Fog Formation Research Articles

Dutch fog: On the observed spatio-temporal variability of fog in the Netherlands.

The Netherlands is characterized by highly variable land use within a small area, and a strong influence of the North Sea on national climate. Devoid of significant topography, it is an excellent location for assessing the relative influence of various factors on fog occurrence in the absence of terrain effects. Using observations from a dense network of weather stations throughout the country, the climatology of fog in the Netherlands is assessed over a period of 45 years. On a national scale, interannual variability is linked to changes in synoptic pressure‐gradient forcing. Within the country, a comprehensive in‐depth analysis of regional differences between fog occurrence is made, together with an assessment of local physical factors which could bias fog formation in one location over another. Regional variability is shown to be strongly related to the mesoscale influences of urbanization and the North Sea. In fact, some locations experience over twice as much fog as others. From this finding, a simple index is presented, which combines the water and urban fraction surrounding a station. This “Regionally Weighted Index” (RWI) is able to accurately sort the stations according to their relative fogginess. Its practical use is encouraged for assessing a given site's climatological favourability, even when in situ meteorological observations are unavailable.

Diffusion layer model for condensation of vapor with the presence of noncondensable gas under natural convective condensation

A diffusion layer model was developed for condensation heat transfer of a vapor with noncondensable gases. The model predicts the heat transfer coefficient for free convection condensation over a vertical surface in the presence of air with consideration of the effect of the variation of the mixture molecular weight and density across the diffusion layer on the vapor diffusion. The model also takes into account the effect of the suction due to the condensation on the sensible heat transfer and fog formation. The model predictions compare well with previous experimental data and correlations. This model more accurately predicts the convection and condensation heat transfer of steam and air along a vertical surface with an average error of 14.74% and with almost all the points within ±20% error bands. The results show that the suction and fog formation significantly affect the heat transfer. For high noncondensable mass fractions, the fog formation should be considered, especially at high pressures.

Impact of water vapor transfer on a Circum-Bohai-Sea heavy fog: Observation and numerical simulation

a wide-range winter frontal fog that happened over Circum-Bohai-Sea (CBS) on December 17–192,016 was observed using satellite cloud image inversion, surface, ocean buoy, and radiosonde data. The evolution of fog processes was investigated with numerical simulations using the Weather Research and Forecasting (WRF) Model together with the FNL (Final) reanalysis data. The results show that the WRF model can reproduce the features of the heavy fog generation and development. The observed results show following characteristics: (1) Before fog appearance, the southwest low-level jet (LLJ) of which the wind speed was over 16 m/s at 925 hPa developed. The LLJ carrying warm and wet air provided a stable inversion layer about 1200 m thick and continuous vapor transport to the CBS area, which is favorable for the formation of heavy fog. With the weak northwestern cold air moving towards the CBS area, a typical frontal fog process was triggered; (2) the weak cold air did not destroy the structure of low-level thermal inversion, and the heavy fog maintained. With the weak front moving towards the east, the foggy coverage continually extended over the sea surface, finally covering the Bohai Sea and the North Yellow Sea. A water-vapor sensitivity test suggests that with the LLJ continually conveying water vapor to the area, the accumulative effect of an increase in water vapor content took place in the low-level atmosphere, which changed the temperature and humidity structure of the boundary layer. The liquid water content, coverage area, and thickness of fog changed drastically.

Impacts of black carbon on the formation of advection–radiation fog during a haze pollution episode in eastern China

Abstract. Aerosols can not only participate in fog formation by acting as condensation nuclei of droplets but also modify the meteorological conditions such as air temperature and moisture, planetary boundary layer height (PBLH) and regional circulation during haze events. The impact of aerosols on fog formation, yet to be revealed, can be critical in understanding and predicting fog–haze events. In this study, we used the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to investigate a heavy fog event during a multiday intense haze pollution episode in early December 2013 in the Yangtze River Delta (YRD) region in eastern China. Using the WRF-Chem model, we conducted four parallel numerical experiments to evaluate the roles of aerosol–radiation interaction (ARI), aerosol–cloud interaction (ACI), black carbon (BC) and non-BC aerosols in the formation and maintenance of the heavy fog event. We find that only when the aerosols' feedback processes are considered can the model capture the haze pollution and the fog event well. And the effects of ARI during the fog–haze episode in early December 2013 played a dominant role, while the effects of ACI were negligible. Furthermore, our analyses show that BC was more important in inducing fog formation in the YRD region on 7 December than non-BC aerosols. The dome effect of BC leads to an increase in air moisture over the sea by reducing PBLH and weakening vertical mixing, thereby confining more water vapor to the near-surface layer. The strengthened daytime onshore flow by a cyclonic wind anomaly, induced by contrast temperature perturbation over land and sea, transported moister air to the YRD region, where the suppressed PBLH and weakened daytime vertical mixing maintained the high moisture level. Then heavy fog formed due to the surface cooling at night. This study highlights the importance of anthropogenic emissions in the formation of advection–radiation fog in the polluted coastal areas.

