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Urban areas usually experience the urban heat island (UHI) effect, that is, they are significantly warmer than surrounding rural areas. The temperature difference is usually larger at night than during the day, [1] and is most apparent when winds are weak, under block conditions, noticeably during the summer and winter. The main cause of the UHI effect is from the modification of land surfaces while waste heat generated by energy usage is a secondary contributor. [2] [3] [4] 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. [5] 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. [6] Urban areas occupy about 0.5% of the Earth's land surface but host more than half of the world's population. [7]
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. [8] The impact in a city can change a lot based on its local environment. Heat can be reduced by tree cover and green space which act as sources of shade and promote evaporative cooling. [9] Other options include green roofs, passive daytime radiative cooling applications, and the use of lighter-colored surfaces and less absorptive building materials. These reflect more sunlight and absorb less heat. [10] [11] [12]
Climate change is not the cause of urban heat islands but it is causing more frequent and more intense heat waves which in turn amplify the urban heat island effect in cities. [13] : 993 Compact, dense urban development may increase the urban heat island effect, leading to higher temperatures and increased exposure. [14]
The effects of the urban heat island effect on people often vary based on socioeconomic factors. In many regions, racial minorities and people of lower economic status are more likely to live in areas with high urban heat indices within cities. [15] [16] Lower-income individuals and families are also less likely to be able to afford resources to mitigate the effects of urban heat, such as air conditioning. As such, some scholars have claimed that urban heat islands are an environmental justice issue.
A definition of urban heat island is: "The relative warmth of a city compared with surrounding rural areas." [17] : 2926 This relative warmth is caused by "heat trapping due to land use, the configuration and design of the built environment, including street layout and building size, the heat-absorbing properties of urban building materials, reduced ventilation, reduced greenery and water features, and domestic and industrial heat emissions generated directly from human activities". [17] : 2926
For most cities, the difference in temperature between the urban and surrounding rural area is largest at night. While temperature difference is significant all year round, the difference is generally bigger in winter. [18] [19] The typical temperature difference is several degrees between the city and surrounding areas. The difference in temperature between an inner city and its surrounding suburbs is frequently mentioned in weather reports, as in "68 °F (20 °C) downtown, 64 °F (18 °C) in the suburbs". In the United States, the difference during the day is between 0.6–3.9 °C (1–7 °F), while the difference during the night is 1.1–2.8 °C (2–5 °F). The difference is larger for bigger cities and areas with a high air humidity. [20] [21]
Though the warmer air temperature within the UHI is generally most apparent at night, urban heat islands exhibit significant and somewhat paradoxical diurnal behavior. The air temperature difference between the UHI and the surrounding environment is large at night and small during the day. [22]
Throughout the daytime, particularly when the skies are cloudless, urban surfaces are warmed by the absorption of solar radiation. Surfaces in the urban areas tend to warm faster than those of the surrounding rural areas. By virtue of their high heat capacities, urban surfaces act as a giant reservoir of heat energy. For example, concrete can hold roughly 2,000 times as much heat as an equivalent volume of air. As a result, the large daytime surface temperature within the UHI is easily seen via thermal remote sensing. [23] As is often the case with daytime heating, this warming also has the effect of generating convective winds within the urban boundary layer. It is theorized that, due to the atmospheric mixing that results, the air temperature perturbation within the UHI is generally minimal or nonexistent during the day, though the surface temperatures can reach extremely high levels. [24]
At night, the situation reverses. The absence of solar heating leads to the decrease of atmospheric convection and the stabilization of urban boundary layer. If enough stabilization occurs, an inversion layer is formed. This traps urban air near the surface, and keeping surface air warm from the still-warm urban surfaces, resulting in warmer nighttime air temperatures within the UHI. Other than the heat retention properties of urban areas, the nighttime maximum in urban canyons could also be due to the blocking of "sky view" during cooling: surfaces lose heat at night principally by radiation to the comparatively cool sky, and this is blocked by the buildings in an urban area. Radiative cooling is more dominant when wind speed is low and the sky is cloudless, and indeed the UHI is found to be largest at night in these conditions. [25] [26]
The urban heat island temperature difference is not only usually larger at night than during the day, but also larger in winter than in summer.[ citation needed ] This is especially true in areas where snow is common, as cities tend to hold snow for shorter periods of time than surrounding rural areas (this is due to the higher insulation capacity of cities, as well as human activities such as plowing). This decreases the albedo of the city and thereby magnifies the heating effect. Higher wind speeds in rural areas, particularly in winter, can also function to make them cooler than urban areas. Regions with distinct wet and dry seasons will exhibit a larger urban heat island effect during the dry season.[ citation needed ]
If a city or town has a good system of taking weather observations the UHI can be measured directly. [27] An alternative is to use a complex simulation of the location to calculate the UHI, or to use an approximate empirical method. [28] [29] Such models allow the UHI to be included in estimates of future temperatures rises within cities due to climate change.
