Hiroshima's climate is classified as humid subtropical, defined by four distinct seasons influenced heavily by its unique geography. Nestled between the Chugoku Mountains to the north and the Shikoku Mountains to the south, the city sits on the shores of the Seto Inland Sea. This location creates a protective "rain shadow," resulting in more sunny days and less extreme snowfall than many other Japanese cities.
Seasonal Climate Variations
Spring (March – May)
Spring is a transitional period where the cold Siberian winds weaken, replaced by temperate air from the south.
- Patterns: Early March remains brisk, but by April, temperatures stabilize into a comfortable range (highs of 15°C to 20°C).
- Phenomena: This season is famous for the Sakura (cherry blossom) bloom, usually peaking in early April. The air is generally dry and clear, though "Yellow Dust" (kosa) occasionally drifts from the Asian continent, creating a hazy sky.
Summer (June – August)
Summer in Hiroshima is intense, characterized by high humidity and the Pacific monsoon.
- The Rainy Season (Tsuyu): From early June to mid-July, the city experiences persistent light to moderate rain and heavy cloud cover. This is the wettest time of year.
- The Heat: Following the rainy season, August brings oppressive heat, with temperatures frequently exceeding 32°C (90°F). Because of the "urban heat island" effect in the city center, nighttime temperatures rarely drop below 25°C, providing little relief.
- Atmosphere: The weather "feels" much hotter than the thermometer suggests due to the extreme humidity, which locals describe as mushi-atsui (steamy hot).
Autumn (September – November)
September marks the peak of the typhoon season. While the Chugoku and Shikoku mountains offer some protection, Hiroshima can still experience heavy rainfall and strong winds from passing tropical cyclones.
- Cooling Trend: By October, the humidity drops significantly, ushering in some of the year's most stable and pleasant weather.
- Aesthetics: November is characterized by crisp air and vibrant autumn foliage (koyo), particularly in the surrounding mountain valleys and on Miyajima Island.
Winter (December – February)
Winters are cold but relatively dry and sunny compared to the Japan Sea coast.
- Temperature: Average daytime highs hover around 10°C, while nights can dip near freezing.
- Snowfall: Actual snow accumulation in the city is rare—averaging only a few days a year—though the northern mountainous regions of the prefecture receive heavy snow, supporting local ski resorts.
Regional Weather Phenomena
The Seto Inland Sea Effect
The Seto Inland Sea acts as a giant heat sink, moderating the city's temperatures. A specific phenomenon known as "Sea Fog" often occurs on calm, cold winter mornings. As moisture-laden air cools over the water, a thick, mystical fog rolls through the islands of the bay, often visible from higher elevations like Mount Misen.
Diurnal Rhythms
In the summer, Hiroshima experiences a "land and sea breeze" cycle. During the day, the land heats up faster than the sea, drawing a cooling breeze inland. At night, the direction reverses. However, in the peak of mid-summer, these breezes can sometimes stall, leading to stagnant, hot air in the downtown river delta.
Historical Context and Local Adaptation
Historically, Hiroshima’s delta geography made it prone to flooding, a challenge locals have managed through extensive canal and river engineering. Today, life in Hiroshima is finely tuned to the seasons:
- Summer Adaptations: Locals utilize the city’s many rivers for "cool-down" events and frequent shaded shopping arcades (Hondori) to escape the sun.
- Tidal Awareness: Because the iconic Itsukushima Shrine is built over the water, locals and visitors alike pay close attention to the tide tables, as the "floating" effect is entirely dependent on the lunar cycle and weather-driven surges.