Years ago when I began cutting firewood I was advised that wood splits best when it's frozen. I've split wood in every kind of weather, but have always found frozen wood easier to split.
Take a chunk of wood with considerable moisture in it--freshly cut, green wood--and sink a splitting maul or wedge into it during warm weather. The steel edge will bury itself, pushing wood fibers to the side and squirting water out. (This is more easily seen in softer wood.) Take the same chunk of wood when temps are in single digits or below zero (or have been for several days), and when struck with a maul or wedge the wood will pop apart smartly.
It's the difference between wood having a partial content of water vs. ice. Ice will break, water won't.
In recent years when I've been using a hydraulic splitter, the weather is immaterial. But when you do it by hand, you'll find frozen wood splits much easier.