In winter fogs, when the polluted air or liquid fog came over, the potential gradient increased positively to several times of normal value. This increase in the potential gradient was explained by the decrease of the air conductivity.
Accompanying with the glaciation of those liquid fogs, the positively enhanced potential gradient decreased to the normal values or even negative values.
About the decrease due to the glaciation following mechanisms are considered. Ice crystals carried preferentially the negative charge. The conductivity of the air increased by the removing submicron aerosols due to the growth of ice crystals (Facy effect).
At times, it was observed that when the snowfall stopped, the potential gradient abruptly increased to positive high values. About the mechanism, it was considered that the residual positive space charge was gravitationally separated from the negative snow crystals, then the positive space charge was brought down to the surface by downdraft between snow clouds.