If you’re looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Karatsu: The hottest months are usually August and July, with an average temperature of 25°C, while the coldest months are January and February, with an average of 9°C. The rainiest months in Karatsu are July, June, August and September, with each month seeing an average of 241 mm of rainfall.