The year in Hiroshima, month by month
The figures come from the Hiroshima station, Japan Meteorological Agency 1991–2020 series: 1,572.2 millimetres a year over 108.6 days, 2,033.1 hours of sun and an annual mean of 16.5 degrees. The scoring crosses rain, rainy days, sunshine, temperature and humidity.
The idea that winter is the wet season comes from the Mediterranean and does not work here. On this coast winter is the dry season, and clearly so: January and December are the two months that receive the least water of the whole year.
What rain there is falls in summer. And not spread out: concentrated into two months that completely change what a trip can be.
01 Two hundred and eighty against forty-six
The Hiroshima station records 279.8 millimetres in July and 46.2 in January. The ratio is six to one, and it does not depend on a freak year: these are the means of the 1991–2020 series.
June comes right behind, with 226.5 millimetres, and adds the annual maximum of rainy days: 11.9. Between them they total 506.3 millimetres, almost a third of the 1,572.2 that fall in the whole year, over barely 23 days.
That is the difference that matters. It does not rain on more days than elsewhere: far more falls on each rainy day. A July afternoon can leave in a few hours what the whole of January never manages to gather.
Two hundred and seventy-nine point eight millimetres in July and forty-six point two in January. Winter is the dry season.
02 August is hotter and drier
Here is the figure that throws almost everyone: the hottest month is not July, it is August, with a mean high of 32.8 degrees against 30.9. And yet August receives only 131.4 millimetres, less than half of July's.
Sunshine follows that switch: 207.3 hours in August against 173.4 in July and 154.6 in June. The sequence is clean: first the water arrives, then the heat, and each has its own month.
For travel, that does not make August a good month. With lows of 25.1 degrees, the night does not cool, and at 69 per cent humidity the sensation does not drop even under a clear sky. It is summer's brightest month and also its hardest for walking.
03 June has less sun than March
The year totals 2,033.1 hours of sun, but distributed in a way that does not resemble other similar latitudes. The maximum is not in high summer: it is in May, with 210.8 hours.
And the contrast arrives right after. June comes to 154.6 hours, below March (176.7) and October (178.6). That is, the month with the longest days of the year is one of the least sunny, because the sky is covered almost all the time.
That mismatch between available light and actual light is what makes early summer uncomfortable. It is not that it rains all day: it is that the day never opens. And at 71 per cent humidity, clothes do not dry either.
04 The dry winter almost nobody expects
January leaves 46.2 millimetres over 6.8 days and December 54.0 over 7.6. They are the two driest months of the calendar, and the only European winter that resembles them is an Atlantic island's turned inside out.
Cold is not the problem either. January's mean high is 9.9 degrees and its low 1.6; the month's snow is measured in some 3 centimetres, and there are years when it simply does not settle on the flat.
What is hard is the light. With 138.6 hours of sun in January, some four and a half a day, and a very early dusk, winter becomes a trip short on usable hours. Dry, mild and dark is an unusual combination, and it has to be told in full.
05 The same prefecture, thirteen hundred metres higher
All of the above applies to the coast. The prefecture covers 8,479 square kilometres and is mostly mountain, and inland the calendar is different: there are ski areas on the northern summits, above 1,300 metres, in the same territory where the capital barely sees 3 centimetres of snow a month.
The reason for that coastal mildness is the inland sea. An enclosed, shallow, swell-free sea between islands moderates the temperatures of the whole coastal strip and takes away much of its extremes.
For planning, this means altitude gives you options. In high summer, going inland changes several degrees and some humidity; in winter, going up changes the landscape completely. Two different calendars an hour's drive apart.
06 How to choose the month
If the trip is about walking, islands or cycling, April, May, October and November. All four combine highs between 17.7 and 24.4 degrees with contained rain, and October and November add the two lowest rainy-day counts of the year after winter: 7.1 and 6.9.
If you can only travel in summer, choose August over July and organise the day in reverse: early and late outside, midday under cover. And take it as given that in September water can arrive concentrated into very few days, with 162.7 millimetres on average.
And if your only window is winter, do not rule it out for fear of rain, because there will not be any: 46.2 millimetres in January. Rule it out, if at all, for the light. With 138.6 hours of sun, the day ends before you notice.
The year's four stretches
From 141.0 to 169.8 millimetres a month over some 10 days, with 191.9 and 210.8 hours of sun and highs from 19.8 to 24.4 degrees. May is the sunniest month of the year and April the least humid, at 61 per cent.
226.5 and 279.8 millimetres, 506.3 across the two months, over 23.5 days. Sunshine falls to 154.6 hours in June, less than in March, and humidity rises to 71 and 73 per cent.
131.4 and 162.7 millimetres with 207.3 and 167.3 hours of sun. August is the hottest month, with highs of 32.8 degrees and lows of 25.1; September drops to 29.1 but can bring storms.
109.2 and 69.3 millimetres over 7.1 and 6.9 days, with 178.6 and 153.3 hours of sun and highs of 23.7 and 17.7 degrees. Fewer rainy days than any month between March and September.
