WHEN SUMMER TEMPERATURES rise to uncomfortable levels, cities take a bigger beating than the rest of the landscape. This urban heat effect is especially brutal in big, dense, concrete-dominated cities like New York.
Armed with a thermal imaging camera that detects infrared radiation, artist Nickolay Lamm spent the afternoon in NYC capturing the city’s heat signatures on August 15, 2013. The results are a compelling illustration of why it feels like you might melt there in the summer.
“The general message is that manmade surfaces tend to be warmer than natural surfaces,” said atmospheric scientist John Frederick of the University of Chicago, who studies energy budgets of various types of surfaces. “This has implications for energy demand (and people’s individual electricity and natural gas bills). In summer, the heat island effect will increase the demand for cooling, and in winter the same effect reduces the demand for heating.”
The complex temperature structure of urban settings shows up clearly in the thermal image above, Frederick says. The trees in Battery Park, whose leaves can cool by transpiration, have the coldest temperatures an appear blue. A black lamppost that absorbs sunlight efficiently is hot and red, while building materials display a range of temperatures depending on their thermal properties and exposure to the sun.
Compare the thermal image to the one at the right. The warm brick wall of the building on the left shows up bright pink, while the windows are green because they are in contact with an air conditioned interior, and remain cooler than the wall. The vertical walls provide added area for absorption of solar radiation and increase urban heating in their immediate vicinity, while vegetation can provide cooling to offset the urban heat island effect, Frederick says.
The thermal image below shows the impact that light and dark colors have on heat absorption. Dark surfaces contribute to the urban heat island effect, while white surfaces have the opposite effect, Frederick says. The white stripes of this crosswalk reflect sunlight and are cooler (yellow, orange) than the street’s dark surface (red). Contact with the air below street level apparently keeps the sewer grate relatively cool (green).
White roofs and light-colored walls will reflect more solar radiation and remain cooler than dark surfaces. But, solar radiation provides only part of the heating, Frederick says. The atmosphere also generates longwave infrared radiation, which is a major factor.
“If we had eyes that responded to longwave infrared radiation, the world would look very different,” Frederick said. “It would remain bright 24 hours per day since the longwave radiation emitted by the objects around us is always there. Since the longwave radiation is invisible to us, most people do not realize how important it is in determining the temperature of the Earth’s surface and as a mechanism for objects to lose energy and cool themselves.”
Metallic surfaces are better at reflecting the longwave part of the spectrum. This is probably why the Empire State sign in the image to the left appears dark and cool. In most cases, reflected energy leaves the immediate environment and ends up being dissipated in the atmosphere or even escaping into space.
But in a densely built urban area, both the solar energy reflected and longwave energy emitted by one surface could be absorbed by other surfaces and cause heating. This leads to a “radiation trapping” effect in urban canyons where tall buildings are densely packed, Frederick says. This effect is probably common in New York City.
Check out the rest of the hot city in more of Lamm’s photos on the following pages. And keep in mind that he took these images on an average August afternoon when termperatures maxed out around 83 °F, an unpleasant level as any New Yorker will tell you, but far from the record of 97 °F.
All photos: Nickolay Lamm / StorageFront.com.
The Jersey City skyline looks like hot coals on a campfire in this image. Sitting by the cool blue of the Hudson River, the south-facing building walls being hit directly by the sun are the warmest surfaces (red), while the east-facing walls are little less hot (yellow, green). Clear portions of the sky are quite cold (black) while low-altitude clouds have temperatures similar to the river. The water can cool by evaporation, but this is not an option for dry building materials which remain warmer than the water during daylight, Frederick says.
The whole thing just looks so innocent in the image below.
All photos: Nickolay Lamm / StorageFront.com.
Structural materials in vertical walls of the Empire State Building become warm when hit by the sun’s radiation (red), while windows in contact with an air conditioned interior stay cooler (blue).
All photos: Nickolay Lamm / StorageFront.com.
