More to the point, they're humid like you've rarely experienced.
After a grueling day in the heat, you might be tempted to blast the A/C in your room or ride in the coldest train car.
I'm here to warn you: don't do it!
While a brief blast of cold air is refreshing, you shouldn't shock your body by subjecting it to extreme temperatures. It's a sure way to catch a cold, and I'm sure you'll agree that you don't want to spend your homestay sneezing and sniffling.
Okay, you might say, I'll keep the A/C off or to a minimum in my room at night, but what about public transportation? Trains, planes, and buses are all kept nice and icy during the summer.
I would suggest packing a light cardigan or a scarf to use in frigid indoor conditions.