Ralph G. Eldridge mist—the transition The MITRE Corporation tram haze to log Bedford, Mass. Abstract Haze particles are defined as "fine dust or salt parti- The transition from haze to fog is manifest by the oc- cles which are so small that they cannot be felt or indi- currence of dissimilar optical phenomena which are a vidually seen with the naked eye, but they diminish hori- consequence of a change in the aerosol distribution of zontal visibility and give the atmosphere a characteristic the scattering medium. Based on these effects, visibilities opalescent appearance that subdues all colors." "Fog is between 0.5 and 1 km are properly called mist. Visi- easily distinguished from haze by its appreciable damp- bilities in excess of 1 km are, generally, characteristic ness and gray color. Mist may be considered as inter- of haze; whereas, fog is limited to visibilities less than mediate between fog and haze. ." Its particles con- 0.5.km. sist "of an aggregate of microscopic and more-or-less 1. Introduction hygroscopic water droplets suspended in the atmo- sphere." Mist "produces, generally, a thin grayish veil Studies of atmospheric scattering have been concerned over the landscape," and "reduces visibility to a lesser with atmospheres of haze having relatively good visibili- ties, or with atmospheres of fog or cloud characterized extent than fog. The relative humidity with mist is often by poor visibilities. "According to international defini- less than 95 per cent." tion, fog reduces visibility below one kilometer" The definition of mist as being composed of an aggre- (Huschke, 1959); therefore, visibilities in haze must be gate of microscopic and hygroscopic water droplets, and greater than one kilometer. Although this definition of that mist is intermediate between haze and fog infers a fog is quite sufficient for operational and synoptic me- distinction between haze and fog based on the defini- teorology, it does not include the transitional state be- tion of their particle size. Haze droplets are "any small tween haze and fog which is of interest to optical and liquid droplets contributing to an atmospheric haze con- aerosol studies. Therefore, several optical phenomena dition. Haze droplets comprise the transition state be- and aerosol distribution characteristics are reviewed and tween condensation nuclei and cloud drops (or fog correlated to define the visibilities associated with the drops). The size of haze droplets . are the order of a transition of haze to fog; that is the visibility regime of few tenths of a micron diameter. The larger haze drop- mist. This study also confirms the generally accepted lets may be termed 'mist droplets.' " Therefore, the visibility regimes of haze and fog. definition of haze particles as consisting of "fine dust 2. Meteorological definitions or salt particles" should be expanded to include sub- micron liquid droplets and condensation nuclei. The definitions of haze, fog, and mist which follow are Two distinguishing characteristics of haze and fog extracted from the GLOSSARY OF METEOROLOGY (Huschke, become evident from their definitions. First, a chromatic 1959) to provide a common basis for discussion. scene viewed through haze tends to become subdued; The terms "fog droplet" and "cloud droplet" will be used interchangeably to describe "a particle of liquid whereas, the same scene is rendered to shades of gray water from a few microns to tens of microns diameter, when viewed through fog. Secondly, haze is composed formed by condensation of atmospheric water vapor." of a heterogeneous mixture of particles which are pre- This usage is justifiable when it is considered that a fog dominately submicron in size, while fog is composed droplet is "physically the same as a cloud droplet." Fur- of water droplets a few microns to tens of microns in thermore, "fog differs from cloud only in that the base diameter. Therefore, a scene viewed through mist is of fog is at the Earth's surface while clouds are above somewhat subdued in color and the mist aerosol distri- the surface." bution is a haze aerosol distribution modified by the growth of some aerosols by the condensation of water 1 The research reported was sponsored by the Electronic Systems Division, Air Force Systems Command, under Con- vapor in an environmental relative humidity in excess tract No. AF19(628)-5165. of 95%. 