Here's a couple of ideas....
Cold days and especially snowy days will result in school closures. School closures will result in more kids accessing the network, slowing it down.
The cold air has caused something to contract and pinch or contract an unreliable section of cable. Your cable company can help with this, althought you would see degradation of a cable tv picture as a logical result. Make sure all your connections are tight.
The physics principle here suggests that the speed would increase with colder weather, but given the temperatures and materials being used here you would be hard pressed to quantify or notice any improvement.