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Why Are Cities Hotter Than Surrounding Areas?

By Nickolay Lamm

Aug

22

2013

Temperatures can increase by as much as 10°F in urban areas due to the heat island effect, which is caused by the following:

Buildings and asphalt with dark surfaces, which absorb light and release heat.
Lack of vegetation, which keeps an area cool through evaporative cooling.
Car and air conditioners, which release heat.

In the U.S., heat kills more people than floods, hurricanes, lightning, and tornadoes combined.

The added heat in urban areas also causes significant property damage. With 35 percent (and rising) of self storage facilities located in cities, you may want to take extra precautions when storing delicate items. Check facilities for amenities such as climate controlled units to help keep your sensitive belongings protected from unruly temperatures.

I went to New York City with a thermal imaging camera to see this effect in action and the images below are the result of what I brought back home. I was inspired to create this project after walking around Manhattan in 95°F heat back in July.

The interpretations below each thermal image are provided by John E. Frederick from the University of Chicago. For complete set of thermal images, please contact nickolaylamm@gmail.com.



The dark sidewalk 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 man made surfaces means that evaporative cooling cannot occur here, and this is a major contributor to the urban heat island effect.



The outer metal surfaces of the two automobiles are warmer (yellow, red) than the shaded street. Tires that experience frictional heating are the warmest areas in the image (red). Note that the dark colored car (red) is warmer than the light colored one. The passenger compartments of both automobiles are relatively cool (green), while the engine area of the car in the foreground is relatively hot (red). The release of heat in combustion contributes to the air in densely populated urban areas being warmer than in rural surroundings.



The white strips of the crosswalk reflect sunlight and are therefore 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). Dark surfaces contribute to the urban heat island effect, while white surfaces have the opposite effect.



The large electric screen generates heat and appears red in the thermal image. The yellow taxi in sunlight (pink, red) on the right is warmer than the white van (green) to its left. The dark-colored automobile in the lower left absorbs sunlight efficiently and appears pink and red. The release of heat via concentrated energy use contributes to the air in urban areas being warmer than in rural surroundings.



Electric lights and objects in direct sunlight are the warmest objects observed (red) in this image. The sides of buildings that are not facing the sun are relatively cool (blue). The plant remains relatively cool (blue, green) due to conversion of water from liquid to vapor. This illustrates that the urban heat island effect has a complex spatial structure owing to the variety of materials that are present.



Highly reflecting surfaces such as the metallic Empire State sign remain cool (black) while portions of the building that receive the most direct solar exposure become warm (red). The local urban heat island effect depends on the reflecting and absorbing properties of the materials that are present in the immediate vicinity.



Sunlight is incident from the right and leads to the right-facing wall (red) in this image being warmer than the front-facing wall (mainly light green). Windows of the front-facing wall are relatively cool (dark green), probably due to contact with the air conditioned interior. The metallic roof reflects sunlight and remains relatively cool, being almost invisible in the thermal image. The vertical walls of skyscrapers provide large surface areas for absorption of solar radiation and thereby increase average temperatures in limited areas of a city, although the shielding of sunlight from other areas acts in the opposite direction.



This thermal image illustrates the temperature structure that exists on vertical walls in urban settings. Walls that receive direct sunlight (red, green) are warmer than walls that are shaded (blue). The background sky (black) is the coldest region of the image. The vertical walls of skyscrapers provide large surface areas for absorption of solar radiation and thereby increase average temperatures in limited areas of a city, although the shielding of sunlight from other areas acts in the opposite direction.



The lower electric screen generates heat and appears red in the thermal image. The upper screen generates less heat (yellow, red). Walls that do not receive direct sunlight remain relatively cool (blue). The release of heat via concentrated energy use contributes to the air in urban areas being warmer than in rural surroundings.



This thermal image illustrates the temperature structure that exists on vertical walls in urban settings. Walls that receive direct sunlight (red) are warmer than walls that are shaded (blue). The background sky (black) is the coldest region of the image. Thermal radiation emitted by the walls of buildings acts to increase the temperature at the ground in areas where many tall structures are present and thereby adds to the urban heat island effect.



