CATASTROPHIC fire danger ratings have been declared in two areas of South Australia as a blast of hot weather sweeps the state.

Many areas were expecting the mercury to top 40C today ahead of a cool change later in the day.

Adelaide was forecast to have a top of 39C before cooler weather tomorrow.

The Bureau of Meteorology said the extreme heat in combination with strong winds had prompted it to declare catastrophic conditions for the Yorke Peninsula and eastern Eyre Peninsula districts.

Five other districts were rated at extreme.

Fire services are on high alert across the state.