Impact of Air Pollution Controls on Radiation Fog Frequency in the Central Valley of California

AbstractIn California's Central Valley, tule fog frequency increased 85% from 1930 to 1970, then declined 76% in the last 36 winters. Throughout these changes, fog frequency exhibited a consistent north‐south trend, with maxima in southern latitudes. We analyzed seven decades of meteorological data and five decades of air pollution data to determine the most likely drivers changing fog, including temperature, dew point depression, precipitation, wind speed, and NOx (oxides of nitrogen) concentration. Climate variables, most critically dew point depression, strongly influence the short‐term (annual) variability in fog frequency; however, the frequency of optimal conditions for fog formation show no observable trend from 1980 to 2016. NOx concentration, which has a decreasing north‐south concentration gradient, declined continuously over this period, consistent with the long‐term temporal and spatial trends in fog. As development in the Central Valley increased direct particle and other pollutant emissions from 1930 to 1970, fog frequency increased. Following the Clean Air Act, particle emissions quickly declined, and NOx emissions declined steadily, reducing the cloud condensation nuclei (CCN) available for fog formation. As a precursor of ammonium nitrate aerosols, which are efficient CCN, we used NOx measurements and emission trends as a proxy for the CCN trend. We conclude that while the short‐term fog variability is dominantly driven by climate fluctuations, the longer‐term temporal and spatial changes in fog have been driven by changes in air pollution. For conditions close to the dew point, a decrease in fog of 5 days/year per 10 parts per billion NOx decrease occurred across the Central Valley.

A WRF Modeling Study on the Effects of Land Use Changes on Fog Off the West Coast of the Korean Peninsula

This study investigates, for the purpose of fog forecasting, the impacts of topography and land use changes on the characteristics of turbulence that directly contribute to the formation and dissipation of fog off the west coast of the Korean Peninsula using the Weather Research and Forecasting model version 3.5.1. During the investigation period, there are 59 coastal ground fog and 29 sea fog events. Local meteorological characteristics of coastal ground fog were similar to those of radiation fog typically seen over the land surface since the reclaimed island was constructed. After the sun rises, relative humidity over the land surface decreases rapidly—within a couple of hours—due to surface heating, which is controlled directly by shortwave radiation. Over the sea surface, however, the sea fog remains, with the relative humidity higher than 95% even during the daytime. For two selected cases, topography and land use were modified to identify turbulence characteristics through numerical modeling. This modification contributed to better forecasting the formation and dissipation of fog by changing characteristics of sensible and latent heat flux in the land surface model and then planetary boundary layer over the reclaimed island.

The enigmatic absence of cyanobacterial biocrusts from the Namib fog belt: Do dew and fog hold the key?

Although considered as pioneering organisms of biocrusts, cyanobacterial biocrusts are scarce and even absent from the Namib fog belt, which is inhabited, however, by luxurious lichen-dominated crusts. While previously proposed that the phenomenon may stem from a low rainfall frequency, it is suggested herein that it may stem from a negative balance of carbon. This explanation is based on published data on photosynthesis of biocrusts along with an analysis of dew and fog formation. Lush cover of lichens at habitats with no cyanobacteria also casts doubt on the widely accepted view that regards cyanobacteria as a required component in biocrust succession.

