I am planning to travel to Kyushu in August/September on a conducted tour and was wondering about the weather as it appears to be quite hot and humid at this time. I plan to travel to Mt Aso, Kurokawa Onsen and the Amakusa islands and wondered if it may be cooler in these particular areas?
Thank you

I visited Kyushu during the first two weeks in September, a couple of years ago. I visited Mt. Aso and Amakusa, but not Kurakawa. I also made some other stops. The weather was somewhat hot, and somewhat humid, but not unbearable as I found it to be in Kyoto on a previous trip. There was occasional rain as well. The main problem I found is that the humidity makes it difficult to do any sort of hiking. It made me feel "weighted down." So, in summary, as long as you don't plan on doing any vigorous hikes, you should be fine.

The Amakusa Islands are absolutely gorgeous. You are so fortunate to be going.

It varies, but quite often the dreadful muggy summer heat isn't quite over. Both Aso and Kurokawa are at higher elevations though so it will not be as bad, and actually the islands will be a bit more breezy. The big sight of Kumamoto, the castle, is still closed due to earthquake damage. Right now going up to the Aso crater rim is hit and miss. But at least there have been a few open days - it was more or less closed for 4 straight years from 2014. You can see a preview here.

Many thanks to both of you for your invaluable feedback. I am reconsidering my travel dates to a slightly cooler month. Thanks Adastra also for your notes on the castle and the crater!