Leonard O. Myrup published the first comprehensive numerical treatment to predict the effects of the urban heat island (UHI) in 1969. [30] The heat island effect was found to be the net result of several competing physical processes. In general, reduced evaporation in the city center and the thermal properties of the city building and paving materials are the dominant parameters. [30] Modern simulation environments include ENVI-met, which simulates all interactions between building and ground surfaces, plants and ambient air. [31]
There are several causes of an urban heat island (UHI); for example, dark surfaces absorb significantly more solar radiation, which causes urban concentrations of roads and buildings to heat more than suburban and rural areas during the day; [2] materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the surrounding rural areas. This causes a change in the energy budget of the urban area, often leading to higher temperatures than surrounding rural areas. [32]
Pavements, parking lots, roads or, more generally speaking transport infrastructure, contribute significantly to the urban heat island effect. [33] For example, pavement infrastructure is a main contributor to urban heat during summer afternoons in Phoenix, United States. [33]
Another major reason is the lack of evapotranspiration (for example, through lack of vegetation) in urban areas. [26] The U.S. Forest Service found in 2018 that cities in the United States are losing 36 million trees each year. [34] With a decreased amount of vegetation, cities also lose the shade and evaporative cooling effect of trees. [35] [36]
Other causes of a UHI are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection and prevents pollutants from dissipating. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. [4] [37] [38]
High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere. [32] UHI not only raises urban temperatures but also increases ozone concentrations because ozone is a greenhouse gas whose formation will accelerate with the increase of temperature. [39]
Climate change is not a cause but an amplifier of the urban heat island effect. The IPCC Sixth Assessment Report from 2022 summarized the available research accordingly: "Climate change increases heat stress risks in cities [...] and amplifies the urban heat island across Asian cities at 1.5°C and 2°C warming levels, both substantially larger than under present climates [...]." [40] : 66
The report goes on to say: "In a warming world, increasing air temperature makes the urban heat island effect in cities worse. One key risk is heatwaves in cities that are likely to affect half of the future global urban population, with negative impacts on human health and economic productivity." [13] : 993
There are unhelpful interactions between heat and built infrastructure: These interactions increase the risk of heat stress for people living in cities. [13] : 993
Patterns of UHIs appear to be constant in neighborhoods that were historically given poor ratings by the Home Owners’ Loan Corporation in the U.S. These neighborhoods often lack the infrastructure necessary to combat the effects of UHIs.
There appears to be a correlation between historically redlined areas in the U.S. and those most vulnerable to heat exposure today. According to Professor Jeremy S. Hoffman and his colleagues, there is a strong positive relationship between historically redlined neighborhoods and the current heat indexes of those neighborhoods compared to non-redlined ones. [16] The researchers found that "nationally, land surface temperatures in redlined areas are approximately 2.6 °C warmer than in non-redlined areas."
Aside from the effect on temperature, UHIs can produce secondary effects on local meteorology, including the altering of local wind patterns, the development of clouds and fog, the humidity, and the rates of precipitation. [41] The extra heat provided by the UHI leads to greater upward motion, which can induce additional shower and thunderstorm activity. In addition, the UHI creates during the day a local low pressure area where relatively moist air from its rural surroundings converges, possibly leading to more favorable conditions for cloud formation. [42] Rainfall rates downwind of cities are increased between 48% and 116%. Partly as a result of this warming, monthly rainfall is about 28% greater between 20 to 40 miles (32 to 64 km) downwind of cities, compared with upwind. [43] Some cities show a total precipitation increase of 51%. [44]
One study concluded that cities change the climate in area 2–4 times larger than their own area. [45] One 1999 comparison between urban and rural areas proposed that urban heat island effects have little influence on global mean temperature trends. [46] Others suggested that urban heat islands affect global climate by impacting the jet stream. [47]
UHIs have the potential to directly influence the health and welfare of urban residents. As UHIs are characterized by increased temperature, they can potentially increase the magnitude and duration of heat waves within cities. The number of individuals exposed to extreme temperatures is increased by the UHI-induced warming. [48] The nighttime effect of UHIs can be particularly harmful during a heat wave, as it deprives urban residents of the cool relief found in rural areas during the night. [49]
Increased temperatures have been reported to cause heat illnesses, such as heat stroke, heat exhaustion, heat syncope, and heat cramps. [50]
Extreme heat is the deadliest form of weather in the U.S. Per a study by Professor Terri Adams-Fuller, heat waves kill more people in the U.S. than hurricanes, floods, and tornadoes combined. [51] These heat illnesses are more common within medium-to-large metro areas than the rest of the U.S., largely in part due to UHIs. Heat illnesses can also be compounded when combined with air pollution which is common in many urban areas.
