Arctic Ice Fog: Its Microphysics and Prediction

Ice fog consists of suspended small ice crystals with maximum sizes less than about 200 μm, having similar fall velocities as fog droplets, and that often reduces visibility to less than 1 km. Its formation is strongly dependent on high number concentrations of available heterogeneous ice nuclei (IN) at temperatures (T) > −40 ºC, homogeneous nucleation below −40 ºC, and available moisture in the air. Radiative cooling, advective cooling, and cold air subsidence, particularly over the Polar region or high elevation mountainous geographical regions, play an important role in its formation and development. Ice fog crystals form at cold T when the relative humidity with respect to ice (RH_i) is ≥100%. Favorable ice nucleation conditions typically occur at T < −15 ºC and its microphysical characteristics and their evolution needs to be better understood for a physically based representation in numerical forecast models. This is likely to be of growing societal importance due to the known sensitivity of the Arctic environment to climate change. Accidents related to low visibility over the northern latitudes may increase tenfold over the Arctic regions because of increasing population and traffic. This suggests that ice fog conditions can have major impacts on aviation and ground/water-based transportation, as well as on climate change and ecosystem. These open issues, as well as challenges related to ice fog measurements and predictions, are discussed in detail, and its importance for evaluating weather and climate conditions over cold environments are emphasized.