A Numerical Study of “Hijikawa-Arashi”: A Thermally Driven Gap Wind Visualized by Nocturnal Fog

AbstractA renowned local wind in Japan, “Hijikawa-Arashi,” is a thermally driven nocturnal gap wind accompanied by fog. The wind is visually identified by the fog along the valley of the Hijikawa River between the Ozu basin and the Seto Inland Sea during the early morning in autumn and winter. A fine-resolution numerical model is employed to reproduce the main observed features of Hijikawa-Arashi. A vertical resolution of 10 m or less at the lowest level is required to express the nocturnal radiative cooling of the land that is required for fog formation in the basin, and fine horizontal resolution is necessary to express a realistic valley through which the fog is advected to the sea. Multiple hydraulic jumps accompanied by supercritical flow occur because of the complex topography. Both moisture transport by the sea breeze during the daytime and evaporation from the land surface are important for accumulating moisture to produce the fog.

Engineering of Durable Antifog Thin Coatings on Plastic Films by UV-Curing of Proteinoid Prepolymers with PEG-Diacrylate Monomers.

Fog formation on transparent surfaces constitutes a major challenge in several optical applications, such as plastic packaging, lenses, mirrors, and windshields. To overcome this problem, we prepared and characterized durable antifog thin coatings on plastic films such as polyethylene terephthalate (PET). Proteinoids are biocompatible random polymers made of α-amino acids by thermal step-growth polymerization. Proteinoid prepolymers were prepared by adding activated double bonds to proteinoids via the Michael addition reaction. A series of thin antifog cross-linked coatings were prepared by spreading on PET films with a Mayer rod various mixtures of the proteinoid prepolymers, polyethylene glycol diacrylate, and a photoinitiator, followed by UV-curing of the dried coatings. The antifog properties of the coatings were determined by the contact angle, roughness, haze, and gloss measurements, as well as hot and cold fog tests, to examine the optical properties of the films under fog formation conditions. Mechanical properties such as adhesion, robustness, and abrasion resistance of the antifog coatings were examined by tape, knife-scratch, and sandpaper abrasion tests. The effect of coating composition, wettability, and roughness on the antifog properties of the coated PET films was elucidated. The formula was optimized, and the corresponding UV-cured antifog cross-linked thin coating exhibited transparency with good adhesion and excellent durable antifog performance.

Condensation heat and mass transfer of steam with non-condensable gases outside a horizontal tube under free convection

The condensation heat and mass transfer of steam-He, steam-N2 and steam-CO2 mixtures on a horizontal tube under free convection was studied experimentally and theoretically. A model was developed to predict the total condensation heat transfer coefficient of these mixtures and the model considered the effects of suction, fog formation and the variation of the mixture density and molecular weight across the diffusion layer. The experiments were conducted to investigate the effects of noncondensable gas component, concentration and surface subcooling. The condensation heat transfer coefficients are significantly reduced due to the presence of noncondensable gases. The condensation heat transfer coefficient decreases slowly as the surface subcooling increases and the subcooling effect decreases with increasing noncondensable gas concentration. For a given noncondensable gas mole fraction and a given surface subcooling, the steam-He mixture has the highest measured condensation heat transfer coefficient, followed by the steam-N2 mixture and the steam-CO2 mixture has the lowest measured condensation heat transfer coefficient. But for a given noncondensable gas mass fraction and a given surface subcooling, the measured condensation heat transfer coefficient of the steam-He mixture is the lowest, followed by the steam-N2 mixture, CO2 mixture. The model predicts the relative errors of the total condensation heat transfer coefficients of these mixtures within ±30%. A condensation heat transfer correlation was developed based on the experimental data and the correlation achieved an uncertainty of less than 20%.

PM2.5 vertical variation during a fog episode in a rural area of the Yangtze River Delta, China

