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Caren Marzban, Stephen Leyton, and Brad Colman

1. Introduction Coastal locations around the United States are susceptible to marine stratus and low ceilings. Occasionally, the stratus extends to the surface, resulting in dense fog conditions and a significant reduction in horizontal visibility. When these adverse conditions occur, they often disrupt daily activities, such as aviation and shipping interests, sometimes to the point of jeopardizing human safety. It is imperative, then, to provide forecasters with the most accurate guidance for

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Qiang Huang, John Hanesiak, Sergiy Savelyev, Tim Papakyriakou, and Peter A. Taylor

1. Introduction Blowing snow is a common weather phenomenon in the Canadian Arctic and prairies. It occurs when the wind is strong enough to raise the snow particles to sufficient heights above ground that horizontal visibility (meteorological optical range, MOR; also described as Vis thereafter) is reduced to 9.7 km (6 mi) or less ( Atmospheric Environment Service 1977 ). It can be a hazard to public safety and transportation since the visibility can be significantly reduced in some blowing

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Richard M. Chmielecki and Adrian E. Raftery

1. Introduction The accurate prediction of visibility even over short-term (0–6 h) forecasting periods is challenging. Since most numerical weather prediction models do not explicitly model visibility, forecasts of visibility must first be derived from other meteorological parameters such as cloud water content, relative humidity, and precipitation. There are a handful of different specifications of visibility as a function of other weather parameters ( Knapp 1999 ; Doran et al. 1999

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Bjarne Hansen

airports. The Meteorological Service of Canada (MSC) is responsible for providing accurate and timely weather forecasts for 190 airports across Canada. The forecasts describe weather conditions expected to affect flight conditions for up to the next 24 h. These conditions include cloud ceiling height, horizontal visibility, precipitation, and wind direction and speed. Forecasters work to keep these forecasts as accurate and current as possible, and will quickly revise forecasts as appropriate for the

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I. Gultepe and J. A. Milbrandt

1. Introduction Visibility (Vis) parameterizations as a function of relative humidity with respect to water (RH w ) and precipitation rates (PR) are important for many weather forecast–related applications, including aviation, transportation, and search and rescue operations ( Gultepe et al. 2007 ; Tardif 2007 ). In past applications, Vis parameterizations have been used in various models that include numerical weather prediction (NWP) models ( Stoelinga and Warner 1999 ) and fog models

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Zhongping Lee and Shaoling Shang

observed and whether an object is visible or not impacts decision-making and management. Subsequently, the term “visibility” has been used to provide a quantitative representation of this information throughout the past decades. However, there is no unified definition of visibility, although it is usually referred to as the distance of “an object will be just visible ” ( Duntley 1948b , p. 237; Malm et al. 1980 ; Middleton 1947 ), as adopted by the International Civil Aviation Authority (ICAA). On

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Gary L. Achtemeier

1. Introduction Dense fog reducing visibility to a few meters has been implicated as a causal factor of multiple-vehicle accidents in all sections of the United States and parts of Canada ( Pagowski et al. 2004 ). An additional factor contributing to the frequency of extreme fog events is the combination of fog with smoke from prescribed burns ( Achtemeier et al. 1998 ). Land managers in the southern United States (states from Virginia to Texas and from the Ohio River southward) use prescribed

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William R. Ryerson and Joshua P. Hacker

1. Introduction Reductions to visibility between the ranges 1 and 7 mi (1 mi = 1.6 km) due to fog are a significant safety concern for many aviation operations. Accurate visibility predictions in this range, hereafter termed light fog, are critical because they dictate restrictions on certain aircraft types and equipment, pilot level of experience, etc. Remote and sparsely observed regions, often of primary interest to the U.S. Air Force, provide a particularly challenging visibility prediction

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R. Boers, H. Klein Baltink, H. J. Hemink, F. C. Bosveld, and M. Moerman

applications. Overlying thin cirrus clouds may additionally obstruct accurate fog detection. Of course, the hazard for traffic is not so much fog itself but its consequence, namely, reduced visibility. Visibility is the end result of a complex interplay between aerosol microphysics, chemistry, radiation cooling, and particle activation ( Elias et al. 2009 ). Before particle activation occurs, visible extinction is not so large, and range variations in visibility can be observed remotely by lidar. However

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I. Gultepe, M. D. Müller, and Z. Boybeyi

1. Introduction Fog formation is directly related to thermodynamical, dynamical, radiative, aerosol, and microphysical processes and to surface conditions. Extinction of light at visible ranges within the fog results in low visibilities that can affect low-level flight conditions, marine traveling, shipping, and transportation. Fog occurrence more than 10% of time in some regions of Canada ( Whiffen 2001 ) demands that fog nowcasting and/or forecasting models should be improved. In particular

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