Summers in Japan last from June to September and bring a mixture of high temperatures and high relative humidity. Temperatures across the country range from warm to very hot. The high humidity makes life in Japan's low-lying cities can feel uncomfortably close. Rain falls throughout the year in Japan, but by June and early July, the increasingly southerly winds bring on the country's main rainy season. For two or three weeks Japan is washed by the "Ume Rain," so-called because that is the time of year the ume, or plum tree, bears fruit. While it doesn't rain continuously, the downpour can go on for days at a time, with high humidity keeping everything damp. Following close on the heels of the heavy rains comes a hot and humid summer. Day after day, temperatures rise to over 30°C (86°F).
Northern Japan (northern Honshu and Hokkaido) experiences warm summers a respite from the heat and humidity further south. Hokkaido, with its vistas of mountains, forests, rivers, wetlands, and meadows, is much drier than the Tokyo area. Sapporo averages a relatively cooler 20.2°C (68°F). Wakkanai, on the northern end of Hokkaido, has an average July high temperature of 19°C (67°F). Rainfall is intermittent with sunshine.
Summer in central Japan is muggy. Tokyo's average high temperature in July is 27°C (82°F). Evenings are a bit more tolerable, with the average temperature dropping to 21°C (71°F). On average, the relative humidity in the Japanese capital in July ranges from 89% in the morning to 72% in the afternoon.
Okinawa, in the Ryukyu Islands, falls well inside the temperate zone, but its climate is subtropical with long, hot, humid summers due to the strong influences of the surrounding ocean, monsoons, typhoons, and the warm "Kuroshio" current. July in Naha, Okinawa, averages 37°C (87°F) for a daytime high temperature, cooling off only by an average of 4°C (7°F) at night.
About 28 typhoons develop in the western Pacific from June to September, and towards the end of summer and into early autumn, two or three regularly roar in from the southwest to strike the Ryukyu Islands. Most however, generally blow themselves out long before reaching Tokyo.