A dense radiation fog event occurred at the Shouxian site, Anhui Province, China, from the evening of January 2 to noon on January 3, 2017. During this event, vertical profiles of particulate matter (PM) and meteorological parameters within the lower troposphere (0–1000 m) were collected using a tethered balloon. This study assessed the evolution of the PM2.5 profile with the planetary boundary layer (PBL) structure and the effects of fog on the PM2.5 concentration. The results showed the following: (1) At the surface, the average diurnal variation in Aitken mode, accumulation mode and coarse mode particles had bimodal patterns before fog formation and was mainly influenced by diurnal variation in the mixing level depth (MLD). The aerosol number concentrations decreased remarkably, and the PM2.5 was strongly scavenged from 150 μg/m3 to 45 μg/m3 during the fog process. (2) In the vertical direction, the PM2.5 distribution was affected by the PBL height and the vertical fog structure. At 05:00 LT (local time) (i.e., early morning before the fog event), the PM2.5 concentration was slightly higher in the stable layer (260 μg/m3) than in the residual layer (200 μg/m3). At 14:00 LT (haze period), PM2.5 was well mixed below 500 m, with a concentration of 310 μg/m3. After 20:00 LT, when fog formed, PM2.5 was scavenged from the surface to the upper layers, and the scavenging height was controlled by the fog top height. (3) The vertical development of fog was promoted by turbulent mixing and radiation cooling at the fog top. Turbulent mixing enhanced the particle scavenging efficiency of fog droplets by the collision-coalescence process. The PM2.5 scavenging height was corresponded to the turbulence height. Therefore, turbulence development in the fog was the essential dynamic factor driving PM2.5 reduction.

Investigation of coastal sea-fog formation using the WIBS (wideband integrated bioaerosol sensor) technique

Abstract. A wideband integrated bioaerosol sensor (WIBS-4) was deployed in Haulbowline Island, Cork Harbour, to detect fluorescence particles in real time during July and September 2011. A scanning mobility particle sizer (SMPS) was also installed providing sizing analysis of the particles over the 10–450 nm range. During the campaign, multiple fog formation events occurred; they coincided with dramatic increases in the recorded fluorescent particle counts. The WIBS sizing and fluorescence intensity profiles indicated that the origin of the signals was potentially non-biological in nature (i.e. PBAPs, primary biological aerosol particles). Furthermore, the data did not support the presence of known fluorescing chemical particles like SOA (secondary organic aerosol). Complementary laboratory studies showed that the field results could potentially be explained by the adsorption of molecular iodine onto water droplets to form I2 ⋅ (H2O)x complexes. The release of iodine into the coastal atmosphere from exposed kelp at low tides has been known for many years. This process leads to the production of small IxOy particles, which can act as cloud condensation nuclei (CCN). While the process of molecular iodine release from coastal kelp sources, subsequent particle formation, and the observations of sea mists and fogs have been studied in detail, this study provides a potential link between the three phenomena. Of mechanistic interest is the fact that molecular iodine included into (rather than on) water droplets does not appear to fluoresce as measured using WIBS instrumentation. The study indicates a previously unsuspected stabilizing transport mechanism for iodine in the marine environment. Hence the stabilization of the molecular form would allow its more extensive distribution throughout the troposphere before eventual photolysis.

How important are aerosol–fog interactions for the successful modelling of nocturnal radiation fog?

Forecasting and modelling fog formation, development, and dissipation is a significant challenge. Fog dynamics involve subtle interactions between small‐scale turbulence, radiative transfer and microphysics. Recent studies have highlighted the role of aerosol and related cloud microphysical properties in the evolution of fog. In this article, we investigate this role using very high‐resolution large eddy simulations coupled with a newly developed multi‐moment cloud microphysics scheme (CASIM), which has been designed to model aerosol–cloud interactions. The simulation results demonstrate the sensitivity of the fog structure to the properties of the aerosol population (e.g. number concentration). This study also demonstrates the importance of the treatment of aerosol activation in fog formation and discusses future work required to improve the representation of aerosol–fog interactions for simulations of fog.

Laboratory and numerical modeling of the formation of superfog from wildland fires

Smoke from human-induced fires such as prescribed fires can occasionally cause significant reduction in visibility on highways in the southern United States. Visibility reduction to less than 3 m has been termed “superfog” and environmental conditions that lead to its formation have been proposed previously. Accurate characterization and prediction of precursor conditions for superfog is needed to prevent dangerous low visibility situations when planning prescribed fires. It is hypothesized that extremely hygroscopic cloud condensation nuclei from the smoldering phase of a fire can produce a large number of droplets smaller in size than in naturally occurring fog. This large number of small droplets can produce superfog conditions with relatively low liquid water content. A thermodynamics-based model for fog formation was developed. Laboratory generated superfog measured by a Phase Doppler Particle Analyzer determined that mean droplet radius was 1.5 μm and the size distribution could be modeled with a lognormal distribution. Experiments in an environmentally-conditioned wind tunnel using longleaf pine (Pinus palustris Mill.) needle fuel beds provided visibility, heat flux, temperature, humidity, and particle data for model validation. Numerical modeling was used to approximate the growth of a superfog boundary-layer with liquid water content values of 2 g m−3 or greater. The model successfully predicted previous superfog events.

