There are several factors affecting the moisture ratio and climate in attics. The outdoor temperature is one factor when the temperature is low and the relative humidity in the outdoor air is high. Even warm, humid air from the indoor environment that reaches the attic is a risk factor. Moisture in the air that is ventilated in the attic during night time is cooling down and condenses on the inside of the ceiling. This is called night radiation.
Night radiation is a phenomenon that arises on clear winter nights when energy radiates from surfaces that faces the night sky. These surfaces are being cooled down due to the radiation exchange with space. One example is that you, during some circumstances, find ice on your front window of the car but not the side windows.
How does this affect your attic?
The outdoor ceiling is being cooled down during clear nights which consequently affect the indoor ceiling in the long run. Night radiation can create large temperature differences and increased risk of condensation. When air thereafter meets the inside of the supporting roof, you get condensation or hoar frost. Next day, the temperature increases again and the outdoor ceiling quickly reaches a higher temperature. This is unfortunately a good start for microbial growth.