Good Question !!! Here are a couple of reasons:

1. Battery - The cold "saps" the battery's ability to hold it's charge - the charge trickles away faster. So more of your gasoline is used for recharging the battery. So although ALL vehicles suffer MPG problems in cold weather, we Hybrid owners suffer even more...

2. Fuel reaction to the cold - The fuel does not vaporize as well in the cold, and the computer will run the engine richer until it is warm, using more fuel.

3. Shorter days means more headlight use, and the additional load on the alternator uses more fuel.

4. Engines are more efficient after warming up - and if your trips are fairly short, you will suffer MORE because the engine will never reach full efficiency before the end of the trip.

5. Cold Tires - Rubber gets stiff in the cold, so tires from overnight parking take longer to warm up and until they do, they have higher rolling resistance, which also contributes to additional fuel usage.

more later....