In Hamamatsu the climate is temperate, with quite 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 prevail, while in summer they are replaced by hot and humid currents of tropical origin.
Hamamatsu is located in the central part of the island of Honshu, on the coast, in the Shizuoka prefecture. North of the city lie the Kiso Mountains, or Central Alps, with a series of peaks almost 3,000 metres (9,800 ft) tall.
Given the relatively low latitude (we are on the 34th parallel), and the protection of the mountains (we are on the southern 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 6/8 °C (43/46 °F) or less, can not be ruled out. Snowfall is very rare: unlike in Tokyo, where sometimes it can snow, here in practice it never snows (but sometimes it can snow in the northern outskirts of the city, see Tenryu-ku). Night frosts are quite rare and usually light, however, at night the temperature often drops close to 0 °C (32 °F).
In summer, sunny and muggy periods alternate with periods of bad weather, with wind and rain. Fortunately, the breeze blows from the sea.
Here are the average temperatures.
Average temperatures - Hamamatsu

Hamamatsu	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Hamamatsu	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	2	3	6	10	15	19	23	24	21	15	10	5
Max (°C)	10	11	14	19	23	26	29	31	28	23	18	13
Min (°F)	36	37	43	50	59	66	73	75	70	59	50	41
Max (°F)	50	52	57	66	73	79	84	88	82	73	64	55

In Hamamatsu, precipitation is abundant, given that it amounts to 1,800 millimetres (70 inches) per year. The driest season is definitely the winter. The rainiest periods are the first part of the rainy season (mid-June to mid-July, the plum rains), and the latest (September, also due to the contribution made by typhoons, see below).
Here is the average precipitation.
Average precipitation - Hamamatsu

Hamamatsu	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Hamamatsu	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	55	80	150	165	190	240	190	150	250	165	120	50	1810
Prec. (in)	2.2	3.1	5.9	6.5	7.5	9.4	7.5	5.9	9.8	6.5	4.7	2	71.3
Days

From June to October (but especially from August to early October), Hamamatsu can be affected by typhoons, the tropical cyclones of the Pacific, which in this area, facing south, are felt in a particular way. The most intense typhoons can bring torrential rains and heavy damage.
The cherry blossoming in Hamamatsu takes place around the 30th of March.
The amount of sunshine in Hamamatsu is good in winter, while it is barely decent in the other seasons, due weather disturbances in spring and fall, and to the summer monsoon. In summer, August is the sunniest month: apart from typhoons and some waves of bad weather, there are several sunny periods.
Sunshine - Hamamatsu

Hamamatsu	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Hamamatsu	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	6	7	6	7	6	5	6	7	5	5	6	6

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 - Hamamatsu

Hamamatsu	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Hamamatsu	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	17	16	16	17	20	22	24	26	25	23	21	19
Sea (°F)	63	61	61	63	68	72	75	79	77	73	70	66