From September to October in Taipei, the mean temperature falls just 2.8°C. It still feels hot; your wardrobe doesn't change.

But absolute humidity drops from 20.6 to 17.4, and then to 14.8 by November. In two months the air loses close to a third of its water.

Which is why 'Taiwan has no autumn' is true for the calendar and false for skin. Temperature and moisture are not the same curve, and moisture moves first.

Why October is where it goes wrong

In September you're still running the summer routine — light, thin, fast-drying — because there were 20.6 grams of water in the air doing part of the work. By October there are 3.2 fewer, and the same routine stops being enough.

But it's still 24°C and you're still sweating, so you don't read it as dryness. You read it as 'why am I oily and tight at the same time'. That's dehydration under oil, caused by the environment, not by your skin changing.

The four levers

1. TimingFrom October, thicken the evening round first. Leave daytime alone — daytime is still hot.
2. QuantityNot a whole autumn wardrobe of products. Add back a little of the hydrating layer you already own. A third less humidity doesn't need a third more product.
3. OrderStop skipping the watery layer this month. It's the base the oil above it needs.
4. IntervalThe day-night gap is opening up. Give it ten minutes before you leave and after you get home.

Don't wait until November when it finally feels dry.
The humidity started leaving in late September. Skin just takes a fortnight to mention it.