Validity of Global Fog‐Day Trends Indicated by the Global Surface Summary of the Day (GSOD) Data Set

Abstract

AbstractIt has long been understood that fog plays an important role in the atmospheric radiation budget and contributes to many transportation related fatalities and injuries. This study utilizes >8,000 Global Surface Summary of the Day (GSOD) ground stations to investigate trends observed in fog‐days over the past 44‐years, and to examine the validity of these trends under varying observational techniques. Results show strong large‐scale regional trends in the GSOD fog‐day data, with the United States (USA) and much of Europe observing ∼20–25% and ∼3–5% decreases respectively in fog‐day occurrence. However, when comparing fog‐day counts to simultaneous visibility, it is evident that several different fog‐day data collection techniques were used throughout the timeseries in many regions. For example, many stations in the USA made data collection changes in the mid 1990s, and again in the mid 2000s. To identify the artifacts from different data collection techniques, a simple method is developed to determine which stations indeed encountered at least one false deviation to the timeseries. After applying the methodology to all GSOD stations, 1,696 stations are identified as being potentially quality long‐term fog‐day stations. Utilizing these stations, more reliable regional trends can be derived over some specific regions. Spain, Australia, and China show statistically significant decreases in fog‐day occurrence. India and Japan show increases in fog day occurrence. Further analysis shows that the driving factors of fog such as temperature and moisture have changed regionally during the last four decades and could be linked to the long‐term regional fog‐day trends.