Do you need to know, when is the best time to go to Japan? Then you should take a look here, what weather you can expect there during the year.
The climate of Japan is very varied. The weather is tropical in south, but it is cool temperate in north. Most parts of Japan belong to the temperate zone, the climate here is humid subtropical with four distinct seasons. You can expect cool humid weather in the north, but warm tropical rainforest climate in the south.
As Japan has a really huge area, the information below relate mostly only to the area around the capital of this country. To see information about other places, please, choose one of these areas: Hokkaido, Chubu, Chugoku, Kansai, Kanto, Kyushu, Okinawa, Shikoku, Tohoku.
Let's look now what weather you can expect in the area around the capital and in many other cities. The warmest month here is August, the average maximum temperature is 30.9 ℃ (87.62 ℉). The average minimum temperature in this month is 25.1 ℃ (77.18 ℉). The coldest month here is January, the average maximum temperature is 10 ℃ (50 ℉). The average minimum temperature in this month is 2.7 ℃ (36.86 ℉). The most rainy month here is June, the average number of rainy days is 11.4. The least rainy month here is December, the average number of rainy days is 4.1. 
When travelling to some places in Japan, always consider the altitude of the the place as well. The highest point here is Mount Fuji (3776 m) and the lowest point is Hachiro-gata (-4 m). So there are huge differences in altitudes in this country and the weather is very different in them therefore. Keep in mind, that the temperature usually decreases by 1.2 - 1.9 ℃ (2.2 - 3.5 ℉) per 200 m (656 ft), depending on the other conditions. (And we should add, that in the very special condition named thermal inversion even the temperature may be higher in higher altitudes.)
Typical (usual) weather in Japan in January is this: The average highest temperature in Japan in January is 10 ℃ (50 ℉). The average lowest temperature is 2.7 ℃ (36.86 ℉). At the beginning of January you can expect higher temperatures, the average highest temperature is around 11.4 ℃ (52.52 ℉). At the end of January you can expect higher temperatures, the average highest temperature is around 10.35 ℃ (50.63 ℉). The average number of rainy days in January is 4.5. The average rainfall is 53.3 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 9:46 (hours and minutes), in the middle of the month 10:00 and at the end of the month 10:23.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Japan in February is this: The average highest temperature in Japan in February is 10.7 ℃ (51.26 ℉). The average lowest temperature is 3.1 ℃ (37.58 ℉). At the beginning of February you can expect lower temperatures, the average highest temperature is around 10.35 ℃ (50.63 ℉). At the end of February you can expect higher temperatures, the average highest temperature is around 12.05 ℃ (53.69 ℉). The average number of rainy days in February is 5.6. The average rainfall is 56.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 10:23 (hours and minutes), in the middle of the month 10:52 and at the end of the month 11:24.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Japan in March is this: The average highest temperature in Japan in March is 13.4 ℃ (56.12 ℉). The average lowest temperature is 5.8 ℃ (42.44 ℉). At the beginning of March you can expect lower temperatures, the average highest temperature is around 12.05 ℃ (53.69 ℉). At the end of March you can expect higher temperatures, the average highest temperature is around 16.3 ℃ (61.34 ℉). The average number of rainy days in March is 9.7. The average rainfall is 119.3 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:24 (hours and minutes), in the middle of the month 11:58 and at the end of the month 12:32.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.
Typical (usual) weather in Japan in April is this: The average highest temperature in Japan in April is 19.2 ℃ (66.56 ℉). The average lowest temperature is 10.9 ℃ (51.62 ℉). At the beginning of April you can expect lower temperatures, the average highest temperature is around 16.3 ℃ (61.34 ℉). At the end of April you can expect higher temperatures, the average highest temperature is around 21.25 ℃ (70.25 ℉). The average number of rainy days in April is 10. The average rainfall is 124.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:32 (hours and minutes), in the middle of the month 13:05 and at the end of the month 13:36.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.
