Loads of you love hot sunny weather, but experts are warning that it could bring misery to lots of people with breathing problems, like asthma.
A smog alert has been issued for parts of England and Wales because the warm, wind-free weather is mixing with city fumes and increasing air pollution.
Smog can make it harder for some people to breathe and is worse in big cities.
Anyone with breathing problems, such as asthma, is being advised to keep their inhalers on them and close the windows.
Smog is a type of air pollution. Fumes from cars and factories rise into the air and react with sunlight to create ozone gas.
In hot still weather, this ozone gas can settle and form a thick hazy layer, which is the smog we can see.