Cairns has two seasons: the wet (November to April) and the dry (May to October). The distinction matters.
The dry season is the classic time to visit. Temperatures sit between 18°C and 26°C, humidity is manageable, and the weather is reliably clear. This is when reef conditions are best, trails are accessible, and Cairns fills up with travellers — book accommodation in advance, especially in July and August.
The wet season brings heavy rain, high humidity, and occasional cyclones. Some roads north of the Daintree River become impassable for days at a time. But the wet season also brings life: waterfalls run full, the rainforest is intensely green, and inland swimming holes are at their best. Prices drop significantly and the crowds thin out. The wet is not a reason to avoid Cairns if you are flexible, but it requires different plans.
Stinger season (October to May) means marine stingers — primarily box jellyfish — make swimming in the open sea dangerous without a stinger suit. Stinger nets protect some beaches north of Cairns (Port Douglas, Palm Cove), but most reef operators provide stinger suits for in-water activities. The Esplanade Lagoon is unaffected year-round.