An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparent when winds are weak. UHI is most noticeable during the summer and winter. The main cause of the UHI effect is from the modification of land surfaces. A study has shown that heat islands can be affected by proximity to different types of land cover, so that proximity to barren land causes urban land to become hotter and proximity to vegetation makes it cooler. Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area and increase its average temperature. The term heat island is also used; the term can be used to refer to any area that is relatively hotter than the surrounding, but generally refers to human-disturbed areas. Monthly rainfall is greater downwind of cities, partially due to the UHI. Increases in heat within urban centers increases the length of growing seasons and decreases the occurrence of weak tornadoes. The UHI decreases air quality by increasing the production of pollutants such as ozone, and decreases water quality as warmer waters flow into area streams and put stress on their ecosystems. Not all cities have a distinct urban heat island, and the heat island characteristics depend strongly on the background climate of the area in which the city is located. Effects within a city can vary significantly depending on local environmental conditions. Heat can be mitigated by tree cover and green space, which act as sources of shade and promote evaporative cooling. Mitigation of the urban heat island effect also can be accomplished through the use of green roofs, passive daytime radiative cooling applications, and the use of lighter-colored surfaces and less absorptive building materials in urban areas, to reflect more sunlight and absorb less heat. Urbanization has made the effects of climate change worse in cities. History The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. Investigations of the urban atmosphere continued throughout the nineteenth century. Between the 1920s and the 1940s, researchers in the emerging field of local climatology or microscale meteorology in Europe, Mexico, India, Japan, and the United States pursued new