For the first 20 days, the temperature still keepstays high.

It onlydoesn't drops down until late September.

Or "It only drops down in late September."

The temperature ranges from 20 to 30 degrees, so the weather is so nice at presentnow.

"At present" works, but it sounds very formal.

In autumn, the sky looks very hlight and blue, i. It seems like it is just washed out, and there are always white clouds within it.

I'm not sure what you mean by high, but hopefully this works.

The air is cool and dry, not like wet and sticky inlike in the summer.

I think we only have two and a half months of nice weather in autumn, t. That is from September, October and until the first part of November, because it becomes cold in the last part of November.

For me, I like taking walks and riding bikes in this seasoautumn.

For the first 20 days, the temperature still keepays high.

It only drops down untilaround late September.

“Until” here gives a feeling of it’s already cold and will rise again when it’s late September.

这句子听起来像“温度一直都是冷的，直到九月下旬它才会上升”。

It's early September now.

I would merge this with the sentence below.

这句子读起来有点儿尴尬。

TIt’s early September now and the temperature ranges from around 20 to 30 degrees, so the weather is sovery nice at presentcurrently/at the moment/now.

这里的“So”也听起来有点尴尬。

In the afternoon (however), it's a little hot, but it's a nice hot, not (type of) heat, unlike the heat during the summer.

In autumns/During the autumn season, the sky looks very high and blue, it seems like it is’s (it has) just been washed, and there are always white clouds within it.

The air is cool and dry, not like wet and sticky inlike in the summer.

I think we only have two and a half months of nice weather in autumn, thatwhich is September, October and the first parthalf of November because it becomegets cold in the last partter half of November.

Autumn is also a nice season for people to do exercise because it is neither hot nor cold.

For the first 20 days, the temperature still keepays high.

It only drops (down until) in late September.

The temperature ranges from 20 to 30 degrees, so the weather is so nice (at present/currently).

In the afternoon, it's a little hot, but it's a nice hoeat, not like the heat during summer.

In autumn, the sky looks very high and blue, it seems like it is just washed, and there are always white clouds within it.

I think we only have two and a half months of nice weather in autumn, that isin September, October and the first part of November because it becomes cold in the last part of November.

FAs for me, I like taking walks and riding bikes in this season.