Evaluating atmospheric electricity changes as an indicator of fog formation

Abstract

Fog is a high-impact weather phenomenon, which is challenging to forecast accurately. Measuring changes in atmospheric electricity has been proposed as a complement to other fog prediction methods, since fog is known to produce changes in the potential gradient (PG). Many aspects of the relationship between PG and fog remain unexplored, to which modern instrumentation for continuous monitoring of PG and visibility brings a new perspective. We describe several automatic methods of detecting fog events and apply them to a large dataset to understand the evolution of the PG during fog development. The median PG increase in fog is found to be 58 V/m, which is statistically significant compared with the 17 V/m standard deviation of PG in the prefog hours. However, the median lead time of this increase before the fog onset was found to be 0.4 h, compared with 0.6 h solely using visibility data. While we were able to predict fewer fog events using PG (55% of cases) than visibility (64% of cases), PG was much more likely to give a longer lead time (of over 2 h) than visibility (30% vs. 13% respectively). This indicates that PG measurements would be a useful additional tool in fog prediction.