Ah, summer. When eInk screens start to suffer
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If I'm getting this right, you're using the black flash on every page, right?
If so, that's pretty much how eInk works. Heat makes the symptoms worse, because the black flash lasts longer.
You can try to trade-off the lighter text for ghosting by disabling the black flash on every page (or using your device in a climate controlled environment).
(As usual with eInk screens, there might be some variances between production runs, but it mostly always behaves the same on that front. If you have access to a Kobo Aura, you can check if their weird grey-checkered flash helps in these conditions, but, err, we can't do that on a Kindle anyway, so that's just as a point of comparison ^^).
Those are some pretty crappy workarounds, I know
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