The climate of Hangzhou (or Hangchow) is temperate, with relatively cold winters, and hot, muggy and rainy summers. Hangzhou is a megalopolis located in eastern China, in the Zhejiang Province, at the mouth of the Qiantang River, which then opens in Hangzhou Bay. 150 km (95 mi) north-east of the city we find Shanghai, while to the northwest we find Nanjing.
Here are the average temperatures.
Hangzhou - Average temperatures
Min (°C)2471218212525211594
Max (°C)81015212629343328231711
Min (°F)363945546470777770594839
Max (°F)465059707984939182736352

Rainfall is quite abundant, and amounts to 1,400 millimeters (57 inches) per year; there is no really dry season, however, the rains fall below 100 mm (4 in) per month from October to February. Summer, when the rains occur mainly in the form of showers or thunderstorms, is the rainiest season. Here is the average rainfall.
Hangzhou - Average precipitation

The winter, from December to February, is quite cold: the average January temperature is 5 °C (41 °F). There may be cold periods, with highs around 5 °C (41 °F) or below, and with some chance of snow. In January 2016, the temperature dropped to -8 °C (17.5 °F). In other periods instead the temperatures are mild (around 15 °C or 59 °F). Sunny days are a bit more frequent in December, then they become rarer. At night, fog can form.

The spring, from March to May, is initially mild, and becomes gradually warmer. In May there are already some hot, summer-like days.

The summer, from June to September, is hot and muggy. The heat island effect of the big city and the humidity from the sea make the heat annoying, especially in July and August. June is the rainiest month of the year. The periods of good weather can be sweltering, especially in July and August, with highs around 37/38 °C (99/100 °F) or more, and tropical nights. In August 2013 and in July 2017, 41 °C (106 °F) were recorded.

The autumn, in October and November, is initially mild, and gradually becomes cooler, and then quite cold at night by November. Rainfall is moderate.

West lake

The amount of sunshine in Hangzhou is not very good; however, it is decent in July and August, while it is poorer from January to March.
Hangzhou - Sunshine

Near Hangzhou, the sea (East China Sea), cold in winter, is warm enough for swimming from July to September; in June and October it is a bit cool but basically acceptable.
Hangzhou - Sea temperature
Temp (°C)978121822262826231813
Temp (°F)484546546472798279736455

From time to time, from June to early November, typhoons, the tropical cyclones of Southeast Asia, can affect this area, though less so than in southern China.

When to go

It is not easy to find the best time to visit Hangzhou. As the spring is more rainy, you can choose autumn, especially from the end of September to the beginning of November, which is generally mild or pleasantly warm; it must be said that the arrival of a typhoon can not be excluded.