Honshu, map

The climate of Hiroshima, a Japanese city in the southwestern part of Honshu Island, is temperate humid, with quite mild winters, and muggy and rainy summers. Like the rest of Japan, the city is affected by the monsoon circulation: in winter the northwest cold currents prevail, while in summer they are replaced by hot and humid currents of tropical origin.
The town is located on the coast, in the Seto Inland Sea, a short distance from the islands of Shikoku and Kyushu.
In winter there are fairly mild periods, with highs above 10 °C (50 °F), alternating with cold periods, with wind and rain, highs around 5/7 °C (41/45 °F) or less, and possible falls of sleet or snow. Snowfalls, however, are generally light, as are nocturnal frosts.
Summers are hot and humid; there can be periods of good weather, with maximum temperatures around 35 °C (95 °F), tropical nights and high humidity, but also periods of bad weather, due to the summer monsoon. August is the hottest month, both for the thermal inertia of the sea, and for the lower frequency of periods of bad weather.
Here are the average temperatures.
Hiroshima - Average temperatures
Min (°C)2251015192425211484
Max (°C)101114202427313229231712
Min (°F)363641505966757770574639
Max (°F)505257687581889084736354

The temperatures of the airport, located in a green zone, about 40 kilometres (25 miles) east of the city, and 330 metres (1,080 feet) above sea level, are much lower than those of the city: for example, in January 2016 the temperature dropped to -8 °C (18 °F), compared to -4 °C (25 °F) in the city.
The rains are abundant, as they amount to 1,500 millimetres (60 inches) per year; winter is undoubtedly the driest season, at least in quantity; the rainiest months are June and July, ie the first period of the rainy season (called Baiu). In August there is a certain decrease in rain frequency: on average, among the summer months, it is the hottest, as well as the driest and sunniest.
Here is the average precipitation.
Hiroshima - Average precipitation

From June to October (but especially from August to early October), Hiroshima can be affected by typhoons, the tropical cyclones of Southeast Asia, though this area of the Seto Inland Sea is relatively protected (however, in September 1945, the city was hit by a typhoon that caused a lot of damage).
The cherry blossoming in Hiroshima occurs around 2/3 April.
The amount of sunshine in Hiroshima is not very good, however, there are two relative maxima, one in spring (April-May), and the other in August, after the Baiu rains, with 6.5/7 hours of sunshine per day.
Hiroshima - Sunshine


The sea in Hiroshima is warm enough for swimming, with a temperature of 23 °C (73 °F) or more, from July to October, and it becomes pleasantly warm in August.
Hiroshima - Sea temperature
Temp (°C)161515161922252726232118
Temp (°F)615959616672778179737064