The scientific community that includes meteorologists, physical scientists, engineers, medical doctors, biologists, and environmentalists has shown interest in a better understanding of fog for years because of its effects on, directly or indirectly, the daily life of human beings. The total economic losses associated with the impact of the presence of fog on aviation, marine and land transportation can be comparable to those of tornadoes or, in some cases, winter storms and hurricanes. The number of articles including the word ``fog'' in Journals of American Meteorological Society alone was found to be about 4700, indicating that there is substantial interest in this subject. In spite of this extensive body of work, our ability to accurately forecast/nowcast fog remains limited due to our incomplete understanding of the fog processes over various time and space scales. Fog processes involve droplet microphysics, aerosol chemistry, radiation, turbulence, large/small-scale dynamics, and surface conditions (e.g., partaining to the presence of ice, snow, liquid, plants, and various types of soil). This review paper summarizes past achievements related to the understanding of fog formation, development and decay, and in this respect, the analysis of observations and the development of forecasting models and remote sensing methods are discussed in detail. Finally, future perspectives for fog-related research are highlighted.

Fog Research: A Review of Past Achievements and Future Perspectives

Studies of a numerical model of fog are presented. These indicate that: 

(i)
For given initial conditions, a reduction in turbulent diffusion leads to earlier and thicker fog formation.

(ii)
The formation and growth of fog on a realistic time scale is simulated by the inclusion of radiative cooling due to water vapour and carbon dioxide and (when formed) fog droplets.

(iii)
The inclusion of gravitational settling is necessary to simulate realistic liquid water contents.

(iv)
The nature of the underlying surface (through its thermal properties) influences the time of onset of fog.





The effect of cloud cover and of solar insolation on dispersing fog was assessed.



These results are broadly consistent with, and help to account for, the results of a parallel field study by Roach, Brown, Caughey, Garland and Readings (1976).

The physics of radiation fog: II – a numerical study

In this study, a dense fog episode that occurred near Windsor, Ontario, Canada, on 3 September 1999 is investigated. The fog patch, with a spatial scale of several kilometers, reduced visibility on a major highway to a few meters and led to a series of collisions and loss of life. Satellite imagery and surface observations are used to analyze the physics of the event, and several hypotheses on the origin of the fog are presented. A series of simulations of the event with the fifth-generation Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model (MM5) suggest that the fog formed because of convergence of land breezes developing along the shores of a lake and subsequent advection of moisture over the site of the accident. Tests indicate that the small scale of the modeled event contributes to sensitivity of the results to a broad range of factors. Sensitivity to the initial and boundary conditions, including initial soil moisture content and parameterization of turbulence, is discussed.

https://journals.ametsoc.org/doi/pdf/10.1175/1520-0450%282004%29043%3C0003%3AAAMOAE%3E2.0.CO%3B2

Analysis and Modeling of an Extremely Dense Fog Event in Southern Ontario

Fog can adversely affect human activity directly and indirectly, resulting in large losses both in terms of the local economy and lives. Much effort has been devoted to studies of fog across many areas of China, and in that context this paper aims to summarize climatic characteristics and review fog field experiments and their major results relating to fog mechanisms, physical properties and chemical characteristics. Progress in the application of remote sensing techniques and numerical simulation in fog research are also discussed. In particular, the effects of urbanization and industrialization on fog are highlighted. To end, perspectives on future fog research are outlined. The goal of this review paper is to introduce fog research in China to the global academic community and thus promote international collaboration on fog research. This is important because most papers on fog in China are published in Chinese, which are unreadable for the vast majority of non-Chinese researchers.

Fog research in China: An overview

A Lagrangian model has been developed to study acidic deposition due to radiation fog. The model couples submodels describing the development and dissipation of radiation fog, the gas-phase chemistry and transfer, and the aqueous-phase chemistry. The model is applied to a radiation fog episode in Bakersfield in the San Joaquin Valley of California over the period January 4–5, 1985. Model predictions for temperature profile, fog development, liquid water content, gas-phase concentrations of SO_2, HNO_3, and NH_3, pH, aqueous-phase concentrations of SO_4^(2−), NH_4^+, and NO_3^−, and finally deposition rates of the above ions are compared with the observed values. The deposition rates of the major ions are predicted to increase significantly during the fog episode, the most notable being the increase of sulfate deposition. Pathways for sulfate production that are of secondary importance in a cloud environment may become significant in a fog. Expressing the mean droplet settling velocity as a function of liquid water content is found to be quite influential in the model's predictions.

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Mathematical modeling of acid deposition due to radiation fog

The system of physical equations describing temperature changes near the ground in fog-free air as well as in radiation fog is solved numerically. The variation of the exchange coefficient with height is taken into account using different models while time variations are still disregarded. Temperature changes due to latent heat effects are incorporated in this study. Moreover, the presence of radiative flux divergence is included in an approximate manner. The solution of the problem is presented in terms of graphs showing the development of temperature and water droplet profiles as function of time and height. Computed liquid water content as well as temperature profiles are in general agreement with observations while the vertical growth of fog usually proceeds too rapidly. Concrete suggestions are given of how to improve the model.

A preliminary prediction analysis of radiation fog

: This study deals with the prediction and maintenance of radiation fog. In conjunction with this study, two theoretical problems are discussed which form the basis for the prediction of radiation fog, its growth and eventually its dispersal by clouds or solar radiation. The first part of this investigation concerns itself with the prediction of temperature and wind profiles. The theory of radiative transfer for nonscattering particles and absorbing gases is included in the differential equations describing the physics of the boundary layer. The second problem discusses the mathematical theory of the distribution of short and long wave radiation in a multiple scattering and absorbing cloud or fog consisting of droplets and gases. A new solution of the equation of radiative transfer theory as formulated by Chandrasekhar (1960) is presented here. No attempt has been made at this time to include the complete transfer theory in the prediction equations. (Author)

Barry C. Nielsen

Papers

A preliminary prediction analysis of radiation fog

Prediction and maintenance of radiation fog.