European cities possess shorter buildings with more open-sky views.
The intensity of the urban heat island with city population shows marked differences between North American and European urban heat islands as depicted in Figure 2.7. No, America hasn’t suspended the laws of thermodynamics. Rather, the urban heat island depends on the “closed-in-ness” of a city, called a city’s skyview factor, the fraction of the upward hemispere open to the sky. Imagine the difference in skyviews between an open prairie and a closed-canopy forest: The skyview factor measures the urban equivalent when viewing the sky from the street, with marked differences between residential suburbs and downtown business cores with tall buildings. As an example, I took the photograph shown in Figure 2.8 in one of the few places in downtown Durham with a smallish skyview factor. Standing between these two buildings, walls fill much of the overhead view, giving a relatively small skyview factor.
Skyview factors resolve the disparity between the UHIs for European and North American cities. Indeed, perhaps Los Angeles’s urban heat island, discussed in Figure 2.7, might be more in line with Europe’s, as my calculation on p. 43 suggested, because it has relatively shorter buildings due to earthquake dangers.
These closed-in city streets are called urban canyons, complete with their own interesting properties related to light and energy. First, I present a short primer on the light coming from the Sun. Just like around a campfire where kids feel the heat of the flames, the Sun’s hot surface bathes Earth with 6,000C light, composed of ultraviolet, visible, and infrared wavelengths. All frequencies of light have the same speed, and the product of frequency and wavelength equals the speed of light. High frequency goes with short wavelength (ultraviolet), and low frequency goes with long wavelength (infrared). Photons from sunlight have relatively high frequency and short wavelength. When concrete or asphalt absorbs this high-energy light, the material heats up and reradiates low-energy, infrared light characteristic of outdoor temperatures around 25C. This light is the same thing as visible sunlight, but has a lower frequency and longer wavelength, and thus lower energy and is not visible.
Out in a parking lot this infrared light radiates from the asphalt in a hemispherical way, up into the sky, but in an urban canyon the light radiated from the wall of a building often moves toward the wall of a neighboring building or toward the road or sidewalk, and gets reabsorbed. This process of absorption and emission goes on and on. Light bouncing around urban canyons delays the cooling of the roads and buildings in a city core. Cities, of course, can solve this problem with strategically placed vegetation (see Figure 2.17).
The fusion reactions taking place within the Sun are much, much hotter, but that energy gets absorbed by the mass of the Sun itself and is then reemitted as light at cooler temperatures. On Earth we see and feel the radiation coming off the Sun’s surface at an almost cold-and-frozen, relatively speaking, 6000C. Scientists often refer to light as being at a certain temperature, though that’s a bit informal. What’s meant is that the atomic and subatomic particles of objects at a certain temperature radiate photons (i.e., light), and the energy distribution of those photons depends on the object’s temperature.
It is true that all frequencies of light have the same speed in an absolute vacuum but through various media that truth breaks down. Think about light going through a prism, and how white light splits into a rainbow. That demonstration shows a frequency-dependent speed of light in the medium making up the prism.