For most of us, tonight and tomorrow night will have us experience temperatures in the mid to low 20's, this proceeded some of what by any stretch of the imagination, would be called a " mild to warm ", winter. For most of us, the queen has started to rapidly expand the brood nest well beyond what would normally be covered by the winter cluster. I know that low temps can kill off the brood if prolonged exposure happens. So how low a temperature and for how long would brood have to be chilled before a kill off is experienced? If in the 20's outside would residual heat from the re-clustering bees be warm enough to save the brood on the frames just outside the cluster?