Map from Google - Okayama

The climate of Okayama, a Japanese city in the southern part of Honshu Island, in the Chogoku region, 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 will prevail, while in summer they will be replaced by the 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 summer monsoon and typhoons. In fact, on the other side of the sea we find the island of Shikoku, which with its mountains protects it from the south winds.
In the winter there are fairly mild periods, with highs above 12 °C (53.5 °F), alternating with cold periods, with wind and rain, and highs around 5/7 °C (41/45 °F) or less. Night frosts are typically light, both for the influence of the sea, and for the heat island effect (although in January 1963 the temperature dropped to -8.9 °C or 16 °F). Snowfall is very rare, even more 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 for the thermal inertia of the sea, and for the lower frequency of periods of bad weather.
Here are the average temperatures.
Average temperatures - Okayama
OkayamaJanFebMarAprMayJunJulAugSepOctNovDec
OkayamaJFMAMJJASOND
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 metres of altitude, are a bit lower.
The rains are quite abundant (although this is one of the driest areas of Japan), as they amount to 1,100 millimetres (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 the frequency of the rains: on average, among the summer months, it is the hottest, as well as the driest and sunniest.
Here is the average precipitation.
Average precipitation - Okayama
OkayamaJanFebMarAprMayJunJulAugSepOctNovDecYear
OkayamaJFMAMJJASONDY
Prec. (mm)35508590125170160851358050301105
Prec. (in)1.423.33.54.96.76.33.35.33.121.243.5
Days

In the latter part of summer and in early autumn, from August to October, Okayama may be affected by typhoons, the tropical cyclones of Southeast Asia, though this area of the Seto Inland Sea is relatively sheltered.
The cherry blossoming in Okayama occurs around April the 3rd.
The main tourist attraction of Okayama, along with the castle, is the Koraku-en, the Japanese garden, considered one of the three most important, along with Kenroku-en in Kanazawa and Kairaku-en in Mito.

Korakuen Garden

The amount of sunshine in Okayama, quite good in winter, never becomes very good, but in the summer it is better than in other parts of Japan: the sunniest month is August, with 6 hours and a half of sunshine per day, and in a year there are 2,000 hours of sunshine, a remarkable result for Japan. No coincidence that this area is known to the Japanese as "The Sunny Country".

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 gets pleasantly warm in August.
Sea temperature - Okayama
OkayamaJanFebMarAprMayJunJulAugSepOctNovDec
OkayamaJFMAMJJASOND
Sea (°C)141313151822252726232017
Sea (°F)575555596472778179736863