Japan gets 1,705 millimeters of rain a year. In terms of annual rainfall, it ranks forty-sixth out of 180 countries. Columbia ranks number one at 3,240 millimeters per year, while Egypt comes in last with only 51 millimeters per year. The world annual average is 807 millimeters. Since Japan gets twice that amount, its annual rainfall can be said to be relatively heavy. Periods of heavy rainfall include the rainy season in June and the typhoon season of September/October. Tokyo's annual rainfall in 2017 was 1,430 millimeters, with the month of June accounting for 122 millimeters, or 8.5%, of the total. Because of direct strikes by relatively large typhoons, September accounted for 14.5%, while October accounted for 37%. Each year's typhoon season is the swing factor in determining that particular year's annual rainfall.
The rainy season in June is created when cold high-pressure systems from mainland China collide with warm thermal drafts over the Pacific Ocean, creating unstable conditions in the atmosphere that result in persistently rainy days. The rainy-season weather front tends to stagnate over Japan for extended periods. Characterized not only by steady rainfall but also by high humidity and rising temperatures, the rainy season is unpleasant and is therefore generally disliked, especially by city dwellers. However, the heavy rains are indispensable to cultivators of rice and other agricultural crops and can be a key factor in determining whether that year's harvest will be plentiful or poor. And, as a source of drinking water, the rains are actually important for city dwellers as well.
Japan has many names and terms for rain. Living as they do in a temperate humid climate with heavy rainfall, the Japanese accept the drawbacks, as well as the blessings, of rain. They are sensitive to the subtle differences of rain, drawing distinctions between rain that falls steadily, rain that falls strongly all at once, and rain that falls despite the lack of rain clouds. There are different names for the same type of rainfall and different names depending on the season. In fact, some say that the Japanese language has as many as 400 different names for rain. Seasonal rains include haru-same, a typical spring rain that falls from the end of February through late March; haku'u, a steady, fine 'white rain' summer rain shower; and aki-same, a cold autumn rain that appears as summer transitions to autumn. Then there is shigure, a kind of late autumn/early winter rain that tends to start and stop and is not very strong. There is also toori-ame, a light rain shower that comes after clear skies suddenly become dark. As a catchword signifying winter, toori-ame is often used in haiku poems.