Fog and Tidal Current Connection at —Early Recognition Alfred H. Woodcock Department of Oceanography University of Hawaii and Recent Measurements' Honolulu, Hawaii 96822

Abstract Bay (in southern ) on the afternoon of 29 June 1946. The weather on that day was characterized by a south- Notes by Gardner Emmons about the initiation of low advective westerly flow of moist tropical air, with intermittent fog over fogs on Cape Cod are presented. Subsequent measurements made in the relatively cool offshore waters bordering the southern these fogs confirm his suggestion that mixing and temperature New England coast. However, in traveling northeastward changes associated with tidal currents account for the fog. Puzzling temperature measurements that are at apparent variance with the toward the head of , the fog was largely evapo- mixing theory of fog formation are presented. It is proposed that rated by a slight heating of the air over the water of the bay, these temperature discrepancies are due to the effects of water vapor which at this season is normally somewhat warmer than the condensation on the sea water surface. offshore waters. At 1400 EST (75th meridian civil time), when the phe- nomenon to be described was first noticed, I was situated 1. Introduction on Tobey Island, near the village of Monument Beach (see Fig. 1). At this time, the following conditions, determined Years ago, Gardner Emmons made visual observations of from visual observation, prevailed northwest of Wings Neck the apparent inception of advective fog over Cape Cod Canal at the approach to Cape Cod Canal (CC Canal): wind, SW, near his home at Monument Beach, Mass. His observations Beaufort 3-4; visibility to the west, north, and east, 4-9 n mi; were never published. hazy; patches of thin stratus ~100 m overhead. No meas- During recent correspondence about his published work urements of air and water temperature were available; but on the lower atmosphere over the sea and fog (Emmons, the air temperature over the water at the head of Buzzards 1947), he sent me his notes summarizing the old, unpublished observations; these contained a suggested explanation of the cause of the Canal fogs. His closing remark was, "You are welcome to use it [the summary], or refer to it, in any way you may wish." Emmons' notes about the Canal fogs, and my recent meas- urements made as a result of his notes, are interesting and useful. They reveal a locale where, by chance, air and water masses associated with a unique natural and man-made con- figuration of land and sea occasionally interact to perform "experiments" in cloud physics. Other workers concerned with marine fog problems may also find this site and these natural experiments useful. Locally these low canal fogs are called "vapor" in order to differentiate them from the deeper advective fogs originating elsewhere. Emmons' 1946 notes about the fogs are presented now, fol- lowed by my recent measurements supporting his explana- tion of the mechanics of their formation.

2. Emmons: Monument Beach—June 1946

A visible demonstration of the initial stages of production of a so-called advection fog took place at the head of Buzzards

FIG. 1. Chart of Buzzards Bay entrance to the Cape Cod Canal, !Hawaii Institute of Geophysics Contribution No. 1212. showing the locations of the canal fog observation sites referred to by Emmons and me. Note that Mashnee and Hog Islands are now 0003-0007/82/020161 -06$05.50 connected (by landfill) to the mainland near Monument Beach, ® 1982 American Meteorological Society Mass.

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Bay was estimated to have been 18-20°C. The water temper- cided roughly with a line connecting the south shore of ature off Wings Neck probably was within this range also. Mashnee Island and Stony Point (East Wareham). The cen- Looking westward from my observation point on Tobey ter of the whitish zone was over the ship channel that leads Island, I noticed that the surface of the water in the area northeastward into the CC Canal proper. Occasionally, where the CC Canal opens into Buzzards Bay appeared to be against the dark background of the land beyond, thin wisps covered by a dense white mist. The depth of the mist could of mist could be seen thrusting above the general level of the not be estimated accurately, since the area in which it was lo- mist blanket. cated was km or so from my position, but it did not seem This was the appearance of the phenomenon looking to be more than ~1 m. The southern edge of the area coin- westward to the left of Mashnee Island. The trees on Mash-