Heat exposure can have adverse effects on mental health. Increases in temperature can contribute to increased aggression, as well as more cases of domestic violence and substance abuse. [52] Greater heat can also negatively impact school performance and education. According to a study by Hyunkuk Cho of Yeungnam University, an increased number of days with extreme heat each year correlates to a decrease in student test scores. [53]
High UHI intensity correlates with increased concentrations of air pollutants that gathered at night, which can affect the next day's air quality. [54] These pollutants include volatile organic compounds, carbon monoxide, nitrogen oxides, and particulate matter. [55] The production of these pollutants combined with the higher temperatures in UHIs can quicken the production of ozone. [54] Ozone at surface level is considered to be a harmful pollutant. [54] Studies suggest that increased temperatures in UHIs can increase polluted days but also note that other factors (e.g. air pressure, cloud cover, wind speed) can also have an effect on pollution. [54]
Studies from Hong Kong have found that areas of the city with poorer outdoor urban air ventilation tended to have stronger urban heat island effects [56] and had significantly higher all-cause mortality [57] compared to areas with better ventilation. Another study employing advanced statistical methods in Babol city, Iran, revealed a significant increase in Surface Urban Heat Island Intensity (SUHII) from 1985 to 2017, influenced by both geographic direction and time. This research, enhancing the understanding of SUHII's spatial and temporal variations, emphasizes the need for precise urban planning to mitigate the health impacts of urban heat islands. [58] Surface UHI's are more prominent during the day and are measured using the land surface temperature and remote sensing. [59]
UHIs also impair water quality. Hot pavement and rooftop surfaces transfer their excess heat to stormwater, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Additionally, increased urban water body temperatures lead to a decrease in biodiversity in the water. [60] For example, in August 2001, rains over Cedar Rapids, Iowa led to a 10.5 °C (18.9 °F) rise in the nearby stream within one hour, resulting in a fish kill which affected an estimated 188 fish. [61] Since the temperature of the rain was comparatively cool, the deaths could be attributed to the hot pavement of the city. Similar events have been documented across the American Midwest, as well as Oregon and California. [62] Rapid temperature changes can be stressful to aquatic ecosystems. [63]
With the temperature of the nearby buildings sometimes reaching a difference of over 50 °F (28 °C) from the near-surface air temperature, precipitation warms rapidly, and run-off into nearby streams, lakes and rivers (or other bodies of water) to provide excessive thermal pollution. The increase in thermal pollution has the potential to increase water temperature by 20 to 30 °F (11 to 17 °C). This increase causes the fish species inhabiting the body of water to undergo thermal stress and shock due to the rapid change in temperature of their habitat. [64]
Permeable pavements may reduce these effects by percolating water through the pavement into subsurface storage areas where it can be dissipated through absorption and evaporation. [65]
Species that are good at colonizing can use conditions provided by urban heat islands to thrive in regions outside of their normal range. Examples of this include the grey-headed flying fox (Pteropus poliocephalus) and the common house gecko (Hemidactylus frenatus). [66] Grey-headed flying foxes, found in Melbourne, Australia, colonized urban habitats following the increase in temperatures there. Increased temperatures, causing warmer winter conditions, made the city more similar in climate to the more northerly wildland habitat of the species.
With temperate climates, urban heat islands will extend the growing season, therefore altering breeding strategies of inhabiting species. [66] This can be best observed in the effects that urban heat islands have on water temperature (see effects on water bodies).
