Rain and cold at the same time
Short answer: Wet clothing carries heat away twenty five times faster than dry. In rain and cold, dryness matters more than thickness.
Why wet clothes are so cold
Water conducts heat far better than air. When the fabric is soaked, the insulating air pockets disappear and the body's heat goes straight into the water. That is why you get cold quickly in the rain.
The order that works
On the outside something that keeps the water out. On the inside something that still warms even when slightly damp. Bamboo viscose and wool hold up better than cotton in that situation. See bamboo underwear for men.
What to carry in your bag
A dry layer in case the first one gets wet. A pair of dry socks does the most of all. See bamboo socks for men and bamboo long sleeved t-shirts for men.
Frequently asked questions
Why do you feel cold even in mild rain
Because evaporation from wet clothing cools the body constantly.
Is a rain jacket enough
Only if it also closes at the neck and sleeves. Water runs in there.
What do you do about wet feet
Change your socks as soon as you can. It changes the whole experience.