Adelaide is having its warmest start to summer in more than 20 years based on the number of days over 30 degrees.

As of Monday, Adelaide has registered five days over 30 degrees so far during December. This figure is forecast to reach seven days by the middle of this week, with tops of 35 and 32 degrees forecast on Tuesday and Wednesday, respectively.

The last time Adelaide recorded seven days at or above 30 degrees during the first 12 days of summer was in 1994, when there were also seven.

On average, Adelaide usually registers 10 days at or above 30 degrees during the entire month of December and 36 days this hot during summer as a whole.

Maximum temperatures will drop back to the low-to-mid twenties for the rest of this week, possibly only reaching the high teens on Friday.