Hardwood Boards

Can anyone tell me, why our hardwood floor gets in the winter gaps between the boards?

Last summer was everything nice and even. Now, there are long gaps between boards. Is it cause winter or just luck of routine maintenance.
Thanks,
Thank you all. I could not even imagine, that dry air takes such a toll on the wood floors. Despite to dry nose, I found thanks to you all another reason to get humidifier in our rooms.
Thanks a lot, I do appreciate all your help!

Your house is drying out. Wood shrinks and swells depending on the ambient humidity. Your house should be about 40% relative humidity. You probably need a humidifier running.

The previous answers are correct, except dirt will fall in the cracks preventing them from closing during the summer,,,,eventually. Best to keep the house at the proper humidity levels all year round to prevent future problems.

Marble Pyramid – Hardwood Board Game