Hong Kong has four distinct seasons, and the difference between the best and worst times to visit is significant.
Autumn (October to December) is by far the best time to visit. Humidity drops to comfortable levels, skies are often clear, and temperatures settle around 20–24°C. Rain is light and infrequent. Pack a jacket for evenings. Note that typhoon season officially extends through November, though autumn typhoons are uncommon.
Winter (December to February) is mild and pleasant. Average temperatures hover around 15–18°C, occasionally dipping to 10°C during cold snaps from the north. It never snows in the city, though it may frost at higher elevations. Humidity is low, rain is rare, and the air is clear. Locals layer up dramatically at temperatures that visitors from northern climates find comfortable.
Spring (March to May) brings heavy rain and rapidly rising humidity. Temperatures climb from 20°C toward 30°C. Rainstorm warnings come in three levels (amber, red, black) — a black rainstorm signal means businesses close and transport stops. The city comes to a standstill several times during the season.
Summer (June to September) is hot (30–35°C), intensely humid, and the worst time to walk around outside. Typhoon season peaks in August and September. Shops and restaurants crank the air conditioning to arctic levels, so bring a layer indoors. If you do visit in summer, the city's indoor attractions — museums, malls, restaurants — are very well air-conditioned.
Typhoon season runs officially from May to November. The Hong Kong Observatory issues signals from T1 (tropical cyclone warning) to T10 (hurricane conditions). A T8 or above signal means most businesses close; get indoors and stay there.