Mist—The Transition Tram Haze To

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 atmosphere." 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 visibilities, 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 submicron 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%.

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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

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).

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Unauthenticated | Downloaded 10/07/21 04:01 AM UTC Vol 50, No. 6,, June 1969 Bullrich (1960) made numerous light scattering measurements 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 between 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 distribution proved satisfactory for reproduction of scattering 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. Bullrich plotted the wavelength exponent of the scattering effi- ciency function as a function of the visibility. The exponent is unity for visibilities in excess of 0.9 km and FIG. 3. Aerosol distributions as a function of lapse time decreases in a regular manner with the decrease in visi- after the initiation of the condensation process. The times, bility for visibilities less than 0.48 km. Therefore, it may in minutes, are indicated at the base of each curve after Nei- be assumed that the transition from haze to fog occurs burger and Chien (1960). between 0.9- and 0.48-km visibility. If Koschmieder's formula (1924) is used, then the transition occurs between attenuation coefficients of 4.4 and 8.2 km-1. an environment of a few tenths of a per cent supersaturation. Haze and fog aerosol distributions. Middleton (1952) The time dependent aerosol distributions shown in commented that "in all of Foitzik's observations of true Fig. 3 are the average of Neiburger and Chien's Stratus fog, the red is slightly more extinguished than the blue. A and B distributions. This figure illustrates how a haze It is difficult to believe that droplets in such dense fog aerosol distribution changes to a cloud (or fog) drop-size would be of the order of size (diameters of 2-3 microns) distribution as a result of droplet growth by condensa- which would produce this effect." Yet, Bullrich (1963) tion. Between 40 and 45 min after the initiation of the found it necessary to postulate a bimodal drop-size dis- growth process, there is a substantial increase in the tribution with the secondary modal maximum at 4-/jl concentration of 1- and 2-fi radius droplets which cause radius in order to reproduce the observed scattering a rapid deterioration of the visibility. After a lapse of functions with Mie theory. The average location of the about 48 min, the distribution begins to assume the secondary modal maximums published by Eldridge bimodal character indicative of fog. Inspection of Nei- (1966) is 4.5-//, radius, and all 18 are located between 3.5- burger and Chien's Figs. 7 and 11, which relates the and 6.0-//, radius. Therefore, fog drop-size distributions visibility to the time after the initiation of the droplet may be distinguished from haze aerosol distributions by growth process, shows that the most rapid decrease in the existence of a prominent secondary modal maximum visibility occurs when the visibility is between 0.9 and between 3- and 6-/JL radius. 0.4 km. Using Koschmieder's relationship (1924), the The paucity of observed visibilities between approxi- resulting attenuation coefficients are 4.3 and 9.8 km"1, mately 0.5 and 1.5 km, as well as studies of cloud drop- respectively. let growth by condensation, suggests that the transition from haze to fog occurs rather rapidly. Neiburger and 4. Conclusions Chien (1960) computed drop-size distributions as a func- From the experiments of Foitzik (1938), Bullrich (1960), tion of time after the initiation of the condensation pro- and Goes (1963, 1964), it is concluded that there is a cess. Their study indicated that droplet growth proceeds significant difference in the manner in which light is at a nearly regular rate as the relative humidity ap- scattered by haze as compared to fog. Based on the proaches saturation. As saturation and supersaturation studies of Neiburger and Chien (1960) and Bullrich occurs, the growth process is accelerated. Thereafter, (1963), this difference is a consequence of the change of the droplet growth continues in a regular manner in the scattering medium aerosol size distribution; that is,

