The climate of Nagasaki, a Japanese city located on the west coast of Kyushu, the southernmost of the major Japanese islands, is temperate humid, 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 hot and humid currents of tropical origin.
The city is located at a low latitude (we are at the 32rd parallel), but close enough to the Asian continent to receive cold air masses in winter, able to lower the temperature to some extent.
In winter the sky is often cloudy, as is common in the Japanese western side; there are fairly mild periods, with highs above 10 °C (50 °F), alternating with cold periods, with wind and rain, highs around 5/7 °C (41/45 °F) or less, and possible falls of sleet or snow. Snowfalls, however, are generally light, as are nocturnal frosts.
Summers are hot and humid; there can be periods of good weather, with maximum temperatures around 35 °C (95 °F), tropical nights 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 - Nagasaki

Nagasaki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Nagasaki	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	4	4	7	12	16	20	24	25	22	16	11	6
Max (°C)	10	12	15	20	24	26	30	32	29	24	18	13
Min (°F)	39	39	45	54	61	68	75	77	72	61	52	43
Max (°F)	50	54	59	68	75	79	86	90	84	75	64	55

The rains are abundant, as they amount to 1,850 millimetres (73 inches) per year; winter is the driest season, but only in quantity; the rainiest months are June and July, ie the first period of the rainy season (called Baiu), which in southern Japan is particularly marked, especially on the west coast where Nagasaki is located: sometimes the rains can be torrential. In the southwest of Japan, autumn is a decent season, and already in October it typically doesn't rain much.
Here is the average precipitation.
Average precipitation - Nagasaki

Nagasaki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Nagasaki	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	65	85	130	150	180	315	315	195	190	85	85	60	1860
Prec. (in)	2.6	3.3	5.1	5.9	7.1	12.4	12.4	7.7	7.5	3.3	3.3	2.4	73.2
Days	11	10	12	11	11	14	12	10	10	6	9	10	125

From June to October (but especially from August to early October), Nagasaki may be affected by typhoons, the tropical cyclones of Southeast Asia.
The cherry blossoming in Nagasaki occurs around 31 March.
The amount of sunshine in Nagasaki is never very good, however, there are two relative maxima, one in spring (April-May), and the other in August, after the Baiu rains. In October, after the summer rains, the sunshine amount increases a little as well.
Sunshine - Nagasaki

Nagasaki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Nagasaki	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	3	4	5	6	6	5	6	7	6	6	5	4

In winter, the sea in Nagasaki is cold but not impossible for the more daring; it 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.
Sea temperature - Nagasaki

Nagasaki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Nagasaki	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	18	17	18	19	21	23	26	28	27	25	23	20
Sea (°F)	64	63	64	66	70	73	79	82	81	77	73	68