Human metabolism releases heat. People emit more thermal radiation (red) than their relatively cool surroundings (mainly green). Electric screens emit heat and appear red, while shaded areas are blue. The combination of energy use and high population density acts to increase the air temperature in a large, concentrated urban area, although the overall effect of human metabolism is small compared to other sources of urban heating.



Structural materials in vertical walls become warm when exposed to sunlight (red), while windows in contact with an air conditioned interior remain cooler (blue). The background sky is the coldest portion of the image and appears black. Absorption of radiant energy by the walls of tall buildings promotes the local urban heat island effect.



A geometrically complex structure partially exposed to sunlight will show a similarly complex pattern of temperature. 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. Temperature changes in a city have a high degree of spatial structure, and the urban heat island includes all of this variation.



This thermal image illustrates the temperature structure that exists in urban settings. Sunlight is incident from the left, and the left-facing walls of buildings are warmer (pink, red, some yellow) than walls that are at a larger angle to the incoming sunlight. Many of the windows are cooler than the nearby wall due to contact with an air conditioned interior. Although windows are relatively cool, 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.



In this thermal image of the Jersey City skyline, the Hudson River is relatively cool (blue) while the buildings are relatively warm. The water temperature shows little response to the daily cycle in heating, while the temperatures of the building surfaces rise and fall over a day. The south-facing building walls that receive direct sunlight are the warmest surfaces (red) while the east-facing walls are less warm (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. Vertical walls provide added area for absorption of solar radiation and increase the urban heat island effect in their immediate vicinity.



The windows of the building in the center are cooler (dark green) than the solid materials in the surrounding wall (yellow, light green). This likely arises from indoor air conditioning and the fact that windows provide a weaker barrier to energy flow than do thick walls. Regions that experience direct solar illumination are relatively warm (red). The vertical walls provide added area for absorption of solar radiation and increase the urban heat island effect in their immediate vicinity.



The south-facing wall of the Freedom Tower receives direct sunlight and is generally warmer (green, yellow, red) 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 (more red) than the south-facing wall of the Freedom Tower. This arises from the metallic nature of the Freedom Tower’s outer surface which reflects solar radiation thereby keeping the structure relatively cool. The vertical walls provide added area for absorption of solar radiation and increase the urban heat island effect in their immediate vicinity, but highly reflecting materials act to counter this heating.



Lights are warmer and therefore emit more longwave thermal radiation than their surroundings. These warm regions appear pink or red in the above image. The release of heat via concentrated energy use contributes to the air in urban areas being warmer than in rural surroundings.



Human metabolism releases heat. People in Grand Central Station therefore emit more thermal radiation (green, yellow) than their relatively cool surroundings (blue). Note that the windows are almost invisible in the thermal image, indicating little temperature contrast between the interior and exterior. In principle, a high population density acts to increase air temperature in a large, concentrated urban area, although this is a very small factor compared to other effects associated with the structure of a city.



The water remains relatively cool (blue) all day while the Statue of Liberty warms up when exposed to the sun (red). A haze layer exits near the ground, and the particles and droplets in this layer emit “longwave thermal radiation” in the far infrared portion of the spectrum. The haze closest to the ground is relatively warm (red, orange), while the temperature decreases with increasing altitude (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.



This thermal image illustrates the spatially complex temperature structure that exists in urban settings.Trees, whose leaves can cool by transpiration, have the coldest temperatures (blue). A black lamppost that absorbs sunlight efficiently is hot (red), while building materials display a range of temperatures depending on their thermal properties and exposure to the sun. Note the warm brick wall (pink) of the building on the left while the windows, which are in contact with an air conditioned interior, remain cooler (green) 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.

John, who wrote the captions, can be reached at frederic@uchicago.edu.
For complete set of thermal images, email nickolaylamm@gmail.com.

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