The summer season (including the good 'ol rainy season) is approaching quickly. Humidity is up, heat is up, and the farmers have finally planted the rice. There are different ways the Japanese handle the humidity, but I stumbled across these the other day at a store - an entire display of "etiquette pads" and shirts with extra padding in the underarm area. Even Uniqlo (a popular clothing store, akin to Old Navy/Gap) is selling undershirts for women with these armpit pads.
Some pads seem to be placed directly on the shirt, although more often they not they are hooked from the bra strap. Or, you can buy a shirt with extra padding from Uniqlo (or a store of choice) like I did (want to see how they work).
Modesty is a virtue here in Japan, exhibiting itself in various capacities. One of these includes the fact that women tend to flush the toilet while doing their business in public restrooms. (Which is why you'll notice the sound-making machines or the strange, fake sound of a toilet flushing if you visit one). Since I don't regularly make appearances to guy's restrooms, I can't attest to their bathroom etiquette, although I've heard they are less inclined to be that polite.
Wearing panty-hose is still considered proper and polite (whereas, in the U.S. at least, younger women are forgoing the 'hose). Cleavage is rarely shown or seen, although yes, short skirts and shorts are common.
Yes, Japan can say its women are quite modest, although one could argue women in other cultures exhibit modesty in different ways. Nonetheless, who can argue that modern women, eastern and western, don't want to let the sweat shine through? Are sweaty armpits a positive attribute in other cultures? - That, I don't know - but I do know the lovely women in Japan can (if they wish) exercise some good etiquette and pad up.
Sorry guys, looks like you'll just have to sweat it out.