Urban heat islands caused by cities have altered the natural selection process. [66] Selective pressures like temporal variation in food, predation and water are relaxed causing a new set of selective forces to roll out. For example, within urban habitats, insects are more abundant than in rural areas. Insects are ectotherms. This means that they depend on the temperature of the environment to control their body temperature, making the warmer climates of the city perfect for their ability to thrive. A study done in Raleigh, North Carolina conducted on Parthenolecanium quercifex (oak scales), showed that this particular species preferred warmer climates and were therefore found in higher abundance in urban habitats than on oak trees in rural habitats. Over time spent living in urban habitats, they have adapted to thrive in warmer climates than in cooler ones. [67]
Another consequence of urban heat islands is the increased energy required for air conditioning and refrigeration in cities that are in comparatively hot climates. The heat island effect costs Los Angeles about US$ 100 million per year in energy (in the year 2000). [68] Through the implementation of heat island reduction strategies, significant annual net energy savings have been calculated for northern locations such as Chicago, Salt Lake City, and Toronto. [69]
Every year in the U.S. 15% of energy goes towards the air conditioning of buildings in these urban heat islands. It was reported in 1998 that "the air conditioning demand has risen 10% within the last 40 years." [70]
Increases in air conditioning use also serve to worsen the effects of UHIs at night. While cooler nights are often a reprieve from heat waves during the day, the residual heat created by the use of air conditioning systems can lead to higher nighttime temperatures. According to a study by Professor Francisco Salamanca Palou and colleagues, this residual heat can cause nighttime increases of up to 1°Celsius in urban areas. [71] Increased energy use from air conditioners also contributes to carbon emissions, which doubly exacerbates the effects of UHIs.
Strategies to improve urban resilience by reducing excessive heat in cities include: Planting trees in cities, white roofs and light-coloured concrete, green infrastructure (including green roofs), passive daytime radiative cooling.[ citation needed ]
The temperature difference between urban areas and the surrounding suburban or rural areas can be as much as 5 °C (9.0 °F). Nearly 40 percent of that increase is due to the prevalence of dark roofs, with the remainder coming from dark-colored pavement and the declining presence of vegetation. The heat island effect can be counteracted slightly by using white or reflective materials to build houses, roofs, pavements, and roads, thus increasing the overall albedo of the city. [72]
Concentric expansion of cities is unfavorable in terms of the urban heat island phenomenon. It is recommended to plan the development of cities in strips, consistent with the hydrographic network, taking into account green areas with various plant species. [73] In this way, it was planned to build urban settlements stretching over large areas, e.g. Kielce, Szczecin and Gdynia in Poland, Copenhagen in Denmark and Hamburg, Berlin and Kiel in Germany.
Planting trees around the city can be another way of increasing albedo and decreasing the urban heat island effect. It is recommended to plant deciduous trees because they can provide many benefits such as more shade in the summer and not blocking warmth in winter. [74] Trees are a necessary feature in combating most of the urban heat island effect because they reduce air temperatures by 10 °F (5.6 °C), [75] and surface temperatures by up to 20–45 °F (11–25 °C). [76] Another benefit of having trees in a city is that trees also help fight global warming by absorbing CO2 from the atmosphere.
Painting rooftops white has become a common strategy to reduce the heat island effect. [77] In cities, there are many dark colored surfaces that absorb the heat of the sun in turn lowering the albedo of the city. [77] White rooftops allow high solar reflectance and high solar emittance, increasing the albedo of the city or area the effect is occurring. [77]
Relative to remedying the other sources of the problem, replacing dark roofing requires the least amount of investment for the most immediate return. A cool roof made from a reflective material such as vinyl reflects at least 75 percent of the sun's rays, and emit at least 70 percent of the solar radiation absorbed by the building envelope. Asphalt built-up roofs (BUR), by comparison, reflect 6 percent to 26 percent of solar radiation. [78]
Using light-colored concrete has proven effective in reflecting up to 50% more light than asphalt and reducing ambient temperature. [79] A low albedo value, characteristic of black asphalt, absorbs a large percentage of solar heat creating warmer near-surface temperatures. Paving with light-colored concrete, in addition to replacing asphalt with light-colored concrete, communities may be able to lower average temperatures. [80] However, research into the interaction between reflective pavements and buildings has found that, unless the nearby buildings are fitted with reflective glass, solar radiation reflected off light-colored pavements can increase building temperatures, increasing air conditioning demands. [81] [82]
There are specific paint formulations for daytime radiative cooling that reflect up to 98.1% of sunlight. [83] [84]
Another option is to increase the amount of well-watered vegetation. These two options can be combined with the implementation of green roofs. Green roofs are excellent insulators during the warm weather months and the plants cool the surrounding environment. Air quality is improved as the plants absorb carbon dioxide with concomitant production of oxygen. [85]
Green roofs may decrease the urban heat island effect. Green roofery is the practice of having vegetation on a roof; such as having trees or a garden. The plants that are on the roof increase the albedo and decreases the urban heat island effect. [77] This method has been studied and criticized for the fact that green roofs are affected by climatic conditions, green roof variables are hard to measure, and are very complex systems. [77]
Stormwater management is another option to help mitigate the effect of the urban heat island. Stormwater management is the controlling the water produced by the storm in a way that protects property and infrastructure. [86] Urban infrastructure like streets, sidewalks, and parking lots do not allow for water to penetrate into the earth's surface causing water to flood. By using stormwater management, you can control the flow of the water in ways that can mitigate UHI effect. One way is using a stormwater management technique called pervious pavement system (PPS). This technique has been used in over 30 countries and found to be successful in stormwater management and UHI mitigation. The PPS allows water to flow through the pavement allowing for the water to be absorbed causing the area to be cooled by evaporation. [87]
The cost efficiency of green roofs is quite high because of several reasons.[ citation needed ] For one, green roofs have over double the lifespan of a conventional roof, effectively decelerating the amount of roof replacements every year. In addition to roof-life, green roofs add stormwater management reducing fees for utilities. The cost for green roofs is more in the beginning, but over a period of time, their efficiency provides financial as well as health benefits. However, "A conventional roof is estimated to be $83.78/m2 while a green roof was estimated at $158.82/m2." [88] [ clarification needed ]
Green parking lots use vegetation and surfaces other than asphalt to limit the urban heat island effect.
