Map from Google - Hong_Kong

The climate of Hong Kong is subtropical, with very mild winters and hot summers, which are rainy and muggy as well.
Here are the average temperatures.
Average temperatures - Hong Kong
Hong KongJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)151517212426272726242016
Max (°C)191921252830313130282420
Min (°F)595963707579818179756861
Max (°F)666670778286888886827568

Rainfall is abundant, being around 2,400 millimetres (94.5 inches) per year, and is even more abundant in the hills around the city, so much so that more than 3,000 mm (118 in) per year fall in Tai Mo Shan, the highest peak of Hong Kong with its 957 metres (3,140 feet). The rainiest period is from May to September, when the rainfall exceeds 300 mm (12 in) per month in the city, while the least rainy period is from November to January, when the rainfall drops below 50 mm (2 in) per month.
Here is the average precipitation.
Average precipitation - Hong Kong
Hong KongJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)25558017530545537543033010040252400

Winter, from December to February, is very mild, with lows around 15 °C (59 °F), and highs around 20 °C (68 °F), but the sky is often cloudy, with possible fogs, drizzles and windy days. Some days can be pleasantly warm, while in others the temperature can drop to around 10/15 °C (50/59 °F) during the day, and the wind can increase the feeling of cold. Occasionally it can even get cold, with lows around 5 °C (41 °F) and highs around 10 °C (50 °F).

Spring, from March to May, is very mild or pleasantly warm, but the sky is almost always cloudy. In March, a cold wind can still blow, with highs around 13/15 °C (55/59 °F), especially in the first half of the month. Starting from mid-March, with the increase in temperature, the first thunderstorms occur. Already in May, there's a sultry heat, with highs around 28 °C (82 °F), even though it's less intense than in the summer months, moreover, the rains become significant.

The long summer, from June to September, is hot, humid and rainy, with heavy showers or thunderstorms, although the sun shines on average for a few hours per day. Daytime temperatures are around 30/32 °C (86/90 °F), and the heat is felt, due to the urban heat island effect, together with moisture coming from the sea. Luckily, there is widespread use of air conditioning. Usually, the heat remains muggy even at night: lows rarely drop below 25 °C (77 °F), but they can also remain around 28/29 °C (82/84 °F). In the hills around the city and in the districts overlooking the sea, at least the breeze blows. In July, there is often a pause, a period of good weather lasting a week or two, in which the temperature can reach 35 °C (95 °F). The probability of typhoons, which bring heavy rains and strong winds for a few days, with the risk of flooding and landslides, is highest in this season.

In the short autumn, in October and November, the rains become rare and generally not abundant, and the sun is a bit more frequent than in the other seasons. In the first part of October the heat can still be muggy, but gradually becomes more pleasant; in this month, however, and sometimes even in the first half of November, tropical cyclones can still arrive. In November, the temperature is generally pleasant, with highs around 24/25 °C (75/77 °F), but cool and windy days gradually become more likely, and sometimes, in the second part of the month, it can get a bit cold.

Hong Kong

The amount of sunshine in Hong Kong is not great all year round, and it's scarce from February to April, when the sky is often overcast, while in the rainy season it's paradoxically higher, given that the rains occur in the form of downpour or thunderstorm, which do not last long, so that the sun will shine again soon, except when typhoons hit the area. In autumn, the sunshine amount remains acceptable or improves slightly, however, cloudy days are possible in this season as well.
Sunshine - Hong Kong
Hong KongJanFebMarAprMayJunJulAugSepOctNovDec
Sun (hours)533355766666

For swimming, the sea is warm enough from April to November, and becomes definitely warm from May to October. In winter, the South China Sea is cool but not cold: it drops to 20 °C (68 °F) in January and February.
Sea temperature - Hong Kong
Hong KongJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)202022232628292928272422
Sea (°F)686872737982848482817572

As mentioned earlier, Hong Kong can be hit by typhoons, tropical cyclones of Southeast Asia. Typically typhoons occur in this area from June to early November, but sometimes they have passed outside this period, as happened in April 2008 with Typhoon Neoguri (or Ambo), or in May 2006 with Typhoon Chanchu (or Caloy).

When to go

There is no real best time to visit Hong Kong. Winter is mild, but it is often cloudy, it can be windy, and sometimes, though rarely, it can be a little cold. Spring is pleasantly warm, but it's still cloudy, and when the sun becomes a little more frequent, the rains begin as well, then starting from May the air becomes muggy and there is some risk of typhoons. In theory, autumn is the best season, but in October a typhoon can still pass, while in November there can be the first cool and windy days, and sometimes at the end of the month, it can already be a little cold. All in all, the best times are from mid-March to mid-April, and from mid-October to late November.

What to pack

In winter: spring/autumn clothes, sweatshirt or sweater, jacket, raincoat or umbrella, hat for colder days.
In summer: tropics-friendly, lightweight clothing, a light sweatshirt for air-conditioned places, light raincoat or umbrella.