An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night than during the day.
The urban heat island (UHI) has become one of the largest problems associated with the urbanization and industrialization of human civilization, as the increased temperatures associated with the UHI tend to exacerbate the threats to human health posed by thermal stress. As a result, the UHI has been a central theme among climatologists, and it is well documented in many metropolitan areas around the world.
A green roof, or rooftop garden, is a vegetative layer grown on a rooftop. Green roofs provide shade and remove heat from the air through evapotranspiration, reducing temperatures of the roof surface and the surrounding air... In addition to mitigating urban heat islands, the benefits of green roofs include: reduced energy use, reduced air pollution and greenhouse gas emissions, and improved human health and comfort.
With a warming climate, [UHI] is generally viewed as a threat to public health that needs mitigating. On the flip side, “Some organisms may thrive on urban conditions,” said tree physiologist Kevin Griffin of Columbia University’s Lamont-Doherty Earth Observatory, who oversaw the study. Griffin said that the city’s hot summer nights, while a misery for humans, are a boon to trees, allowing them to perform more of the chemical reactions needed for photosynthesis when the sun comes back up.
Warm temperatures in cities increase the need for air conditioning (A/C) to cool buildings. This elevated demand can strain the electrical grid on a hot summer afternoon, making it more susceptible to brown-outs and black-outs.
Urban heat islands can have worse air and water quality than their rural neighbors. UHIs often have lower air quality because there are more pollutants (waste products from vehicles, industry, and people) being pumped into the air. These pollutants are blocked from scattering and becoming less toxic by the urban landscape: buildings, roads, sidewalks, and parking lots.
The concept of an urban heat island in nothing new. In fact, the phenomenon has been studied since the early 19th century, though it was not formally named until later on. However, the overall heating of the Earth, caused in part by greenhouse gas emissions and shifts in wind patterns and pressure belts due to the melting of the icecaps, and the equally steady rise in urban populations combine to exacerbate their effects.
"Waste heat" also contributes to a UHI. People and their tools, such as cars and factories, are always burning off energy, whether they’re jogging, driving, or just living their day-to-day lives. The energy people burn off usually escapes in the form of heat. And if there are a lot of people in one area, that's a lot of heat.
Roofs and pavements can constitute about 60% of the surface area of a U.S. city. These surfaces are typically dark in color and thus absorb at least 80% of sunlight, causing them to get warmer than lighter colored surfaces.
The urban heat island effect (UHI) is the phenomenon that cities are usually much warmer than surrounding rural areas, especially at night. The main cause of UHI is that in cities, the land development, concrete and asphalt, absorbs heat during the day and gives it up during the night.
The National Weather Service says “The mechanisms which lead to the creation of an urban heat island (UHI) have been studied for nearly a century and are well understood….The effects of the UHI are most pronounced during the summer (June-July-August) months.”