Loss to Aviation Economy Due to Winter Fog in New Delhi during the Winter of 2011–2016

Stable and clear atmospheric conditions, lower surface temperatures, an ample moisture supply, and a strong low-level inversion persisting for most of the night usually facilitates the formation of dense fog during winter in Delhi. This severely hinders the flight operations at India’s busiest airport, the Indira Gandhi International (IGI) Airport, where more than 900 flight operations occur per day and an interruption can cause significant financial losses to the aviation industry. It is important to undertake a quantitative study of the estimated losses. This study, undertaken for the first time in India, aimed to evaluate the impact of dense fog at IGI Airport on economic losses which occurred during the winter season between 2011 and 2016. The breakdown of charges for different segments of flight operations for the domestic and international sectors was obtained from India’s Ministry of Civil Aviation and the Center for Asia Pacific Aviation (CAPA) India. A total of 653 h of dense fog between 2011 and 2016 at IGI Airport caused economic losses of approximately 3.9 million USD (248 million Indian rupees) to the airlines. The analysis further found that from 2014–2015 onwards, there has been a reduction in the number of flight delays, diversions, and cancellations by approximately 88%, 55%, and 36%, respectively, due to the strict implementation of guidelines to facilitate the Category (CAT)-III landing for aircraft during dense fog.

Evaluation of the Global and Regional Assimilation and Prediction System for Predicting Sea Fog over the South China Sea

In the South China Sea, sea fog brings severe disasters every year, but forecasters have yet to implement an effective sea-fog forecast. To address this issue, we test a liquid-water-content-only (LWC-only) operational sea-fog prediction method based on a regional mesoscale numerical model with a horizontal resolution of about 3 km, the Global and Regional Assimilation and Prediction System (GRAPES), hereafter GRAPES-3km. GRAPES-3km models the LWC over the sea, from which we infer the visibility that is then used to identify fog. We test the GRAPES-3km here against measurements in 2016 and 2017 from coastal-station observations, as well as from buoy data, data from the Integrated Observation Platform for Marine Meteorology, and retrieved fog and cloud patterns from Himawari-8 satellite data. For two cases that we examine in detail, the forecast region of sea fog overlaps well with the multi-observational data within 72 h. Considering forecasting for 0–24 h, GRAPES-3km has a 2-year-average equitable threat score (ETS) of 0.20 and a Heidke skill score (HSS) of 0.335, which is about 5.6% (ETS) and 6.4% (HSS) better than our previous method (GRAPES-MOS). Moreover, the stations near the particularly foggy region around the Leizhou Peninsula have relatively high forecast scores compared to other sea areas. Overall, the results show that GRAPES-3km can roughly predict the formation, evolution, and dissipation of sea fog on the southern China coast.

Case Study of Fog Predictability for an Event with Cold-Front Synoptic Pattern

Fog has recently become a frequent high-impact weather phenomenon along the coastal regions of North China. Accurate fog forecasting remains challenging due to limited understanding of the predictability and mechanism of fog formation associated with synoptic-scale circulation. One frequent synoptic pattern of fog formation in this area is associated with cold front passage (cold-front synoptic pattern, CFSP). This paper explored the predictability of a typical CFSP fog event from the perspective of analyzing key characteristics of synoptic-scale circulation determining fog forecasting performance and the possible mechanism. The event was ensemble forecasted with the Weather Research and Forecasting model. Two groups of ensemble members with good and bad forecasting performance were selected and composited. Results showed that the predictability of this case was largely determined by the simulated strengths of the cold-front circulation (i.e., trough and ridge and the associated surface high). The bad-performing members tended to have a weaker ridge behind a stronger trough, and associated higher pressure over land and a weaker surface high over the sea, leading to an adverse impact on strength and direction of steering flows that inhibit warm moist advection and enhance cold dry advection transported to the focus region. Associated with this cold dry advection, adverse synoptic conditions of stratification and moisture for fog formation were produced, consequently causing failure of fog forecasting in the focus region. This study highlights the importance of accurate synoptic-scale information for improved CFSP fog forecasting, and enhances understanding of fog predictability from perspective of synoptic-scale circulation.

