During heatwaves, city centres are on average 4-6C (39-43F) warmer than surrounding areas according to the European Commission Joint Research Centre, external.
In extreme cases the difference can be as much as 10C (50F), especially in very calm weather with little wind to mix the air.
A key reason for this contrast is the hard, dark-coloured surfaces, like buildings and roads, that dominate urban landscapes.
They absorb the Sun's energy rather than reflecting it, meaning they store heat. This is then slowly released into the surrounding air leading to higher temperatures, especially at night.
Waste heat generated by air conditioning units, vehicles - and humans - adds to this even further.