In the thermal image above, the dark sidewalk at Battery Park is warmer (red) than the lighter concrete border (green, yellow) which in turn is warmer than the grass (blue) which can cool by transpiration. Water in the background is the coolest surface (dark blue) in the image. The absence of liquid water on manmade surfaces means that evaporative cooling cannot occur here, and this is a major contributor to the urban heat island effect, says Frederick.
All photos: Nickolay Lamm / StorageFront.com.
Sunlight is is heating up the right-facing wall (red) of the Chrysler Building in this image, making it hotter than the front-facing wall (green). The metallic roof reflects sunlight and remains relatively cool, making it almost invisible in the thermal image. This image illustrates how the vertical walls of skyscrapers provide large surface areas for absorption of solar radiation and thereby increase average temperatures in the vicinity. However, Frederick notes, the shielding of sunlight from other areas provides some cooling.
All photos: Nickolay Lamm / StorageFront.com.
Guess what? You’re part of the problem too. Human metabolism releases heat, which shows up clearly in this photo of crowded Times Square. People emit more thermal radiation (red) than their cooler surroundings (mainly green). The huge advertising screens emit heat and appear red, while shaded areas in the square are blue. The combination of energy use and high population density acts to increase the air temperature in a large, concentrated urban area, Frederick says, although the overall effect of human metabolism is small compared to other sources of urban heating.
All photos: Nickolay Lamm / StorageFront.com.
The south-facing wall of the Freedom Tower receives direct sunlight and is warmer (green) than the other vertical surfaces of this building (mainly blue). However, the south-facing walls of the older buildings in the image tend to be warmer (red) than the south-facing wall of the Freedom Tower. According to Frederick, this arises from the metallic nature of the Freedom Tower’s outer surface which reflects solar radiation thereby keeping the structure relatively cool. Highly reflecting materials on the surfaces of buildings act to reduce the urban heat island effect. Trees in the foreground are the coolest objects due to shading and the loss of water via transpiration.
All photos: Nickolay Lamm / StorageFront.com.
This thermal image of a building near Central Park South illustrates the temperature structure of urban settings. Sunlight is coming in from the left, and the left-facing walls of buildings are warmer (pink, red, some yellow) than walls that are angled away from the sunlight. Although windows are relatively cool due to contact with cooler indoor air, Frederick says, the energy required to operate air conditioning systems leads to a net addition of heat to the atmosphere, and contributes to higher air temperatures in dense urban areas than in rural surroundings.
All photos: Nickolay Lamm / StorageFront.com.
The water remains relatively cool (blue) all day while the Statue of Liberty warms up when exposed to the sun (red). This image shows a haze layer near the ground, where particles and droplets emit longwave thermal radiation in the far infrared portion of the spectrum, says Frederick. The haze closest to the ground is warmest and gradually cools down as altitude (red to yellow to green to blue). The ground is heated both by sunlight and the longwave radiation emitted by the atmosphere. Haze layers over urban areas increase the longwave heating, especially overnight, and promote warmer temperatures.
All photos: Nickolay Lamm / StorageFront.com.
The direct release of heat to the air in cities is called anthropogenic heat release. The area near the engine at the rear of the bus above is relatively warm (green) compared to the street and the passenger area (blue). The warmest regions are near the vents (red) that release heat directly from the engine where energy is released in combustion. The release of heat in combustion contributes to the air in densely populated urban areas being warmer than in rural surroundings, says Frederick.
All photos: Nickolay Lamm / StorageFront.com.
The geometrically complex structure on top of Grand Central Station, partially exposed to sunlight, also has a complex pattern of temperature, Frederick says. In general, areas that receive direct sun exposure are the warmest (red) while shaded regions are blue. Note the building on the right, where the red wall receives direct sun exposure and the blue wall is shaded. The white trim along the top of the building reflects sunlight and remains relatively cool (blue) even on the wall that faces the sun.
All photos: Nickolay Lamm / StorageFront.com.
