Cities can change the local weather because of the gases they send into the air during still weather. Cars, factories, power stations and heating systems are all responsible.
Some of these gases combine when sunlight is strong and make a kind of yellow-brown haze known as smog.
The gases can also be carried into the clouds, where they mix with water droplets and make them acid. When this rain falls it is called acid rain and it can be very harmful to plants.