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Unauthenticated | Downloaded 10/07/21 04:01 AM UTC Bulletin American Meteorological Society from a monomodal distribution of the type evolved by with visual ranges less than 1 km) as special cases. . . ." Junge (1955) to a bimodal distribution of the type in- He also commented on the correlation of the variations ferred by Eldridge (1966). The transition from a mono- of the selectivity of red and blue lights with visibility modal haze distribution to a bimodal fog distribution changes (Foitzik, 1938) and states that "it is regrettable occurs rather rapidly in a continuous manner. that a simultaneous record of the relative humidity is If the environmental relative humidity is favorable not available, as it is difficult to escape the conclusion for droplet growth by condensation, then the visibility that changes in the radii of haze particles must have regime of mist may be defined in terms of the attenua- been taking place." Therefore, it would appear that tion coefficients for the transition from haze to fog as the occurrences of special situations, that is, unusual indicated by the above studies. These values are sum- visibilities in haze or fog, is primarily dependent upon marized in Table 1; O-MLN is the average attenuation CO- insufficient environmental relative humidity. An increase in the concentration of haze particles in an un- TABLE 1. Transition attenuation coefficients. saturated environment results in a decrease in the visibility. In the case of fog, an unsaturated environment Reference Omin(km-1) ffmai (km causes the smaller droplets to evaporate resulting in an increase in visibility without destroying the bimodal Foitzik (1938) 3.3 6.0 Goes (1963, 1964) 3.4 6.5 character of the drop-size distribution (see Fig. 7, Bullrich (1963) 4.4 8.2 Eldridge, 1966). Neiburger & Chien (1960) 4.3 9.8 6. Discussion of applications Average 3.85 7.65 Standard deviation 0.50 1.48 Although the visibility regime of mist exists for a relatively short period of time, the transition from haze to fog is of interest in fog and cloud formation studies. efficient at which condensation begins, and owx is the The occurrence of mist also is a necessary parameter attenuation coefficient at which the aerosol distribution in thermal transfer calculations based on visibility be- becomes bimodal. The application of Koschmieder's cause it indicates a significant change in the aerosol formula (1924) indicates that the transition from haze distribution of the scattering medium which affects the to fog occurs between visibilities of 0.51 ± 0.10 and disposition of the scattered energy. 1.02 ±0.14 km. Therefore, the scattering medium is haze This review of the transmission properties of haze when the visibility is greater than one kilometer, and and fog also suggests a modification to runway trans- fog when the visibility is less than a half kilometer. missometers which will allow automatic determinations Because mist is intermediate between haze and fog, of visibility in haze. Runway transmissometers have visibilities between 0.5 and 1 km define the visibility been installed at many airports; however, these instru- regime of mist. ments have proven unsatisfactory in good (haze) conditions. Foitzik (1938) has shown that the ratio of red 5. Discussion of exceptions and blue light attenuations are related to visibility. Middleton (1952) has shown that the "selective" nature While the visibility regimes of haze, mist, and fog ap- of haze extends to the Rayleigh limit; therefore, by using ply for the majority of atmospheric conditions, there the ratio of the red and blue attenuations in haze, are occasions when the environment is not susceptible visibilities in haze may be determined over reasonable to droplet growth. For example, Foitzik (1938) made transmissometer path lengths. When mist occurs, the colored light transmission measurements in a medium ratio becomes unity; with the beginning of fog, the having a visibility of 0.34 km which exhibited the transmission measurement becomes a reliable indica- strong selectivity characteristic of haze. Other examples tion of visibility. By combining the "selective" attenua- of optical measurements in scattering media, properly tion of blue and red rights in haze with the normal described as haze, with visibilities less than 1 km are attenuation measurements in fog, a fully automatic visi- reported by Bullrich (1960). Conversely, fogs with visibility observation becomes possible. bilities greater than 0.5 km, as might be expected in a dissipating fog (Eldridge, 1966), have exhibited the ap- References propriate scattering phenomena of fog (Bullrich, 1960). Therefore, lacking sufficient water vapor, active nuclei, Bullrich, K., 1960: Streulichtmessungen in Dunst und Nebel. etc., haze may have visibilities less than 1 km. Also, dis- Meteor. Rund., 13, 1-9. sipating fog may have visibilities greater than 0.5 km , 1963: Der Beginn der Nebelbildung und seine optische (but probably less than 1 km) when the environment Auswirkung. Z. /. angew. Mathem & Phys., 14, 434-441. becomes unsaturated. Eldridge, R. G., 1966: Haze and fog aerosol distributions. J. With regards to the exceptions to the general case, Atmos. Sci., 23, 605-613. Middleton (1952) has commented "that we should treat Foitzikgetrubte, L.n, 1938Atmosphar: Tiber e diei mLichtdurchlassigkei sichtbaren Spektralbereicht der stark. occasions of dense haze (selective haze or "blue fog" Wiss. Abh. Reichsamt f. Wetterdienst, Berlin, 4, No. 5.

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Unauthenticated | Downloaded 10/07/21 04:01 AM UTC Vol 50, No. 6,, June 1969 Goes, O. W., 1963: Registrierung der Durchlassigkeit in Koschmieder, H., 1924: Theorie der horizontalen Sichtweite. verschiedenen Spektralbereichen in der Atmosphare. Beitr, Beitr. Phys. freien Atmos., 12, 33-53. zur Phys. der Atmos., 36, 127-147. Middleton, W. E. K., 1952: Vision Through the Atmosphere. , 1964: Registrierung der Durchlassigkeit in ver- Toronto, University of Toronto Press, 246 pp. schiedenen Spektralbereichen in der Atmosphare. Beitr, zur Phys. der Atmos., 37, 119-131. Neiburger, M., and C. W. Chien, 1960: Computations of the Huschke, R. E., 1959: Glossary of Meteorology. Boston, Amer. growth of cloud drops by condensation using an electronic Meteor. Soc., 638 pp. digital computer. Physics of Precipitation, Geophys. Junge, C., 1955: The size distribution and aging of natural Monogr. No. 5, Washington, D. C., Amer. Geophys. Union aerosols as determined from electrical and optical data 191-210. of the atmosphere. /. Meteor., 12, 13-25.

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