Friday night is set to be the hottest of the year, with temperatures likely to stay above 20° from sundown to sunrise, according to weather bureau Weeronline.

The temperature will rise during the day and by nightfall on Friday, it will still be around 25 Celsius outdoors, weather forecasters say.

People who suffer from the heat should be sure to keep windows and doors closed during the day, and use curtains or blinds as an extra barrier against the warmth, says Weeronine’s Jaco van Wezel. Early morning, when it is coolest, is the best time to ventilate.

Weeronline expects the tropical temperatures of around 30° to continue over the weekend and up to the middle of next week. In the far south and east, the temperature could reach as much as 35°.

The high temperatures may also result in isolated thunderstorms, particularly on Saturday.

Weeronline says September temperatures are likely to be around the 20° average for the time of year, with showers and occasional thunderstorms.

The KNMI, however, puts the likelihood of temperatures dropping to below the seasonal average by late August at 60-70%.