Honshu, map

In Yokohama, the climate is temperate, with fairly mild, 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 is a good number of sunny days, and it's usually not too 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 (95 °F) and high humidity, alternate with periods of bad weather, with wind and rain, and sometimes (especially in June and July) temperatures a bit cool at night. 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.
Yokohama - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)22510151922242115105
Max (°C)101013182225293127221712
Min (°F)363641505966727570595041
Max (°F)505055647277848881726354

Rainfall is abundant, in fact, it amounts to 1,700 millimeters (67 inches) per year. Winter is undoubtedly the driest season; the rainiest months are September and October, also because of the contribution given by typhoons. In August, there is a certain decrease in rain frequency: on average, among the summer months, it is the driest and sunniest as well as the hottest. Here is the average precipitation.
Yokohama - Average precipitation
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)6065140145150190170165235205105551690
Prec.(in)2.42.65.55.75.97.56.76.59.38.14.12.266.5
Days6712111214129131286119

As mentioned, from June to October (but especially from August to early October), Yokohama can be affected by typhoons, the tropical cyclones of South-East Asia, which mostly affect the southern part of the country but can also arrive here. The most intense typhoons can bring strong winds, torrential rains and heavy damage.
The cherry blossoming (Sakura) in Yokohama takes place around March 30: 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 because of the weather disturbances that pass in spring and autumn, as well as of the summer monsoon. However, among those of summer, August is the sunniest month. Here are the average sunshine hours per day.
Yokohama - Sunshine
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Hours665664574556

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.
Yokohama - Sea temperature
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Temp (°C)171616172022242625232119
Temp (°F)636161636872757977737066