Urban heat islands happen when cities become warmer than nearby rural areas because pavement, buildings, traffic, and waste heat trap and release energy. This cheat sheet helps students connect city design to temperature, air quality, energy use, water runoff, and human health. It is useful for reviewing causes, effects, measurements, and solutions for more sustainable cities.
The most important ideas include heat absorption, albedo, evapotranspiration, shade, and impervious surface area. Urban heat island intensity is often measured as UHI intensity = city temperature - rural temperature. Sustainable cities reduce heat by using trees, parks, cool roofs, permeable pavement, public transit, and efficient buildings.
These strategies work best when they are planned together and placed where people are most exposed to heat.
Key Facts
- Urban heat island intensity is calculated as UHI intensity = urban air temperature - nearby rural air temperature.
- Dark surfaces usually have low albedo, which means they absorb more sunlight and raise local temperatures.
- Albedo can be written as albedo = reflected sunlight / incoming sunlight, with higher values meaning more reflection.
- Impervious surfaces such as asphalt, concrete, and rooftops increase heat storage and reduce water soaking into the ground.
- Trees cool cities by providing shade and by evapotranspiration, which moves water vapor from plants into the air.
- Cool roofs and cool pavements reduce heat gain because they reflect more sunlight and often emit heat more effectively.
- Heat risk is higher when high temperatures combine with vulnerable populations, limited shade, poor housing, and low access to cooling.
- Sustainable city design uses land, energy, water, transportation, and green space planning to reduce environmental impact and improve quality of life.
Vocabulary
- Urban heat island
- An urban area that is warmer than surrounding rural areas because human-built surfaces absorb and release more heat.
- Albedo
- The fraction of incoming sunlight a surface reflects, with light surfaces usually having higher albedo than dark surfaces.
- Impervious surface
- A surface such as pavement or a roof that prevents water from soaking into the ground.
- Evapotranspiration
- The movement of water from soil and plants into the air, which can cool the surrounding environment.
- Green infrastructure
- Natural or planted systems such as trees, rain gardens, parks, and green roofs that manage heat, water, and air quality.
- Sustainable city
- A city designed to reduce pollution, conserve resources, protect ecosystems, and support healthy living for current and future residents.
Common Mistakes to Avoid
- Confusing weather with climate is incorrect because a single hot day does not prove a long-term urban heat island pattern.
- Assuming all cities have the same heat risk is wrong because heat exposure depends on tree cover, building density, surface materials, income, health, and access to cooling.
- Thinking shade and albedo are the same is inaccurate because shade blocks sunlight, while albedo describes how much sunlight a surface reflects.
- Ignoring nighttime temperatures is a mistake because urban materials can release stored heat after sunset and keep cities dangerously warm overnight.
- Counting any green space as equally helpful is wrong because small, poorly placed, or unhealthy plantings may cool less than connected tree canopy, parks, and green roofs near heat-exposed people.
Practice Questions
- 1 A downtown sensor reads 34°C and a nearby rural sensor reads 29°C. What is the urban heat island intensity?
- 2 A surface reflects 30 units of sunlight out of 100 incoming units. What is its albedo?
- 3 A city block has 8,000 m2 of total land area and 5,600 m2 of impervious surface. What percent of the block is impervious?
- 4 A city can fund either a cool-roof program downtown or tree planting in low-shade neighborhoods near schools and apartments. Explain which choice might reduce heat risk more and what evidence you would use.