Correlation between the number of cats on the bed and weather outside.

We have quickly learned there is a correlation between the number of cats that sleep on our beds and the weather conditions outside.
If its warm, we rarely see ANY cats except when the come in to eat and get a drink and I think just to check in with us just to get attention. Yet as the weather gets colder and the precipitation increases, they migrate to our beds. Mainly to the master bedroom. The record so far is at least 10 when the girls doors are closed. But if they open up, when its cold and heaters are running, then everyone has a population of at least 5, with the bulk residing still on our bed. Good thing I assembly the advantage the first of every month and wash the bedding frequently. But they are inexpensive and a green source of energy.