It is rainy season in Hanoi & the north right now, and it runs May to September. 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 September
Hot, wet and heavy, with most of the year's rain falling between July and September.
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 packRoom for humidity
Everything stays slightly damp. Pack fewer clothes and plan to wash them, and keep anything that must stay dry in a sealed bag.
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 pack
Build a packing plan around the season you are actually travelling in.