This paper studies the temporal dynamics of power-line frequency electric and magnetic fields in Saint-Petersburg and environs. It is found that electromagnetic fields generated by high-voltage transmission lines (HVTL) constantly change, depending on their loading and weather conditions. The dependence on weather conditions involves both the direct effect of air electrical conductivity as a function of its humidity, and indirect effects including the dependence of energy consumption on temperature and also the correlations between meteorological characteristics. An attempt has been made to evaluate the role of influencing factors.