The weather in chapter 23 is a continuation of the weather from chapter 22. It is hot. Unbearably hot. Readers are told that it is the kind of heat that seems to press down on you from all sides and take your breath away. Additionally, there isn't even the slightest hint of a breeze to give even a modicum of cooling.
It was the longest day: mindlessly hot, unspeakably hot, too hot to move or even think. The countryside, the village of Treegap, the wood—all lay defeated. Nothing stirred.
The text doesn't say the word "humid," but I imagine it is quite humid too. That's why Winnie's grandmother and mother sit there, fanning themselves, with "unsettled" hair. The proximity of the forest also tells readers that the heat isn't likely a dry, desert-type heat.
Then the weather and heat get worse. The air starts to feel "heavier":
The air was noticeably heavier. It pressed on Winnie's chest and made her breathing difficult.
Winnie correctly interprets that a storm is approaching, but at that moment all that has happened is an increase in relative humidity or pressure. The temperature hasn't started falling, and the breeze hasn't picked up. Everybody decides to go to bed early and try to sleep through the misery of the weather, and it is at this point that the air starts to stir with a breeze and cool down ever so slightly.