Yes, a Sun Oven can be used on a clear winter day. The most important factor in using a Sun Oven is the brightness of the sun, not the outside air temperature. Often, a 40-degree, clear, low-humidity day will allow food to cook faster than a 100-degree day with high humidity. The Sun Oven has been used very successfully at the base camp of a Mt. Everest expedition where the temperatures are often well below zero. There are, however, more cooking hours available in the summer than in the winter. In the summer, it is not unusual to cook from 8:00 AM to 6:00 PM, whereas during the early winter, effective cooking is limited to 10:00 AM to 2:00 PM.