Misreporting of Freezing Fog during Snowfall Conditions in U.S. METAR Observations

Misreported weather conditions at airports can cause significant and unnecessary flight delays and cancellations, while increasing costs to the airlines. In 2022, updates to the Federal Aviation Administration (FAA) holdover time tables for aircraft ground deicing operations included guidance for snow (SN) mixed with freezing fog (FZFG). Holdover time tables provide information on the length of time (i.e., holdover time) anti-icing fluids will protect the aircraft prior to takeoff under various winter weather conditions. The new holdover times for SN mixed with FZFG are significantly shorter than the holdover times for SN or FZFG reported individually. Prior to the introduction of this guidance, pilots would often assess the SN and FZFG conditions individually and use the most conservative holdover time between the two weather conditions. The new guidance has led pilots and ground deicing crews to express concern that FZFG conditions are often reported with SN when FZFG is not present. To assess this, 1-min-observation data from selected Automated Surface Observing System (ASOS) locations prone to SN and FZFG conditions were analyzed to determine if an FZFG signal could be detected using measurements other than visibility during SN conditions. Additionally, Meteorological Aerodrome Reports (METARs) from two nearly collocated airports (one in the United States and one in Canada) were analyzed since Canada relies on human observers to report obscurations, including FZFG. The outcome of both methods indicates a significant number (~85%) of misreported FZFG reports during SN conditions and provides a basis for improving the automated weather-reporting algorithms. SIGNIFICANCE STATEMENT: The purpose of this work is to demonstrate a problem with the method for automatically reporting freezing fog with snow conditions using the current automated weather observing systems in the United States. The key findings show that freezing fog is misreported with snow conditions at least 85% of the time. Incorrect reporting of freezing fog conditions with snow can have significant impacts on aircraft ground deicing operations, resulting in unnecessary flight delays and cancellations, and may require the aircraft to be deiced multiple times unnecessarily, leading to increased costs and decreased efficiency during winter conditions for airline operators.