Cool Your City: Beat the Urban Heat Island
Understand the urban heat island: dark surfaces absorb and store heat, packed buildings trap it and little water evaporates, so the city stays warmer, especially at night; trees, water and reflective roofs reverse each effect.
Before you try
What if every roof in Delhi were painted white? How much cooler would the nights be?
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What happened?
Move a control to see what changes and why.
Typical effect sizes for each factor, not measurements of any one city; real heat islands vary with wind, humidity and layout.
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Try these ideas
- Compare the Concrete jungle and Green city presets at night.
- Keep everything else fixed and raise light roofs from 0% to 100%.
- Find the cheapest change that cuts the night-time difference below 2 °C.
Why it works
Walk from a park into a city centre on a summer evening and the air gets hotter. Dark roofs and roads soak up sunlight all day and release the heat slowly after dark; tightly packed buildings block the breeze and hold the warmth between them; and with little soil or water, there is almost no evaporation to carry heat away. The result is the urban heat island, often 3 to 5 °C warmer than the countryside at night.
The fixes work in reverse. Trees shade the ground and release water vapour that cools the air; ponds and fountains evaporate; white or reflective roofs bounce sunlight away before it becomes heat. Ahmedabad’s cool-roof programme is a real example. In this simulator you change tree cover, roof colour, water and building density and read the day and night temperatures against the countryside.
Check your understanding
Question 1 of 4
The urban heat island is usually biggest
Trees cool a city mainly by
A white roof helps because it
Which city is likely to have the biggest heat island?
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
For parents & teachers
- Measure the temperature over grass and over a car park at the same time.
- Ask: why are heat waves more dangerous in cities?
- Look up your city’s heat action plan.