Green infrastructure can often lead to "green gentrification," whereby the construction of these spaces leads to increased housing costs in the nearby area. This phenomenon often leads to the people who inhabit the area not being able to afford housing and being displaced from their homes. For example, a study from economics professors Katie Jo Black and Mallory Richards found that the construction of New York's "High Line" has led to cost increases of over 35% for homes adjacent to the project. [94]
A passive daytime radiative cooling roof application can double the energy savings of a white roof, attributed to high solar reflectance and thermal emittance in the infrared window, [95] with the highest cooling potential in hot and dry cities such as Phoenix and Las Vegas. [96] When installed on roofs in dense urban areas, passive daytime radiative cooling panels can significantly lower outdoor surface temperatures at the pedestrian level. [11] [12]
The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. [97] A description of the very first report of the UHI by Luke Howard said that the urban center of London was warmer at night than the surrounding countryside by 2.1 °C (3.7 °F). [98]
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 methods to understand the phenomenon.
In 1929, Albert Peppler used the term in a German publication believed to be the first instance of an equivalent to urban heat island: städtische Wärmeinsel (which is urban heat island in German). [99] Between 1990 and 2000, about 30 studies were published annually; by 2010, that number had increased to 100, and by 2015, it was more than 300. [100]
Leonard O. Myrup published the first comprehensive numerical treatment to predict the effects of the urban heat island (UHI) in 1969. [30] His paper surveys UHI and criticizes then-existing theories as being excessively qualitative.
Some studies suggest that the effects of UHIs on health may be disproportionate, since the impacts may be unevenly distributed based on a variety of factors such as age, [55] [101] ethnicity and socioeconomic status. [102] This raises the possibility of health impacts from UHIs being an environmental justice issue. Studies have shown that communities of color in the United States have been disproportionately affected by UHI. [103] [104] [105]
There is a correlation between neighborhood income and tree canopy cover. [106] Low-income neighborhoods tend to have significantly fewer trees than neighborhoods with higher incomes. [107] Researchers hypothesized that less-well-off neighborhoods do not have the financial resources to plant and maintain trees. Affluent neighborhoods can afford more trees, on "both public and private property." [108] One reason for this discrepancy is that wealthier homeowners and communities can afford more land, which can be kept open as green space, whereas poorer housing often takes the form of rentals, where landowners try to maximize their profit by putting as much housing density as possible on their land. [109]
There is also a correlation in the U.S. between ethnicity and exposure to UHIs. Within most U.S. cities, people of color are more likely to live in areas of high Surface Urban Heat Island Intensity than white people in the same cities. According to a study by climatologist Angel Hsu and colleagues, "the average person of color lives in a census tract with higher SUHI intensity than non-Hispanic whites in all but 6 of the 175 largest urbanized areas" in the U.S. [15]
Economic status also plays a role in the human effects on UHIs. Not only are people of lower economic classes more likely to live in UHIs, but they are also less likely to be able to afford commodities such as air conditioning. Like the correlation between SUHI intensity and ethnicity, a similar pattern can be seen when comparing households under the poverty line against those with an income more than double the poverty line. [15]
UHIs can have particularly strong effects on African Americans with chronic diseases. African Americans have higher rates of various chronic diseases, such as asthma and diabetes, than the general population. Per Professor Pamela Jackson and colleagues, these diseases can be exacerbated by extreme heat, leading to health problems such as hypertension or stroke. [110]
On a global scale, there are discrepancies in the effects of UHIs in different regions of the world. While overall heat exposure is increasing worldwide, its effects have increased faster in the Global South in recent decades, per a study by Professor Kanging Huang and colleagues. [111]
The disproportionate impact of UHIs on the Global South exacerbates already occurring environmental injustices. Because many countries in the equator are naturally hot and humid, these areas are particularly susceptible to the effects of UHIs. One World Bank study has found a 7.0° disparity between the hottest and coolest neighborhoods in Bandung, Indonesia. [112]
Researchers have also noted that the spread of impervious surfaces, such as concrete, tar, and asphalt, is correlated with neighborhoods of low socioeconomic status across various U.S. cities and states. [113] The presence of these materials serves as a predictor of "intra-urban variation in temperature".
