This is a very good question.
The answer is: sometimes.
As long as it is a sunny day, you can nearly always cook using a solar oven. That's because the oven operates on the principle of focusing the sun's rays, not on using the outside temperature.
If you are in some of the Northern states, however, colder temps usually mean that daylight is very limited.
You'll have to make sure you do two things:
1. Make sure you position the oven so it is facing directly towards the sun.
2. Make sure you cook during the brightest/warmest time of the day. (Usually this is between 10 am and 2 pm.)
3. Make sure you adjust the position of your solar oven every hour to maintain higher temps.
Here in Charleston, SC, the temperature really isn't much of a factor. There are only a few weeks where it gets really cold here, and it is usually sunny.
But I do have a backup system, just in case. :)
