Within about tTwenty days after autumn startsthe start of autumn, it is still hot. Twenty days after the start of autumn, it is still hot.

I rephrased the first half of your sentence so that it sounds more fluent.

But this "hot" is different from the "hot" in summer. But this "hot" is different from the "hot" in summer.

Alternatively: "But this heat is different from the heat in summer."

The "hot" iIn autumn it is a dry "hot" while in summer it is a humid "hot" in summerone. In autumn it is a dry "hot" while in summer it is a humid one.

(1) I tweaked your sentence so that "hot" isn't used so many times. Unnecessary repetition can break the flow of the sentence. (2) I moved "in summer" forward so that we have parallel structure: "in autumn it is... in summer it is..."

In summer, the air contains more water, which meanso the air humidity is high, and the high humiditywhich makes it feel hotter. In summer, the air contains more water, so the air humidity is high, which makes it feel hotter.

I changed some parts of your sentence so that there is no need to repeat "the higher humidity". This might allow the sentence to flow better.

But when it is autumn, the air humidity is not (as) high, so the lower air humidity makes iit doesn't feel not very hot even if it's the same temperature as that in summer. But when it is autumn, the air humidity is not (as) high, so it doesn't feel very hot even if it's the same temperature as in summer.

Same as above.