What causes the "heat island" effect?

Here's a basic answer,

Normal temperatures of Tokyo go up more than those of the surrounding area. An urban heat island (UHI) is a city or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities ... The main cause of the urban heat island effect is from the modification of land surfaces.

I didn't answer this, but Google did.

Urbanization and by natural surfaces by built surfaces cause the heat island affect and is also in Toyko