There should be some sort of shelter for them. I wouldn't be surprised if there wasn't though, horses are tough animals, can survive through all kinds of weather conditions.
Horses know when the weather is coming and they grow a nice thick coat to protect them. Even in the snow, when I brought my kids into the stall and brushed them out, underneath the wet over coat, their undercoat was still dusty! As long as they have hay to munch, they will be fine, shelter is best as well, but munching hay keeps horses the warmest. They look drafty to me, and I think they will be just fine.