Hong Kong's climate is subtropical, with hot, humid summers that can reach temperatures of 95 F/35 C and humidity close to 100%. Heavy rains May-September make the season a bad time to visit. May-October, typhoons with strong winds are likely. Ferries stop operating once a typhoon signal 8 is hoisted (signals range in escalating order: 1, 3, 8 and—rarely—9 or even 10 for hurricane-force winds), so don't get caught on an outlying island. Signal warnings are posted at ferry piers, on TV and radio, and at the entrances to many hotels and shopping centers.
From October, temperatures drop to the cool 60s F/20s C. The coldest time of year is December-February. Hong Kong does not have much, if any, central heating (just air-conditioning), which can make homes and shops cold when the temperature drops to 50 F/10 C. Clear, sunny days make October and November the best time to visit, though distant scenery may be blurred by smog that is both created locally and that blows south from China.