Urban heat islands have been created over time here in the United States and around the world. In these cities, the temperature on the hottest summer day is rising by up to 1°F each decade. In Baltimore, Phoenix, Tucson, Washington, Shanghai, and Tokyo, for example, scientific data show that July's maximum temperatures during the last 30 to 80 years have been steadily increasing at a rate of one-half to one degree Fahrenheit every ten years.

Cities all over the world have been warming up in the summer over the years.

Data from these cities indicate increases in urban temperatures ranging from 0.2°F to 0.8°F per decade.

Los Angeles is a striking example of how a city was transformed into an urban heat island.

With increasing irrigation and orchards, Los Angeles cooled until major urban development began. (The pronounced temperature depression in the late 1880-90's is due to the a volcanic eruption.) Temperatures are ten-year averages, including the previous four years and the following five.

In the 1930s, Los Angeles was an area covered with irrigated orchards. The high temperature in the summer of 1934 was 97°F. Then, as pavement, commercial buildings, and homes replaced trees, Los Angeles warmed steadily, reaching 105°F and higher in the 1990s. Link to: Vegetation , Cool Roofs , and Cool Pavements