An anatomy of fog over Nova Scotia under a ridge on 8 September 2021

Abstract Interaction of processes over land, sea and synoptic conditions resulted in a complex fog event over and along the Nova Scotia coast on 8 September 2021. This study was undertaken to understand how this occurred. A ridge moving eastward along Nova Scotia caused the fog to evolve in a unique three‐phase pattern under an air temperature inversion at 150 m height. In the first phase, fog formed over the Nova Scotia coastline due to the slowing of onshore surface winds caused by the increased drag which generated convergence, lifting, and saturation. Weather Research and Forecasting (WRF) simulation of this phase was successful. In the second phase, changed wind direction to from the west over land advected initially land‐formed fog over warmer water. In the third phase, surface winds advected a colder fog over warmer sea water which was unstable and facilitated the advance of the fog's leading edge parallel to the Point Nova Scotia coast. WRF simulations of fog cloud development and expansion over water in later stages were relatively poor due to the prescription of the sea surface temperature and not due to the configurations of the physics parameterizations.