Hiroshima sits on the sheltered Seto Inland Sea coast and enjoys a mild climate with hot summers and relatively warm winters compared to other Japanese cities at similar latitude.
Spring (late March–May) is the most popular time to visit. Cherry blossom (sakura) typically peaks in late March to early April; the Peace Park is lined with flowering trees, and Miyajima's hillsides turn pink. Temperatures are comfortable (15–22°C) and the days are long. Crowds build in April.
Summer (June–August) brings the rainy season in June and early July, followed by intense heat and humidity (30–35°C) in August. 6 August is the anniversary of the bombing and draws large crowds for the Peace Memorial Ceremony; if you want to attend, book accommodation well in advance. The city's oyster season is over by this point, but summer festivals and fireworks make evenings lively.
Autumn (September–November) is excellent: comfortable temperatures, low humidity, and autumn foliage (koyo) on Miyajima and at Shukkei-en from late October into November.
Winter (December–February) is mild and rarely snowy in the city. Oyster season is at its peak. This is the quietest season and hotel rates are lower; a good time if the memorials are your main reason to visit and you prefer smaller crowds.