Incidence of fog and low stratus clouds over Begumpet airport during winter months

Abstract. In this paper a new technique for the detection of fog and low stratus in 1 km resolution from MSG SEVIRI data is presented. The method relies on the pan-sharpening of 3 km narrow-band channels using the 1 km high-resolution visible (HRV) channel. As solar and thermal channels had to be sharpened for the technique, a new approach based on an existing pan-sharpening method was developed using local regressions. A fog and low stratus detection scheme originally developed for 3 km SEVIRI data was used as the basis to derive 1 km resolution fog and low stratus masks from the sharpened channels. The sharpened channels and the fog and low stratus masks based on them were evaluated visually and by various statistical measures. The sharpened channels deviate only slightly from reference images regarding their pixel values as well as spatial features. The 1 km fog and low stratus masks are therefore deemed of high quality. They contain many details, especially where fog is restricted by complex terrain in its extent, that cannot be detected in the 3 km resolution.

off. flumen fog A visible aerosol of minute water droplets or ice crystals that is suspended in the air at or near the Earth's surface. Fog is often considered This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields. fog A visible aerosol of minute water droplets or ice crystals that is suspended in the air at or near the Earth's surface. Fog is often considered a type of low-lying cloud and is heavily influenced by local topography, nearby bodies of water, and wind conditions. overcast The condition of cloud clover wherein clouds obscure at least 95% of the sky. The type of cloud cover that qualifies as overcast is distinguished from obscuring surface-level phenomena such as fog. polar vortex Either of the two very large, persistent, rotating, upper-level low-pressure areas suspended in the Earth's atmosphere near the geographic poles. The polar vortices predictably strengthen during their local winter and weaken during their local summer as the temperature contrast between the poles and the Equator changes. When either vortex is weak, high-pressure zones of lower latitudes may push poleward, driving the vortex, jet stream, and masses of cold, dry polar air into the mid-latitudes, which can cause sudden, dramatic drops in temperature known as cold waves. radiation fog Fog formed over land, generally at night in moist, calm air under clear skies. The most common type of fog, it is caused by the radiative cooling of the Earth's surface and the lowest layers of the atmosphere when the temperature of the air near the ground decreases below its dew point. Radiation fog occurs most often in the autumn and winter, and is often deepest around sunrise but usually disperses after dawn when heated by solar radiation. warm front A type of front located at the leading edge of a warmer air mass as it overtakes a cooler air mass that is moving more slowly in the same direction. Warm fronts lie within broader troughs of low pressure than cold fronts, which sometimes follow them, and the temperature difference between the air masses they separate is often greater. Stratiform clouds, fog, and steady rain with occasional thunderstorms often precede the boundary as it moves. In surface weather analysis, warm fronts are symbolized by a red line with semicircles pointing in the direction of travel.

<p>            The safe operation of aviation and shipping, particularly in areas of insufficient coverage of automatic meteorological stations in the Arctic requires accurate interpretation of satellite images. Operational detection of fog and low stratus clouds and recognizing of them on the background of snow and ice cover and cloudiness of the upper layer is very important challenge. </p><p>           The verified images obtained by Aqua and Terra satellites with a scanning radiometer MODIS, which operates in 36 spectral bands, with wavelengths from 0.4 µm to 14.4 µm, were collected.  With the Beam VISAT 5.0 software, which was designed to work with satellite data in raster format, thematic digital techniques of satellite multispectral information, based on difference in the values of the integral brightness of the images, both in optical and far-infrared ranges of the spectrum, have been developed.  These techniques, models of additive color synthesis, improve the quality of interpretation of fogs and low stratus clouds in terms of the complex structure of cloudiness and underlying surface in polar regions. Developed RGB combinations, which are based on the selected MODIS bands are:</p><ol><li>RGB (1.6 µm; 0.8 µm; 0.6 µm)</li> <li>RGB (0.8 µm; 3.9-8.7 µm; 10.8 µm)</li> <li>RGB (0.8 µm; 1.6 µm; 3.9-8.7 µm)</li> <li>RGB ((0-12)-(0-11) µm, (0-11)-(0-3.8) µm, (0-11) µm)</li> </ol><p>          Analysis of the obtained images has shown that the developed models of color synthesis help to distinguish the fog/low stratus clouds under different conditions of cloudiness and underlying surface accurately.</p><p>Key words: remote sensing, satellite imagery, additive color synthesis, fog, low stratus clouds, polar regions</p>

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. 

Unlike fog droplets, drizzle falls to the ground. It usually falls from low stratus clouds and is frequently accompanied by low visibility and fog . See

full. The tops of the woods in the horizon, seen above the fog , look exactly like long, low , black clouds , the fog being the color of the sky. Oct. 8, 1857