Urban heat island

An urban heat island describes a temperature contrast associated with built-up land: a city or neighborhood is warmer than less-developed surroundings, or warmer than greener and more open places within the same urban area. The term does not mean that every street is hotter than every nearby rural location. Conditions vary across a city and change with weather, season, time of day and the way temperatures are measured.

Researchers distinguish surface heat islands from atmospheric ones. A surface heat island concerns the temperature of roads, roofs, soil and other surfaces, often mapped with satellite or infrared measurements. An atmospheric, or canopy-layer, heat island concerns the air temperature in the urban environment near the ground and among buildings and trees. Surface readings and air readings measure different things and should not be treated as interchangeable. Air temperature is particularly relevant to conditions people experience; surface data can help reveal fine-scale differences between features such as pavement, roofs and parks.

Urban development changes the surface and energy balance of a place. Dark or conventional paving and roofing can reflect less sunlight and absorb and store more heat than many natural surfaces. Trees and other vegetation provide shade, while plants and moist ground cool the air through evapotranspiration; where greenery and water are scarce, these cooling processes are reduced. Vehicles, air-conditioning equipment, buildings and industry also release waste heat.

The layout of streets and buildings matters as well. Dense structures can restrict air movement, and surrounding buildings can slow the release of stored heat. As a result, heat may build during the day and remain in the urban environment after sunset. Calm, clear weather tends to favor stronger heat-island conditions, while cloud cover and stronger winds can weaken them. The contribution of each factor varies with local materials, vegetation, density, geography and weather.

A study of an urban heat island begins by deciding what contrast matters: the whole city compared with its surroundings, or hotter and cooler locations within a city. Researchers can measure air temperature with fixed weather stations or mobile sensors, and use satellites, aircraft or ground-based thermal instruments to assess surface temperatures. Each method has limits. A satellite can cover a broad area but observes only at particular times and may not capture shaded surfaces beneath trees or buildings; a few weather stations may miss neighborhood-scale hot spots. Combining methods can give a fuller picture.

The U.S. Environmental Protection Agency summarizes research finding urban temperatures roughly 1–7°F (about 0.6–3.9°C) higher than outlying areas during the day and 2–5°F (about 1.1–2.8°C) higher at night in U.S. studies. These are broad ranges, not a fixed value for every city or a comparison between identical surface types. Local contrasts depend on the study area, measurement type, weather and surrounding land cover. Community mapping campaigns, including NOAA-supported projects, have used sensors carried through neighborhoods to help identify localized hot spots and inform planning.

Higher local temperatures can add to heat stress during hot weather, reduce nighttime relief, increase demand for air-conditioning and contribute to air-quality problems. These effects are not shared evenly: exposure varies with neighborhood design and tree cover, while the ability to find shade or cool an indoor space also depends on people's circumstances. For this reason, citywide averages can hide locations and populations that face greater heat exposure. Urban heat islands are a local climate effect, distinct from global climate change, although a warmer background climate can intensify the consequences of urban heat.

Common responses include retaining or planting trees, increasing vegetation, using green roofs, and choosing reflective or otherwise cooler materials for roofs and pavements. Shade and better urban ventilation can also be part of planning. Such measures change the conditions that create heat islands, but their effects depend on where and how they are used; mapping local temperatures helps communities identify where interventions may be most useful.