Beginning in the 2020s, a number of cities worldwide began creating Chief Heat Officer positions to organize and manage work counteracting the urban heat island effect. [114] [115]
The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject.(March 2023) |
Bill S.4280, [116] introduced to the U.S. Senate in 2020, would authorize the National Integrated Heat Health Information System Interagency Committee (NIHHIS) to tackle extreme heat in the United States. [117] Successful passage of this legislation would fund NIHHIS for five years and would instate a $100 million grant program within NIHHIS to encourage and fund urban heat reduction projects, including those using cools roofs and pavements and those improving HVAC systems. As of July 22, 2020 the bill has not moved past introduction to Congress.
The city of New York determined that the cooling potential per area was highest for street trees, followed by living roofs, light covered surface, and open space planting. From the standpoint of cost effectiveness, light surfaces, light roofs, and curbside planting have lower costs per temperature reduction. [118]
A hypothetical "cool communities" program in Los Angeles has projected in 1997 that urban temperatures could be reduced by approximately 3 °C (5 °F) after planting ten million trees, reroofing five million homes, and painting one-quarter of the roads at an estimated cost of US$1 billion, giving estimated annual benefits of US$170 million from reduced air-conditioning costs and US$360 million in smog related health savings. [74]
In a case study of the Los Angeles Basin in 1998, simulations showed that even when trees are not strategically placed in these urban heat islands, they can still aid in minimization of pollutants and energy reduction. It is estimated that with this wide-scale implementation, the city of Los Angeles can annually save $100M with most of the savings coming from cool roofs, lighter colored pavement, and the planting of trees. With a citywide implementation, added benefits from the lowering smog-level would result in at least one billion dollars of saving per year. [70]
Los Angeles TreePeople is an example of how tree planting can empower a community. Tree people provides the opportunity for people to come together, build capacity, community pride and the opportunity to collaborate and network with each other. [119]
Los Angeles has also begun to implement a Heat Action Plan to address the city's needs at a more granular level than the solutions provided by the state of California. The city uses the LA Equity Index in an effort to ensure that the effects of extreme heat are mitigated in an equitable manner. [120]
In 2021, Climate Adaptation Planning Analysis (CAPA) received funding from the National Oceanic and Atmospheric Administration to conduct Heat Mapping across the United States. [121] Ten areas from Virginia - Abington, Arlington, Charlottesville, Farmville, Harrisonburg, Lynchburg, Petersburg, Richmond, Salem, Virginia Beach and Winchester - participated in the heat watch campaign. This campaign consisted of 213 Volunteers brought together by campaign organizers who made 490,423 Heat Measurements across 70 Routes total. After taking measurements throughout the day, equipment and data was sent back to CAPA where it was analyzed using machine learning algorithms. After analysis of the data, CAPA came back together with campaign organizers from each area to discuss potential plans for each town in the future.
New York City implemented its "Cool Neighborhoods NYC" program in 2017 intending to mitigate the effects of extreme urban heat. One of the plan's ambitions was to increase funding for the city's Low-Income Home Energy Assistance Program. Specifically, the plan sought to increase funding for cooling solutions for lower-income families. [122] Arizona
Tempe Arizona has had many programs aimed at reducing the urban heat island. One of those programs is called TreeBate where the City of Tempe will provide a rebate up to $75 toward a purchase of a native desert plant. This program is active every year from July 1 to the June 30. Another program is called the Rain to Roots Master Plan where their goal is to implement, maintain, and fund green stormwater infrastructure. This program started in September 2023 and is planned to be completed by fall of 2024. [123]
Athens, the capital of Greece, has undertaken initiatives to reduce the urban heat island effect and reduce the impact of pollution from vehicles. To create green spaces that offer cooling, small unused plots of land are being reconfigured into pocket parks. [124]
Sydney, Australia has one of the worst urban heat islands ever. One example is from 4 January 2020 when Sydney's western suburbs were considered the "hottest place on Earth". Suburbs such as Bankstown and Parramatta recorded temperatures of 47.3 and 47.0 °C (117.1 and 116.6 °F) with Observatory Hill recording 43.7 °C (110.7 °F). The hottest suburb at the time was Penrith, which had a reading of 48.9 °C (120.0 °F). Despite all this, Sydney's central area was sitting at a comfortably hot 35 °C (95 °F) with coastal areas seldom reaching 30 °C (86 °F).
