Most cities in the US are warming at twice the rate of the outlying rural areas and the planet as a whole. While an individual heat wave event may be responsible for the majority of warming in an urban area at a given time, a phenomenon called the urban heat island effect may exacerbate the warming and play a role in additional heat-related mortality, an increase in tropospheric ozone, economic losses that can total in the billions of dollars from excess energy consumption, and environmental degradation of our urban environment.
Urban heat islands (UHIs) occur when urban and suburban areas experience elevated temperatures relative to their rural surroundings because of differences in vegetation cover, buildings and other development, and infrastructure (Fig. 1 and 2).
This difference in temperature is proportional to the size of the city and can be in excess of 2.5-5.5°C (5-10°F) during the daytime and as much as 11°C (20°F) at night. In megacities (cities with a population exceeding 10 million people) like Tokyo, the urban heat island effect may contribute to daytime temperatures that are more than 11°C (20°F) warmer than their rural surroundings.
Few who live in heavily developed urban centers would dispute the presence of UHIs. In fact, the dominating influence of UHIs can even be seen in National Weather Service watch/warning products as shown in Figure 3.
When discussing UHIs it is important to distinguish between surface and atmospheric UHIs.
Surface UHIs represent the surface temperature of exposed objects like roofs, pavement, and lawns. For example, pavement might have surface temperatures 30-55°C (50-100°F) warmer than the air temperature. They are present both in the daytime and nighttime, although they are strongest during the day. Shaded objects have lower surface temperatures. The urban temperature image in Figure 1 is measuring the surface urban heat island effect.
Atmospheric UHIs are determined by comparing the temperature of the air between urban and rural areas. Atmospheric UHIs are weak or absent during the daytime and strongest at night or early in the morning due to the delayed release of heat from the urban infrastructure. This can have important consequences for individuals seeking nighttime relief during heat wave events. Atmospheric UHIs can be further subdivided into canopy layer UHIs and boundary layer UHIs. Canopy layer UHIs use air temperature in the layer of air where humans exist and includes the layer of air from the surface to the tops of trees and rooftops. Boundary layer UHIs use air temperature in the layer of air from the tops of trees and rooftops up to a point where the urban environment no longer influences the atmosphere. This typically extends no higher than 1.5 km (1 mi) up in the atmosphere. Figure 2 depicts a canopy layer UHI taken from sheltered air temperature stations 1-2 meters above the ground. In general, canopy layer UHIs are most commonly referred to when discussing the effects of cities on development of UHIs.
Urban areas are typically warmer than their rural surroundings because of reduced vegetation cover and increased buildings, roads, and other infrastructure. In rural areas, the presence of trees and other types of vegetation provide shade and reduce air temperatures through evapotranspiration – a process by which water is either evaporated from the surface or released by plants during photosynthesis, thus dissipating heat through the phase change of water. In contrast, urban areas are often highly developed landscapes with less vegetation than their rural surroundings with dry and impervious surfaces that contribute little to no cooling through evapotranspiration. In addition, urban areas are warmer than their surroundings because materials used in the construction of buildings and infrastructure contribute to heat gain depending on how the sun’s energy is reflected, emitted, and absorbed. Dark rooftops and asphalt roadways have a low reflectivity and thus, absorb a large fraction of the sun’s energy incident on them whereas a white tile roof may reflect more radiation away and remain cooler.