Japan has a rainy season around June every year, and September is also a common time for typhoons.

Ways to cope with the rainy season
  • Always have an umbrella - there are smaller umbrellas that can easily fit in a backpack.
  • Buy waterproof clothes (cheap ponchos/coats/trousers can be bought from 100 yen shops, convenience stores and supermarkets. Proper gear can be bought from clothes/outdoor/sports shops.)
  • Bring spare socks with you, and sandals for changing if your shoes get wet, or if you know it's going to rain, pack a change of clothes.
  • Wearing shorts/skirts can be more comfortable, as wet trouser legs at the bottom are uncomfortable in the heat/humidity.
  • Buy Wellington/Rain boots (can be found cheaply in home centres)
  • Buy a bicycle attachment that can hook an umbrella to your handle bars (cyling while holding an umbrella is common, but dangerous and frowned upon)
  • Buy a car

  • If you have a garden, weeds will grow extremely fast and tall in a short amount of time. You can ask your school to place weed killer in your garden, or garden on your own.
  • It doesn't rain everyday, but when a typhoon comes, there will be a period of rain that can last for a few days, or a week or so.
  • If there is a typhoon approaching, public transport may be suspended.
  • Tourist spots become less busy during rainy weather, so it's a good time to travel if you can put up with the weather.
  • Avoid the yellow lines for helping the blind, since those bumps are slippery and dangerous when wet.
  • For more tips, check out SurvivingInJapan's 40 rainy season tips here.

The warm, humid atmosphere makes perfect conditions for mould(mold) to grow.

Cleaning mould from surfaces
Bathrooms and kitchens are prone to mould growth. If it's growing on top of a hard surface, it shouldn't be hard to clean.
You can buy 'Kabi Killer' (Kabi Kira- カビキラー) to kill mould. Kabi Killer also kills many other things, like cockroaches.

Preventing mould
Mould is an organism which grows in the presence of water. So, removing the humidity will help to prevent mould growth.
You can buy an electric dehumidifier, but this is an expensive option. There are also dehumidifier packs with beads in them that you place around the room. They aren't as good as the electric ones, but they're better than nothing.
Sometimes, air conditioners have a 'dry' (ドライ)function, which can help with the humidity.
You need to take care with your clothes, if they grow mould, it's almost impossible to save them.
You can buy moisture absorbing packs, beads and hanging things to put inside your wardrobes and cupboards to help.
A DIY solution is to put rolled up news papers in your cupboards, they will absorb some moisture, and if you replace them regularly, can make a difference.

Cleaning regularly will also help keep mold from building up.

Tatami Mats
When it's hot, it's crucial to fold up your futon and put everything away (or, at least move it somewhere different), since your futon will trap the heat and cause mold to grow, which in turn, will cause ダニ (dani) or tatami bugs to infest the mats you sleep on in. The 100 yen stores and home stores also sell wipes (like for Swiffer-esque brooms) that you can clean your tatami with.

Also, airing out your futon will prevent mold from growing on your futon. You can either air them outside, or inside near a window that gets sunlight. For extra cleanliness, you can also beat your futon so the dust goes out of it.