You've also got the change in viscosity inside your hubs to think about. The grease or oil (probably grease) is much more viscous in cold weather, making internal drag within the hubs greater. Same thing with your bottom bracket bearings, the grease will become more viscous. Between the hubs and the bottom bracket you get a pretty substantial increase in mechanical drag. Then, if you're using a heavy oil or wax on your chain, the lube becomes more viscous which means the chain doesn't flex as easily which means it takes more effort to get it to conform to chainring and cog. This is to say nothing of biological factors that work against you. As your body inhales cold air, and throat becomes cold, your body's natural reaction is to increase mucus production; which can restrict breathing. Then, as mentioned already, you have to propel yourself through denser air. And, your tires become stiffer and don't conform as easily to bumps in the terrain, which means a loss of efficiency. The list of factors working against you during cold weather just goes on and on.