Chiang Mai & the north is in its dry stretch right now, November to March. This is the easy window: plan around sun and heat rather than around rain.
Going in September
The monsoon over the hills, wettest from July to September. It rarely rains all day, but roads and trails turn to mud.
What this means for your bag
A packable rain shell
Not a winter coat — it stays warm through the rain. A light hooded shell that folds into its own pocket ends up in the day bag instead of the hotel room.
See the Weather Protection 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 packInsect protection
Standing water means mosquitoes, and the ones out at dusk are the ones that matter. Repellent with DEET or picaridin, and long sleeves in the evening.
See the Health & Wellness packShoes that survive being soaked
Streets flood faster than they drain. Sandals or mesh trainers dry overnight; leather does not, and one wet pair means walking in wet shoes all week.
See the Active & Adventure pack
Build a packing plan around the season you are actually travelling in.