Fog and low-level stratus characteristics at the airport of Lviv from surface observations

Fog and low-level stratus have been recognised as a hazardous weather phenomenon leading to several losses in time, money and even human life above all in aviation but also in other forms of transportation, such as navigation and land transportation. The forecast of low-level stratus and fog is one of the most difficult issues of aviation meteorology due to spatial and temporal variability of their characteristics and high dependence on local conditions. So, weather observations can be used for statistical dependencies of fog/low-level stratus characteristics on numerical model outputs.

To study fog and low-level stratus characteristics occurring at the airport of Lviv, Ukraine, three-hourly meteorological observations in the period of 2010-2020 are used. Applying a statistical approach annual, seasonal and diurnal distribution of fog and low stratus and their frequency distribution associated with various meteorological parameters are obtained.

It was shown that at the airport of Lviv low-level stratus more frequently (60% of all cases) forms in the November-December-January-February period, whereas in July and August it is least frequently observed (less than 5% of all cases). Distribution of the fog observations with respect to months also is inhomogeneous. Fog mainly forms from October to January (56% of all cases) with the maximum (19%) in November. From March to September fog happen rarely with minimum in June (3% of all cases). No distinct diurnal cycle of the low-level stratus occurrence can be revealed from the data, however low-level stratus more frequently occurs during the nighttime and in morning. As opposed to low-level stratus for fog the highest frequency is observed in the hours before sunrise, while when the day sets in, frequencies are declining and increasing at night.

Low-level stratus is the most commonly associated with surface temperatures of 0.0 to +8.0°С and relative humidity of 80 to 95% (32% of all cases), by comparison with 35% of all fog cases observed at temperatures of 0.0 to +6.0°С and relative humidity of 96 to 100%.

In all seasons of the year, the highest frequency of low-level stratus (from 35 to 40% of all cases) is in interval of 3...4 m/s, whereas fog is the most frequently observed in calm weather (from 37 to 77% of all cases). In autumn and winter, both under low-level stratus and fog, surface wind is characterized by high occurrence frequency of west, north-west (around 25% of low-level stratus cases and 10% of fog cases) and south, south-east (around 30% of all cases) directions. In spring and summer, north, north-west and west directions are most frequently reported in low-level stratus cases (47% of low-level stratus cases in spring and 69% in summer). For fog in spring and summer the same wind directions are typical (18% of fog cases in spring and 10% in summer). However, it should be noticed that in spring the east direction is often observed for both fog and low-level stratus.