My chickens aren't laying...

[amyltc]

It's been 3 weeks since we've seen an egg we have 6 chickens, they aren't molting, they are all between 9 mo- 18 mo old. It's been very cold ( between 5-20 degrees) and very snowy so they can't free range at all ( hard to do in 2 ft of snow!) they have a snow free run though. Is it just the shorter days and the cold? Is there something I can do to encourage them to lay?