Heat islands

Causes of heat islands

Thermal islands are formed due to several factors:

1. Reduction of natural landscapes in urban areas. Trees, vegetation and bodies of water tend to cool the air by providing shade, transpiring water from plant leaves and evaporating surface water. Hard, dry surfaces in urban areas - such as rooftops, sidewalks, streets, buildings and parking lots - offer less shade and moisture than natural landscapes and therefore contribute to higher temperatures.
2. Properties of urban materials. Conventional man-made materials used in urban environments, such as pavements or roofing, tend to reflect less solar energy, and absorb and emit more heat from the sun than trees, vegetation and other natural surfaces. Often, heat islands form during the day and become more pronounced after sunset due to the slow release of heat from urban materials.
3. Urban geometry. The size and layout of buildings within a city influence wind flow and the ability of urban materials to absorb and release solar energy. In highly developed areas, surfaces and structures obstructed by nearby buildings become large thermal masses that cannot easily release their heat. Cities with many narrow streets and tall buildings become urban canyons, which can block natural wind flow that would bring cooling effects.
4. Heat generated by human activities. Vehicles, air conditioning units, buildings and industrial plants all emit heat into the urban environment. These sources of human-generated, or anthropogenic, heat can contribute to heat island effects.
5. Weather and Geography. Calm, clear weather conditions result in more severe heat islands by maximizing the amount of solar energy that reaches urban surfaces and minimizing the amount of heat that can be carried away. In contrast, strong winds and cloud cover suppress heat island formation. Geographical features can also influence the heat island effect. For example, nearby mountains can block wind from reaching a city, or create wind patterns that pass through a city.

Impact of heat islands

High emissions of air pollutants and greenhouse gases

As described above, heat islands increase electricity demand in the summer. Electricity companies typically rely on fossil fuel power plants to meet much of this demand, which in turn leads to increased emissions of air pollutants and greenhouse gases.

These pollutants are harmful to human health and also contribute to complex air quality problems such as the formation of ground-level ozone (smog), fine particulate matter, and acid rain. The increased use of fossil fuel-fired power plants also increases emissions of greenhouse gases, such as carbon dioxide, which contribute to global climate change.

In addition to their impact on energy-related emissions, elevated temperatures can directly increase the rate of ozone formation at ground level. Ground-level ozone forms when nitrogen oxides and volatile organic compounds react in the presence of sunlight and heat. If all other variables are equal, such as the level of precursor emissions in the air and the speed and direction of the wind, more ozone will form at ground level as the environment becomes sunnier and warmer.

Compromising human health and comfort

Heat islands contribute to higher daytime temperatures, reduce nighttime cooling and increase air pollution levels. These factors, in turn, contribute to heat-related deaths and heat-related illnesses such as general discomfort, difficulty breathing, heat cramps, heat exhaustion, and non-fatal heat stroke.

Heat islands can also accentuate the impact of natural heat waves, which are periods of abnormally hot and often humid weather. Sensitive populations are particularly at risk during these events.

• The elderly are among the most vulnerable to extreme heat events. Many physiological, psychological and socio-economic factors contribute to this danger. Older people are more likely to be in poor health, to be less mobile and more isolated, to be more sensitive to heat and to live on low incomes.
• Young children tend to be more susceptible to extreme heat due to their smaller size and other characteristics. Their faster breathing rate for their body size, time spent outdoors, and their developing respiratory systems make them more likely to suffer from aggravated asthma and other lung diseases caused by air pollution from ozone and smog, which usually increases during heat waves.
• Low-income populations are at greater risk of heat-related illnesses due to poor housing conditions, including lack of air conditioning and small living spaces, and insufficient resources to find alternative shelter during a heat wave.
• People who spend their working hours outdoors are more prone to conditions such as heat exhaustion and heat stroke. They have higher exposures to ozone air pollution and heat stress, especially if work activities involve great physical effort.
• People in poor health, including those with chronic conditions, disabilities, mobility limitations, and those taking certain medications, are vulnerable to extreme temperatures. People with diabetes, physical impairments and cognitive deficits are particularly at risk during heat waves.

Excessive heat events, or sudden and dramatic increases in temperature, are particularly dangerous and can result in above-average mortality rates. From 2004 to 2018, the Centers for Disease Control and Prevention recorded 10,527 heat-related deaths in the United States, an average of 702 per year. These numbers include deaths where heat was the primary cause and deaths where heat was a contributing cause.

Compromised water quality

High temperatures on sidewalk and roof surfaces can heat storm runoff, which flows into storm drains and raises the temperature of the water as it is released into streams, rivers, ponds and lakes. Water temperature affects all aspects of aquatic life, particularly the metabolism and reproduction of many aquatic species. Rapid temperature changes in aquatic ecosystems resulting from warm runoff water can be particularly stressful, and even fatal, to aquatic life.

One study found that urban streams are warmer on average than streams in forested areas, and that temperatures in urban streams increased more than 4°C during small storms due to runoff water heated by urban materials.

Green infrastructure is an option for cooling storm runoff and improving water quality. May include the use of downspout disconnects, rain gardens, plant boxes, bioswales, permeable paving, green streets and alleys, green car parks and green roofs; as well as land conservation efforts.

Source: Learn About Heat Islands | US EPA