Multiple factors that cause this can be due to it being far from the coast, kilometres of identical houses with black roofs along with greenery being replaced by tarmac and black asphalt (absorbing more heat and radiating it) and little room for yards to grow trees (therefore less shade).
Though this is not the only time the urban heat island has played its role in Sydney's west, the mercury has climbed to 45.2 °C (113.4 °F) on 1 January 2006, 42.9 °C (109.2 °F) on 23 January 2010, 46.5 °C (115.7 °F) on 18 January 2013, 46.9 °C (116.4 °F) on 11 February 2017 and 47.3 °C (117.1 °F) on 7 January 2018.
These urban heat differences usually only range from 2 to 7 °C (36 to 45 °F), rarely exceeding 10 °C (50 °F). Sydney's biggest urban heat island difference was recorded on 1 February 2020, roughly a month after Penrith's maximum record. A reading of 34.8 °C (94.6 °F) was recorded at Sydney's Observatory Hill; meanwhile, the mercury skyrocketed to 46.9 °C (116.4 °F) in Penrith Lakes. What makes this remarkable is that it is a difference of 12.1 °C (53.8 °F) along with southwestern/mid-western Sydney suburbs seldom reaching 37 °C (99 °F).
Albedo is the fraction of sunlight that is diffusely reflected by a body. It is measured on a scale from 0 to 1. Surface albedo is defined as the ratio of radiosity Je to the irradiance Ee received by a surface. The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface. These factors vary with atmospheric composition, geographic location, and time.
A microclimate is a local set of atmospheric conditions that differ from those in the surrounding areas, often slightly but sometimes substantially. The term may refer to areas as small as a few square meters or smaller or as large as many square kilometers. Because climate is statistical, which implies spatial and temporal variation of the mean values of the describing parameters, within a region there can occur and persist over time sets of statistically distinct conditions, that is, microclimates. Microclimates can be found in most places but are most pronounced in topographically dynamic zones such as mountainous areas, islands, and coastal areas.
Energy-efficient landscaping is a type of landscaping designed for the purpose of conserving energy. There is a distinction between the embedded energy of materials and constructing the landscape, and the energy consumed by the maintenance and operations of a landscape.
A heat wave, sometimes described as extreme heat, is a period of abnormally hot weather. Definitions vary but are similar. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season. Temperatures that humans from a hotter climate consider normal, can be regarded as a heat wave in a cooler area. This would be the case if the warm temperatures are outside the normal climate pattern for that area. High humidity often occurs during heat waves as well. This is especially the case in oceanic climate countries. Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s. Heat waves occur from climate change.
The climate in urban areas differs from that in neighboring rural areas, as a result of urban development. Urbanization greatly changes the form of the landscape, and also produces changes in an area's air. The study of urban climate is urban climatology.
Urban climatology is the study of urban climate. It is a branch of climatology that concerns interactions between urban areas and the atmosphere, the effects they have on one another, and the varying spatial and temporal scales at which these processes occur.
Reflective surfaces, or ground-based albedo modification (GBAM), is a solar radiation management method of enhancing Earth's albedo. The IPCC described this method as "whitening roofs, changes in land use management, change of albedo at a larger scale ."
An urban forest is a forest, or a collection of trees, that grow within a city, town or a suburb. In a wider sense, it may include any kind of woody plant vegetation growing in and around human settlements. As opposed to a forest park, whose ecosystems are also inherited from wilderness leftovers, urban forests often lack amenities like public bathrooms, paved paths, or sometimes clear borders which are distinct features of parks. Care and management of urban forests is called urban forestry. Urban forests can be privately and publicly owned. Some municipal forests may be located outside of the town or city to which they belong.
Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility than the original energy source. Sources of waste heat include all manner of human activities, natural systems, and all organisms, for example, incandescent light bulbs get hot, a refrigerator warms the room air, a building gets hot during peak hours, an internal combustion engine generates high-temperature exhaust gases, and electronic components get warm when in operation.
Sustainable drainage systems are a collection of water management practices that aim to align modern drainage systems with natural water processes and are part of a larger green infrastructure strategy. SuDS efforts make urban drainage systems more compatible with components of the natural water cycle such as storm surge overflows, soil percolation, and bio-filtration. These efforts hope to mitigate the effect human development has had or may have on the natural water cycle, particularly surface runoff and water pollution trends.
