Map from Google - Yokohama

In Yokohama the climate is temperate, with fairly mild and sunny winters, and hot, humid 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 hot and humid currents of tropical origin.
Yokohama overlooks the sea in Tokyo Bay, not far from the Japanese capital. Given the relatively low latitude (we are on the 35th parallel), and the protection of the mountains (we are on the east side), in winter the currents of Siberian origin are felt little, so there are several sunny days, and usually it's not very cold; however, cold and windy periods, with highs around 5/7 °C (41/45 °F) or less and possible snowfalls, can not be ruled out. Night frosts are rare and usually light.
In summer, hot and muggy periods (with highs around 35 °C or 95 °F and high air humidity) alternate with periods of bad weather, with wind and rain, and sometimes (especially in June and July) a little cool at night. 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 - Yokohama
YokohamaJanFebMarAprMayJunJulAugSepOctNovDec
YokohamaJFMAMJJASOND
Min (°C)22510151922242115105
Max (°C)101013182225293127221712
Min (°F)363641505966727570595041
Max (°F)505055647277848881726354

The rains are abundant, as they amount to 1,700 millimetres (67 inches) per year; winter is undoubtedly the driest season; the rainiest months are September and October, also due to the contribution made by the typhoons. In August there is a certain decrease in the frequency of rains: among the summer months it is on average, as well as the hottest, the driest and sunniest.
Here is the average precipitation.
Average precipitation - Yokohama
YokohamaJanFebMarAprMayJunJulAugSepOctNovDecYear
YokohamaJFMAMJJASONDY
Prec. (mm)6065140145150190170165235205105551690
Prec. (in)2.42.65.55.75.97.56.76.59.38.14.12.266.5
Days6712111214129131286119

As mentioned, in late summer and early autumn, from August to October, Yokohama can be affected by typhoons, the tropical cyclones coming from the southwest, which mostly affect the southern part of the country, but can also arrive here. The most intense typhoons can bring torrential rains and heavy damage.
The cherry blossoming (Sakura) in Yokohama takes place around the 30th of March: there are dedicated events in the streets and parks where you can admire them.
The amount of sunshine in Yokohama is good in winter (there are no less than 6 hours of sunshine per day in December and January), while it is just mediocre or decent in the other seasons, due to spring and fall disturbances and the summer monsoon: in summer, there are only 4 hours of sunshine per day in June, 5 hours in July and 6 hours and a half in August.

Mount Fuji from Yokohama

The sea is warm enough for swimming, with a temperature of 23 °C (73 °F) or more, from July to October, as you can see in the following table.
Sea temperature - Yokohama
YokohamaJanFebMarAprMayJunJulAugSepOctNovDec
YokohamaJFMAMJJASOND
Sea (°C)171616172022242625232119
Sea (°F)636161636872757977737066