Cold weather and Prius gas mileage

I had heard that cold weather reduces the mileage on the Prius. Fall is in full effect in Chicago and temps are regularly in the the 30's and 40's now. I have noticed that my average mileage is down by 3-5 MPG since the temp changes.

My route or driving habits have no changed, just the temp. I will be keeping an eye on this as winter sets in to see if it goes down more or will level off. The mileage is still very good. Have also heard that people have used engine block heaters to off set this drop in MPG.