Honshu, map

The climate of Okayama, a Japanese city in the southern part of Honshu Island, in the region called Chogoku, is temperate, with quite mild winters and hot, moist, 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.
Okayama is located on the coast, in the Seto Inland Sea, an area relatively protected both from the winter monsoon (since we are on the south side) and from the summer monsoon as well as typhoons. In fact, on the other side of the sea, we find the island of Shikoku, which protects it from the south winds with its mountains.
In winter, there are fairly mild periods, with highs above 12 °C (54 °F), alternating with cold periods, with wind, rain, and highs around 5/7 °C (41/45 °F) or less. Night frosts are typically light, both because of the influence of the sea and because of the heat island effect. However, in January 1963, the temperature dropped to -8.9 °C (16 °F). Snowfall is very rare, even more so than in Tokyo.
Summers are hot and humid; there can be periods of good weather, with maximum temperatures around 35 °C (95 °F) or above and high humidity, but also periods of bad weather due to the summer monsoon. August is the hottest month, both because of the thermal inertia of the sea and because of the lower frequency of periods of bad weather.
Here are the average temperatures.
Okayama - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)1141015192425211483
Max (°C)91013202428313328221712
Min (°F)343439505966757770574637
Max (°F)485055687582889182726354

The temperatures of the airport, located north of the city, in a green area, and at 240 meters (800 feet) of altitude, are a bit lower.
Rainfall is quite abundant (although this is one of the driest areas of Japan), in fact it amounts to 1,100 millimeters (43 inches) per year. Winter is undoubtedly the driest season. 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.
Okayama - Average precipitation
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)35508590125170160851358050301105
Prec.(in)1.423.33.54.96.76.33.35.33.121.243.5
Days

From June to October (but especially from August to early October), Okayama may be affected by typhoons, the tropical cyclones of Southeast Asia, though, as mentioned, this area of the Seto Inland Sea is relatively sheltered.
The cherry blossoming in Okayama occurs around April 3rd.
The main tourist attraction of Okayama, together with the castle, is the Koraku-en, the Japanese garden, considered as one of the three most important, together with the Kenroku-en in Kanazawa and the Kairaku-en in Mito.

Korakuen Garden

The amount of sunshine in Okayama, quite good in winter, never becomes very good, but in summer, it is higher than in other parts of Japan. The sunniest month is August, with almost 7 hours of sunshine per day, and there are more than 2,000 hours of sunshine per year, a remarkable result for Japan. No coincidence that this area is known to the Japanese as "The Sunny Country". Here are the average sunshine hours per day.
Okayama - Sunshine
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Hours555665675655

The sea in Okayama 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. Here are the average sea temperatures.
Okayama - Sea temperature
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Temp (°C)141313151822252726232017
Temp (°F)575555596472778179736863