If it snows, they are stuck in the barn. If it is below 15,, they get stuck in the barn. If it is REALLY windy and the wind chill hits around 10, they get stuck in the barn.
If it is raining and a decent day, they decide. If it is cold and not windy and above 15, they decide. If it is windy, yet the wind chill is above freezing, they decide. If it is foggy, misty or sleeting, they decide.
I have a few brave birds that go out in just about anything, but most are "temp whimps" and they choose to enjoy the barn for the day.
Enjoying the barn means they have FULL excess to layer, mash, poultry grain, fresh water, their dust bath areas, their roosts and the nest boxes.
At night they are locked up in wire mesh stalls with NO water, NO food and NO dust bath areas. I do this to keep any predator out of their enclosure. They have numerous roosts, both metal and wood in the stalls.
The Guineas have excess to the barn always as they roost in the rafters of the barn, WAY up high.