The Climatology, Frequency, and Distribution of Cold Season Fog Events in Northern Utah

The distribution and frequency of fog events during the cold season in northern Utah is studied using surface Mesowest data from 2004 to 2014. Fog is identified using the reported weather conditions, relative humidity, and visibility. The length and time of each fog occurrence is calculated. Statistics are performed on daily, monthly, and seasonal timescales. To ensure representativeness of the data and to account for the relatively small sample size, "near-fog" conditions are included in some statistics. Results show that there is significant variability among the valleys in northern Utah in terms of both quantity and timing of fog events. Fog occurs more frequently in locations close to lakes such as the Great Salt Lake or Utah Lake than in locations farther away. It is also noted that small, enclosed valleys have higher amounts of fog than broader, open valleys. Throughout the region, there is a distinct peak in fog in late January for most stations. A strong peak in fog occurrences near dawn is also found for all cold season months. In addition, the influence of local, mesoscale conditions on the fog distribution is evident in many stations. It is found that the existence of fog at one location is a very poor predictor of fog at nearby locations on a daily timescale, which implies serious forecasting difficulties over complex terrain. However, it is also found that on an annual timescale the amount of fog at one location can be used to estimate the amount of fog at another location. The controlling factors that contribute to the variability of fog events over northern Utah (a mountainous region) are discussed.