On the Predictability of Radiation Fog Formation in a Mesoscale Model: A Case Study in Heterogeneous Terrain

This study evaluates the predictability of the formation phase of a radiation fog event observed during the night of 31 October 2015 to 01 November 2015 in the north-east of France at three sites managed by OPE (Observatoire Pérenne de l’Environnement). The fog layer shows significantly different behaviors at the three areas, which are located only a few kilometers apart. Three fog life cycles were observed: the formation of a dense adiabatic fog, the formation of a thin patchy fog, or no fog formation despite favorable conditions. This event was studied with the Meso-NH numerical mesoscale model at two horizontal resolutions, 500 m and 50 m. Simulations at 50 m allow estimation of the spread of the predicted parameters over the heterogeneous terrain studied. These numerical simulations strongly suggest that this event involved numerous interactions and complex circulations. The wind above the nocturnal boundary layer greatly affects the transition of shallow patchy fog into thick adiabatic fog. These numerical simulations also show that the occurrence and type of fog could be very different over a small but heterogeneous area. It is also interesting to note that the spread of the simulated parameters was very high during the transition from shallow fog to a deep fog layer. The spread was concentrated during the regime transition between the fog formation and its maturity. This appeared to be the result of the complex interplay of processes at numerous ranges of scale. A new concept called “pseudo-process diagram” is presented. These pseudo-process diagrams are very good tools to analyze fog, and allow a good illustration of the spread of fog during this chaotic phase. This kind of concept seems a promising tool to analyze fog predictability in depth.

On the Formation and Development of Radiation Fog: An Observational Study

Observations from the Local and Non-local Fog EXperiment field campaign are used to investigate the formation and initial development of radiation fog. Data from six locations have been selected: five sites in Shropshire, and one in Bedfordshire, UK. These data show that during night-time clear-sky conditions, thermally-stable radiation fog only forms when the vertical velocity variance near the ground (at 2 m) decreases to 0.002–0.005 m2 s−2 or less. The variation in this threshold appears to be a function of the vertical relative humidity gradient near the ground. When the vertical velocity variance exceeds these values, saturation may still occur near the ground, but fog formation is inhibited due to the more efficient transport of moisture to the ground in the form of dew. Furthermore, once stable radiation fog forms, any increase in the vertical velocity variance to values greater than 0.002–0.005 m2 s−2 may cause the fog to dissipate. The subsequent development of shallow stable fog into deeper, weakly-unstable fog is also examined. Possible mechanisms for this are discussed, including in situ processes and non-local larger scale processes. Clear evidence of the latter is presented, but the relative importance of in situ and non-local processes for fog deepening requires further clarification.

Impact of high‐resolution ocean–atmosphere coupling on fog formation over the North Sea

Sea‐surface temperature (SST) is a key driver for various interactions and feedbacks between components of the Earth System and can control local weather and climate. The formation of marine fog, for example, can be sensitive to small changes in SST at a scale of a few kilometres. As a contribution to understanding processes at the interface between air and sea, this article discusses results from a state‐of‐the‐art fully coupled regional atmosphere–land–ocean–wave prediction system for the UK at km scale. This study focuses on the impact of the changes in surface forcing resulting from coupling SST in the marine boundary layer and formation of summertime coastal fog over the North Sea.A study from July 2013 provided a good case to evaluate the role of SST in fog evolution. The benefit of an evolving SST in the coupled simulation is shown in capturing a warming trend in observed SST over the five‐day case study period, with a root‐mean‐square error (RMSE) against in situ observations of 1.1 K. In contrast, in uncoupled atmosphere‐only simulations, the initial‐condition SST is persisted for the duration of the case, as is more typical in current operational numerical weather prediction (NWP). In the uncoupled simulations, a cold bias develops over the modelling period and the RMSE against observed SST is 2.4 K.The impact of coupling is shown to propagate into the overlying marine boundary layer and therefore affect the formation of coastal fog. Increased heat flux from a relatively warmer sea surface in the coupled simulations led to near‐surface atmospheric instability, hampering stratus lowering and destroying the fog‐promoting inversion layer. This significantly reduced fog fractions in selected regions. The value of model coupling was assessed by comparing coupled and uncoupled simulations initialized at different times ahead of fog development.