<p>Fog is traditionally defined by the reduction of the horizontal visibility (x_v) caused by the presence of hydrometeors. Among other factors, the number concentration of hydrated aerosol particles and their size reduce visibility. Therefore, air humidity and air pollution are important drivers of fog. We take a closer look at the physical conditions and processes of fog. Fog condensation nuclei (FCN) can exist as stable particles or be activated into larger particles in an unstable state by water vapor uptake. We use Köhler and Mie theories to computationally analyze two fog episodes in mountainous Taiwan with respect to the role of activation in reducing x_v. In both episodes analyzed, the contribution of activated particles to the reduction of x_v is substantial. Activation is a key process in the formation of fog. The agreement between calculated and measured x_v under fog conditions is very good. Typically, a distinct mode of activated FCN is established within the droplet size distribution. Due to inhomogeneities in the foggy air mass, it is difficult to directly observe the fraction of activated (versus non-activated) nuclei in the field. Our analysis shows that any activation, even at low proportions, leads to a reduction in x_v. Activation is a strong driver of fog. During periods of reduced air pollution, and therefore less tendency to produce low visibility conditions at relative humidities below 100%, activation will play an increasing role in the formation of fog almost worldwide.</p> Graphical Abstract <p></p>