What is the lowest temperature they can operate in; and can they be used in open area (i.e.: under an open carport)?

So far they have worked at -15 degrees.

Since they're aren't any bulbs to replace you may be able to use these in a car port as long as they aren't exposed to blowing rain. I've operated this fixture at 20F and waiting for it to get colder to see how low they will go. I don't expect it to be a problem even well below zero.

I don't know the details on this unit, but I have easily converted 8 other 4ft flourescent fixtures to LED bulbs. The 3 LED bulb vendors state -10 degrees F. My underhouse garage always had problems starting the flourescent bulbs on the cold winter days. All my LED lights now start every time. I am purchashing this fixture since I broke one while converting it to LED.

I have read all the info on this light and there is nothing noting the temp range the light works best in. However I have them set up in my garage and the current temp in the garage is 31 degrees. As for using under a carport, I think the light would meet your needs during low light or dark outside conditions.