Chickens wear a down coat. As long as that down coat stays dry they can handle temperatures a lot colder than you will see anywhere in Mississippi. That down coat works by trapping tiny pockets of air in the down and feathers. A strong breeze can ruffle the feathers and release those pockets of air. You may notice your chickens don't mind a warm wind but they generally avoid a cold wind.
If the air temperature is below freezing there is a risk of frostbite. That risk is made a lot worse with high moisture. You can't get the moisture inside the coop lower than the outside but your goal is to keep it at least that dry. That should work. Wind chill can make it worse too. So you need to keep them dry and out of a strong breeze.
I can't tell where the roosts are from your photo or how exposed to rain or wind they are. It might help to put up plastic, plywood, or siding to create a protected spot for them to roost. I'd add it to the top of the wire sides if necessary. Do not enclose the whole thing, you need good ventilation to keep the moisture from building up. It should not take much to give them a protected corner.
Growing up in he Cumberland Gap area of East Tennessee I saw chickens sleep in trees when it was below zero Fahrenheit. They were in a protected valley and the tree was more like a thicket so wind wasn't an issue. You could not get any better ventilation. It was not raining at those temps. Those chickens did not have any problems with frostbite or anything else. They can handle cold temperatures much better than most people think if you just give them a little help.
