Drum brakes - ineffective when cold I have city bike with Sturmey-Archer X-FDD front hub with drum brake.

Last week, after cold and wet days (drizzle, 2 C), the temperature dropped below -10 C.
I store this bike outdoors, I ride it every day 10-30 km.
Because of u-lock malfunction it was grounded for 3 days. When I eventually managed to free it, I observed very poor front brake operation. I suppose it is  about 30% usual effectiveness.
I ruled out the cable - braking cord is moving back and forth with no problem. 

Has something wrong happened to the brake linning? What could cause such behaviour?

Edit:
As I'm lazy person, I didn't want to disassembly the hub and see what's wrong. 
I supposed there is some ice on the surface of the lining, but after few dozens of hard brakings thing went even worse, ie. brake was less and less effective.

What I eventually did was riding for some not so long distance, using brake when pedalling with mudulate force. After that brake suddenly gain it full effectiveness.

@Emyr is probably right. There sholud be a lot of ice. Long braking melted it and let the brake dry.
Moisture may have collected on the friction surfaces and frozen.

Warm the bike up and re-test.