A Yellow Heat Health Alert is in force across the South until tomorrow morning.
It started today at 01:00 (28/08/2023) and ends at 9am on Thursday.
Temperatures are expected to reach 23C or more in some areas.
Chichester, Bognor, Littlehampton and Portsmouth can expect 23C with sunshine and cloud. But in Horsham it's expected to be 26C with cloud.
The government's warning service says minor impacts are probable across the health and social care sector.
People are being reminded to check in on vulnerable people and drink plenty of water.