It is rainy season in Chiang Mai & the north right now, and it runs May to October. The rain tends to come hard and short rather than all day, and it stays warm through it — so the thing to bring is a shell you can fold into a day bag, not a coat.
Going in November
The best months up here: dry, clear and genuinely cool at night — down to single digits in the hills, which surprises people packing for Thailand.
What this means for your bag
Layers, not one thick coat
Three thin layers beat one heavy one: they cover a wider range and pack smaller. A base layer, a fleece and a shell.
See the Winter Warmth Kit packReal sun protection
High-factor sunscreen, a hat with a brim and sunglasses. Reef-safe sunscreen is worth buying before you go — it is expensive and hard to find at the beach.
See the Beach & Pool Kit packQuick-dry fabrics
Cotton takes two days to dry in this air and starts to smell before it does. Synthetics and merino wash in a sink and are wearable by morning.
See the Summer Sun Kit pack
Build a packing plan around the season you are actually travelling in.