See http://www.japan-guide.com/e/e2273.html for average monthly temperatures and rainfall in major Japanese cities.

Winter

Winter days in Japan are typically sunny but chilly.

Spring

Spring is warm and pleasant.

Rainy season

In June or July there is a rainy season, though it does not rain every day.

Summer

The summer, from July to the end of September, is hot and humid.

Typhoon season

Typhoons and storms can occur during any season.

Autumn

The autumn in October and November is relatively warm and very pleasant.

Humidity

There is a tendency for mold to grown in Japan, especially in the humid summers. To avoid this, it is best to keep your apartment well ventilated, or use the air conditioner to remove moisture. Supermarkets sell moisture-absorbing products for putting in drawers and cupboards.