422 Vol 50, No. 6, June 1969 Unauthenticated | Downloaded 10/07/21 04:01 AM UTC Bulletin American Meteorological Society 3. The distinction between haze and fog in the "selectivity" of the relative attenuation during Foitzik (1938), Bullrich (1960), and Goes (1963, 1964) the transition from haze to fog. have reported significantly different scattering proper- Commenting on Foitzik's results, Middleton (1952) ties of haze and fog. Neiburger and Chien (1960) and states that "the most striking feature of the diagram is Bullrich (1963) have shown that a haze aerosol distri- the sudden change in selectivity near <j — 4 km-1. This bution may be transformed into a fog drop-size distribu- is the change from haze to fog, the point where hygro- tion by the condensation of water vapor on active nu- scopic nuclei may be supposed to start increasing in clei. The occurrence of these changes is sufficient to radius in an unstable manner." Further inspection of distinguish haze from fog; and the visibilities at which the shape of the curves indicates a regular change in the these changes occur indicate the limiting visibilities of relative attenuation with a decrease in visibility to <r = each medium. The visibilities associated with the transi- 3.3 km-1. Beyond <r = 6.0 km-1, all three curves become tion from haze to fog define the visibility regime of mist. constant with further decreases in visibility. Therefore, Optical measurements through haze and, jog. The dis- the scattering coefficients 3.3 and 6.0 km-1 bracket the tinction between haze and fog, based upon observations transition from haze to fog, which, by definition, is mist. of the decrease of the chromatic character of a land- More recently, Goes (1963) made simultaneous mea- scape, is dependent upon individual judgments. This surements of the transmissivity of the atmosphere at subjective criteria has led to a diversity of opinions over two wavelengths to determine the dependence of the the years (see Middleton, 1952) regarding the interpreta- near infrared transmission upon visibility. Continuous tion of observational data. Much of the disagreement measurements of transmission through haze and fog was caused by an inadequate definition of haze or fog were made at wavelengths of 0.55 and 0.85 microns. based on the observed visibility. It was not until Foitzik The following year Goes (1964) made transmissivity (1938) showed that it was possible for haze and fog to measurements at 0.55 and 1.2-ju wavelength. Goes plot- have similar visibilities that an objective distinction be- ted the infrared attenuation coefficients for wavelengths tween haze and fog was demonstrated. of 0.85 and 1.2 ju as a function of the visible attenua- Foitzik (1938) made measurements of the relative at- tion coefficient in Fig. 4 (1963) and 2 (1964), respectively. tenuation of blue, green, and red lights in dense haze Curves representing the average value of these relation- and fog over a considerable period of time. His data ships were drawn through Goes' data and are presented provided an objective distinction between haze and fog as Fig. 2. It may be seen that the transitions between based on the "selective" attenuation of blue and red haze and fog, indicated by arrows, occur at visible atten- lights. For example, the attenuation of the blue and red uation coefficients of 3.5 and 5.6 km-1 for a wavelength lights varied about ±13% from the green light attenu- of 0.85 ju, and between 3.2 and 7.5 km-1 for a wavelength ation in dense haze; whereas, in fog the variation was of 1.2 ju. approximately ±2%. The relative attenuation of blue, green, and red lights as a function of the visible attenu- ation coefficient is summarized in Fig. 1 (after Foitzik, 1938). It may be noted that there is a significant change FIG. 1. The relative attenuation of red, green, and blue lights as a function of the mean attenuation coefficient (km-1) measured with a visual telephotometer (Foitzik, 1938). The two arrows indicate the mean attenuation coefficients at FIG. 2. The attenuation of near infrared radiation (X =z 0.85 which a change in the trend of the colored lights relative and 1.2 ju) as a function of the attenuation of visible light attenuation occurs. (X = 0.55) after Goes (1963, 1964). 423 Unauthenticated | Downloaded 10/07/21 04:01 AM UTC Vol 50, No. 6,, June 1969 Bullrich (1960) made numerous light scattering mea- surements of the relative intensity and polarization as a function of the scattering angle in both haze and fog. He found a small spectral dependence in the scattering function in haze, but none could be found in fog. He also showed that there is a considerable difference be- tween the polarization functions for haze and fog. Bull- rich concluded that the polarization function is a more sensitive criterion for the distinction of haze from fog than either the attenuation or scattering functions. Because Bullrich (1963) had not observed the change of the polarization function, which is quite sensitive to changes in aerosol and drop-size distributions during the transition from haze to fog, he undertook a study to de- termine what changes in the scattering medium are necessary to simulate the observed optical phenomena. He found that a Junge (1955) haze model aerosol dis- tribution proved satisfactory for reproduction of scat- tering and polarization functions appropriate for haze; however, it was necessary to fabricate a bimodal drop- size distribution in order to reproduce the fog bows which strongly polarize the scattered light.