Why jeans fail outdoors
Cotton soaks up water, stops insulating once it's wet and takes most of a day to dry. In Iceland's wind that wet fabric pulls heat from your legs the whole time — the reason every local guide says no cotton for days outside.
The places visitors go are wet even when it isn't raining: spray at Seljalandsfoss and Skógafoss, sea mist on the black-sand beaches, sleet on the Golden Circle.
When jeans are fine
Evenings in Reykjavík or Akureyri, restaurants, museums, a dry summer day in town. Iceland is casual, and you'll see locals in jeans in all weathers in the city — they're never far from a warm car or café.
What to wear instead
Quick-drying synthetic hiking or softshell trousers, with thermal leggings underneath from October to April. Over them, waterproof over-trousers that pull on over your boots when the rain comes sideways.