National Center of Excellence ASU
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Urban Climate

Climate Study and UHI

Urban regions are among the most rapidly changing environments on earth. As cities grow, they impact local and regional climates, including temperature averages and extremes. Urban areas are known to alter mean annual air temperatures by 2-5°F per 100 years and up to 20°F at night. Temperature changes affect urban dweller in many ways, influencing their health and comfort, energy costs, air quality and visibility levels, water availability and quality, ecological services, recreation, and overall quality of life.

The climate section of the UHI initiative is about Analyzing Local and Regional Climate Changes: Past, Present-Day, and Prospects for the Future. In addition to analyzing temperature data - analysis of the circumstances surrounding historical and present day collection sites (location, area characteristics) is key.

Project Contact: Anthony Brazel

How have temperatures changed over the last 1000 years?

Graph 1

1000 Year Temperature Anomaly Trend

Rapid Urbanization and the UHI Trend

Graph 2

Phoenix-Casa Grande Annual Average Minimum Temperatures 1960 to present. Data from the Western Regional Climate Center. Note the large increase in Phoenix temperatures compared to the National monument site of Casa Grande.


Example of typical urban effects on the local climate system

Graph 3

Urban climate effects for a mid-latitude city with about 1 million inhabitants
(values for summer unless otherwise noted).


Background - What is the Urban Heat Island (UHI)?

For nearly 100 years, it has been believed that urban areas affect the local climate, mainly in terms of the temperature. The urban effect is due to changes in the thermal properties, moisture and aerodynamic character of the built environment. These changes create a distinct urban boundary layer, or heat dome. This heat dome extends vertically above the city, and in windy conditions can be located downwind as a plume. The temperatures within the heat dome can be 10°F (6°C) higher than the surrounding areas.

At a given time of day, a balance of incoming energy from the sun and outgoing heat from the surface determines the surface temperature. Solar radiation strikes the surface, and reflects a portion back to space and with the remainder both heating the surface and evaporating any water that may be present. The heat is transferred upwards, in part by thermal (infrared) radiation and by turbulence due to the wind flowing over the surface.

In built urban areas, there is generally less water on the surface, as compared to the outlying rural areas. In addition, the walls of buildings radiate horizontally instead of vertically, which traps the heat near the surface. Both of these factors result in the elevation of temperature that is the urban heat island (refer to figure 1).

FIGURE 1.

The low level heating over the cities also has low pressure associated with it. This results in a flow from the rural areas toward the urban center, with the air converging and rising over the city. This rising air can, if conditions are favorable, result in the triggering of thunderstorms over the city (refer to figure 2).

FIGURE 2.

The higher temperatures within the urban dome can also increase the rate of some chemical reactions, and in particular, the formation of low-level ozone. As a result, the urban heat island can have a profound effect on human comfort and health.

Urban Heat Island in Desert Cities

Special circumstances within desert cities alter the character of the urban heat island, both in general and its character throughout the day. In general, the surface within a desert metropolitan area has more available surface water than the natural desert due to irrigated landscapes. This results in a higher percentage of solar energy evaporating surface water, creating a cooler environment. Further, surfaces within the urban center described as hard and rough, compared to the sparsely vegetated natural desert. This urban surface roughness increases the turbulent transport of surface heat, which also helps keep the surface cool. These factors lead to a reduction of temperatures within the urban center that is known as the oasis effect.

FIGURE 3.

The highest daytime temperatures in the region occur in the natural desert areas (see figure 3). There is a slight mitigation of the temperatures in the urban center and xeriscaped residential areas due to evaporation of surface water and shading. Heavily irrigated residential areas are coolest, due both to evaporating surface water and the shading effect of trees. The agricultural areas have the highest surface moisture, but without shading of the surface, the temperatures are higher than in the mesic residential areas.

FIGURE 4.

FIGURE 5.

The situation at night is different from the day (see figure 4). At night, the surface temperature decreases as heat is radiated away from the surface. The most dramatic difference is seen in the desert areas where the cooling is the most complete. Within the built urban area, radiation trapping by the buildings inhibits cooling, and this results in the urban area being warmer than the surrounding areas at night. Temperatures within the urban core are the highest, with the residential areas being somewhat cooler and open areas being coolest. The mesic residential areas, which are cooler than the agricultural areas during the day, are warmer at night. The trees that shade the surface during the day trap radiation at night, and inhibit the cooling of the surface (see figure 5).