People who live at high altitudes make adjustments all the time as they bake to compensate for the lower air pressure. In general, people are not as aware of the changes they have to make in baking as the seasons change. With regard to pizza dough or other breads, there are two atmospheric factors that you really have to take into account.
The first is humidity, which has a dramatic impact on your flour. We sometimes notice how much humidity changes the nature of our flour from a humid summer day to a drier day. In the winter, when all humidity goes south, you really need to compensate. You should add a little water to your dough until it reaches the consistency you get in the summer.
The second factor is temperature, and the effect it has on your yeast. Your kitchen is likely to be cooler in the winter than in the summer (especially little out-of-the-way places where you may be letting your dough rise), so the yeast takes longer to raise the dough. The best solution is to be patient and give it the time it needs. Finding a warm spot away from drafts to let the dough rise will help, too.