In land-use planning, urban green space is open-space areas reserved for parks and other "green spaces", including plant life, water features - also referred to as blue spaces - and other kinds of natural environment. Most urban open spaces are green spaces, but occasionally include other kinds of open areas. The landscape of urban open spaces can range from playing fields to highly maintained environments to relatively natural landscapes.
An urban thermal plume describes rising air in the lower altitudes of the Earth's atmosphere caused by urban areas being warmer than surrounding areas. Over the past thirty years there has been increasing interest in what have been called urban heat islands (UHI), but it is only since 2007 that thought has been given to the rising columns of warm air, or ‘thermal plumes’ that they produce. Common on-shore breezes at the seaside on a warm day, and off-shore breezes at night are caused by the land heating up faster on a sunny day and cooling faster after sunset, respectively. Thermals, or warm airs, that rise from the land and sea affect the local microscale meteorology; and perhaps at times the mesometeorology. Urban thermal plumes have as powerful although less localized an effect.
Urban resilience has conventionally been defined as the "measurable ability of any urban system, with its inhabitants, to maintain continuity through all shocks and stresses, while positively adapting and transforming towards sustainability".
Cool pavement is a road surface that uses additives to reflect solar radiation unlike conventional dark pavement. Conventional dark pavements contribute to urban heat islands as they absorb 80–95% of sunlight and warm the local air. Cool pavements are made with different materials to increase albedo, thereby reflecting shortwave radiation out of the atmosphere. Increasing albedo reduces heat transfer to the surface and can hypothetically cause local cooling if the spatial scale of the albedo reduction is sufficiently large. The EPA reports "that if pavement reflectance throughout a city were increased from 10 to 35 percent, the air temperature could potentially be reduced by 1°F (0.6°C)." Existing dark pavement can be altered to increase albedo through whitetopping or by adding reflective coats and seals. New pavement can be constructed to increase albedo by using modified mixes, permeable pavements, and vegetated pavements.
The oasis effect refers to the creation of a local microclimate that is cooler than the surrounding dry area due to evaporation or evapotranspiration of a water source or plant life and higher albedo of plant life than bare ground. The oasis effect is so-named because it occurs in desert oases. Urban planners can design a city's layout to optimize the oasis effect to combat the urban heat island effect. Since it depends on evaporation, the oasis effect differs by season.
A heat dome is a weather phenomenon consisting of extreme heat that is caused when the atmosphere traps hot ocean air, as if bounded by a lid or cap. Heat domes happen when strong high pressure atmospheric conditions remain stationary for an unusual amount of time, preventing convection and precipitation and keeping hot air "trapped" within a region. This can be caused by multiple factors, including sea surface temperature anomalies and the influence of a La Niña. The upper air weather patterns are slow to move, referred to by meteorologists as an Omega block.
Passive daytime radiative cooling (PDRC) is a zero-energy building cooling method proposed as a solution to reduce air conditioning, lower urban heat island effect, cool human body temperatures in extreme heat, move toward carbon neutrality and control global warming by enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zero energy consumption or pollution. In contrast to compression-based cooling systems that are prevalently used, consume substantial amounts of energy, have a net heating effect, require ready access to electricity and often require coolants that are ozone-depleting or have a strong greenhouse effect, application of PDRCs may also increase the efficiency of systems benefiting from a better cooling, such like photovoltaic systems, dew collection techniques, and thermoelectric generators.
Transpirational cooling is the cooling provided as plants transpire water. Excess heat generated from solar radiation is damaging to plant cells and thermal injury occurs during drought or when there is rapid transpiration which produces wilting. Green vegetation contributes to moderating climate by being cooler than adjacent bare earth or constructed areas. As plant leaves transpire they use energy to evaporate water aggregating up to a huge volume globally every day.
Urban forest inequity, also know as shade inequity or tree canopy inequity, is the inequitable distribution of trees, with their associated benefits, across metropolitan areas. This phenomenon has a number of follow-on effects, including but not limited to measurable impacts on faunal biodiversity and the urban heat island effect. Urban heat inequity occurs when intra-urban heat islands, with their associated negative physical and emotional health consequences, are more common and more intense in lower-income communities.
Urban heat inequity, also termed thermal inequity, is an unequal distribution of heat in urban areas or neighborhoods within them, which causes disproportionate impacts to people living in those communities. Unequal threat of heat stress in urban environments is often correlated with differences in demographics, including racial and ethnic background, income, education level, and age. While the general impacts of urban heat inequity depend on the city studied, negative effects typically act on historically marginalized communities. The idea is closely tied to the urban heat island effect, where a major cause to urban heat inequity is increased urbanization.
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