Typical (usual) weather in Japan in May is this: The average highest temperature in Japan in May is 23.3 ℃ (73.94 ℉). The average lowest temperature is 15.4 ℃ (59.72 ℉). At the beginning of May you can expect lower temperatures, the average highest temperature is around 21.25 ℃ (70.25 ℉). At the end of May you can expect higher temperatures, the average highest temperature is around 24.8 ℃ (76.64 ℉). The average number of rainy days in May is 10.3. The average rainfall is 134.9 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 13:36 (hours and minutes), in the middle of the month 14:02 and at the end of the month 14:22.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Japan in June is this: The average highest temperature in Japan in June is 26.3 ℃ (79.34 ℉). The average lowest temperature is 19.7 ℃ (67.46 ℉). At the beginning of June you can expect lower temperatures, the average highest temperature is around 24.8 ℃ (76.64 ℉). At the end of June you can expect higher temperatures, the average highest temperature is around 28.05 ℃ (82.49 ℉). The average number of rainy days in June is 11.4. The average rainfall is 165.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 14:22 (hours and minutes), in the middle of the month 14:33 and at the end of the month 14:33.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.
Typical (usual) weather in Japan in July is this: The average highest temperature in Japan in July is 29.8 ℃ (85.64 ℉). The average lowest temperature is 23.3 ℃ (73.94 ℉). At the beginning of July you can expect lower temperatures, the average highest temperature is around 28.05 ℃ (82.49 ℉). At the end of July you can expect higher temperatures, the average highest temperature is around 30.35 ℃ (86.63 ℉). The average number of rainy days in July is 10.5. The average rainfall is 151.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 14:33 (hours and minutes), in the middle of the month 14:21 and at the end of the month 14:01.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Japan in August is this: The average highest temperature in Japan in August is 30.9 ℃ (87.62 ℉). The average lowest temperature is 25.1 ℃ (77.18 ℉). At the beginning of August you can expect lower temperatures, the average highest temperature is around 30.35 ℃ (86.63 ℉). At the end of August you can expect lower temperatures, the average highest temperature is around 29.1 ℃ (84.38 ℉). The average number of rainy days in August is 7.8. The average rainfall is 170.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 14:01 (hours and minutes), in the middle of the month 13:35 and at the end of the month 13:04.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Japan in September is this: The average highest temperature in Japan in September is 27.3 ℃ (81.14 ℉). The average lowest temperature is 21 ℃ (69.8 ℉). At the beginning of September you can expect higher temperatures, the average highest temperature is around 29.1 ℃ (84.38 ℉). At the end of September you can expect lower temperatures, the average highest temperature is around 24.8 ℃ (76.64 ℉). The average number of rainy days in September is 11.2. The average rainfall is 205.3 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 13:04 (hours and minutes), in the middle of the month 12:31 and at the end of the month 11:58.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Japan in October is this: The average highest temperature in Japan in October is 22.3 ℃ (72.14 ℉). The average lowest temperature is 15.5 ℃ (59.9 ℉). At the beginning of October you can expect higher temperatures, the average highest temperature is around 24.8 ℃ (76.64 ℉). At the end of October you can expect lower temperatures, the average highest temperature is around 19.6 ℃ (67.28 ℉). The average number of rainy days in October is 9.7. The average rainfall is 197.9 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:58 (hours and minutes), in the middle of the month 11:25 and at the end of the month 10:53.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Japan in November is this: The average highest temperature in Japan in November is 16.9 ℃ (62.42 ℉). The average lowest temperature is 10.2 ℃ (50.36 ℉). At the beginning of November you can expect higher temperatures, the average highest temperature is around 19.6 ℃ (67.28 ℉). At the end of November you can expect lower temperatures, the average highest temperature is around 14.85 ℃ (58.73 ℉). The average number of rainy days in November is 6.7. The average rainfall is 91.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 10:53 (hours and minutes), in the middle of the month 10:24 and at the end of the month 10:01.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Japan in December is this: The average highest temperature in Japan in December is 12.8 ℃ (55.04 ℉). The average lowest temperature is 5.2 ℃ (41.36 ℉). At the beginning of December you can expect higher temperatures, the average highest temperature is around 14.85 ℃ (58.73 ℉). At the end of December you can expect lower temperatures, the average highest temperature is around 11.4 ℃ (52.52 ℉). The average number of rainy days in December is 4.1. The average rainfall is 51.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 10:01 (hours and minutes), in the middle of the month 9:47 and at the end of the month 9:44.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.