Cities are typically 2–8°C warmer than surrounding rural areas because asphalt, concrete, and buildings absorb heat during the day and release it at night. This is the urban heat island effect.
Why it matters
The effect is strongest in summer and at night — exactly when you need relief. Two cities at the same latitude can feel very different after dark. A dense, heavily paved city retains heat long into the evening, while a city with more green space and water cools down faster.
This means raw temperature data (which often comes from airport weather stations outside the urban core) can understate how hot a city actually feels to someone living in the center.
How we measure it
We use satellite land-surface temperature data to calculate the heat differential between urban areas and their surroundings. This captures the actual thermal environment of the built-up area, not just the nearest weather station.
What it doesn't capture
- Urban heat retention varies within a city. A tree-lined neighborhood and a commercial district can differ by several degrees.
- Building material, street width, and green infrastructure all affect local heat — we measure the city-wide average, not block-by-block.
See it in action
Urban heat retention appears in the Climate Lab on every city page, showing how much extra heat the city traps compared to its surroundings.