Keeping the cows close to the calves at weaning increases the noise level, but lessons the impact of social stress on the calf. This includes castration, tagging, vaccination, implanting, dehorning and any other process where the animal is restrained, handled and subjected to a painful process. If the calves are handled roughly, deworming with a pour-on can cause stress too. By performing the typical health practices while the calves still have their dam to return to for comfort reduces the impact of the physical stress on the calves. This is much more than just the weather.
Environmental stress can include changes in diet, space and location.
While calves can do fine in a limited space such as a lot, if it is new to them it will cause additional stress. Weather is the obvious environmental stressor. It is the one stressor over which you have no control. Weather during weaning is the only stress factor over which you have no control. With the day to day variability in weather during the fall, your best strategy at weaning is to minimize all other stress factors and accept the weather as it is.