Answer
The outer surface temperature of the glass may drop below the dew point temperature of the surrounding air, causing the moisture in the vicinity of the glass to condense, after a while the condensate may start dropping down because of gravity.
Work Step by Step
The outer surface temperature of the glass may drop below the dew point temperature of the surrounding air, causing the moisture in the vicinity of the glass to condense, after a while the condensate may start dropping down because of gravity.