FIG. 2. Example of fog over canal near the railroad bridge (Fig. 1). Horizontal visibility at camera level ~3.7 km, and —0.15 km near the canal. (Photograph by Ron Schloerb.) Unauthenticated | Downloaded 10/07/21 07:07 PM UTC Bulletin American Meteorological Society 163 nee Island obscured the waters of the Canal behind it, so that 3. Woodcock: Buzzards Bay—June 1980 and 1981 the change in the character of the mist layer in the downwind direction could not be viewed continuously. However, an I visited the Cape Cod Canal prepared to make the meas- unobstructed view of the canal waters between Mashnee Is- urements needed to test Emmons* explanation of the low land and Hog Island (north of Mashnee) could be had, and fogs over the canal and channel waters near the head of Buz- here it was seen that the white blanket over the water had de- zards Bay. During the visits, I encountered several fogs over veloped into a well-defined band or ribbon of fog 6-9 m in the canal at the Hog Island site (Fig. 1, site B), and at the height, which was almost level with the treetops on the East nearby U.S. Army Corps of Engineers Headquarters (site A), Wareham shore, and which completely obscured objects just west of the railroad bridge. Figure 2 shows an example of below that height. This vertical growth evidently took place a fog over the canal between the highway and railroad within a distance of almost 0.6 km. The fog kept drifting with bridges. the wind in a band along the axis of the dredged channel, and Emmons' explanation of the mechanics of the canal fog beyond Hog Island, until it was carried ashore and then dis- formation suggested that they occur over the canal waters sipated near the village of Buzzards Bay. near the end of the west-flowing ebb tide, when the coldest The phenomenon persisted for approximately one hour. It water was present. A further requirement was that the air be is unquestionably significant that the interval of its existence nearly saturated and at a temperature many degrees above coincided with the last hour of the west-going ebb current, that of the sea water. which flows through the CC Canal into Buzzards Bay. At All of the measurements made during local fogs at site A that stage of the tide, the water temperature in the Buzzards and B (Fig. 1) showed the presence of warm moist air over the Bay end of the Canal undoubtedly has a minimum, for the cold waters at the end of the ebbing tide. A total of nine low water has been flowing from the very cold reservoir in Cape fogs were seen during visits in 1980 and 1981. Thus Emmons' Cod Bay (CC Bay) for a maximum length of time. When the prediction that the local advective fog formation is asso- ebb current ceases, the cold water that has been introduced ciated with the cold-water tidal period is well supported. into the head of Buzzards Bay loses its identity at the surface Among these measurements only two series are presented by sinking beneath the warmer water of the bay, or else flows here: the first to illustrate the overall relationship of the fog back through the Canal toward CC Bay with the beginning to the presence of the cold-water stage of the ebbing tide; and of the flood current. the second to give details of the conditions associated with Although confirming evidence is not available, due to a the onset of fog, using a temperature and mixing ratio (Tand complete lack of water temperature measurements in Buz- u>) diagram. zards Bay, CC Canal, and CC Bay on 29 June 1946, it is be- lieved that this was a case of fog formed in the air that in- a. Fog and the cold-water stage of the ebbing tide itially was nearly at saturation and that moved over considerably colder water. (Scattered observations in the Table 1 shows observations made before, during, and after a western part of CC Bay on 25 June indicated a surface water canal fog. Figure 3 reveals a portion of these data more temperature of about 16°C.) The rapidity of the formation of clearly. Note in Table 1 and Fig. 3 that the low fog observed the fog suggests that vertical mixing of parcels of air, modi- was confined to the cold-water part of the tidal cycle, when fied at the surface of the cold water with warmer air a few the air temperature (Td) and dewpoint temperature (7DP) meters up, played an important role in its development. were the same.

TABLE 1. Data from observations made on a pier over the canal, June 1980, at site A (Fig. 1). All air temperatures taken at ~4 m height; surface water temperatures taken by dip bucket. Note that the low fog occurred when the air and dew point temperatures were the same, and the initial canal water temperatures were much lower. EST = 75th meridian civil time; Td = dry-bulb temperature; 7DP = dew point tempera- ture; Tcw = temperature, canal water. Thermometer error corrections applied; sensitivity 0.2°C.

Date Time Td 7DP Id-DP Wind force June EST °C °C °C and direction Tcw Td ~ Tc

25 1710 19.8 17.0 2.8 3 SW 19.6 0.2 26 0130 17.4 16.1 1.3 3 SW 10.0 7.4 26 0405 18.0 16.7 1.3 3 SW 14.5 3.5 26 0525 18.6 16.7 1.9 3 SW 17.8 0.8 26 0945 19.7 17.7 2.0 4 SW 19.2 0.5 26 1300 21.0 16.7 3.3 4 SW 10.8 10.2 26 1435 21.6 16.6 4.0 4 WSW 10.6 11.0 26 1545 22.0 16.9 5.1 3 SW 15.1 6.9 26 1630 21.2 18.5 2.7 4 SW 17.4 3.8 26 2030 20.3 19.1 1.2 2 WSW 20.0 0.3 27 0220 17.5 17.5 0.0" 3 WSW 10.6 6.9 27 0340 17.2 17.2 0.0 fog 2 WSW 12.5 4.7 27 0510 17.8 17.8 0.0 2 WSW 16.7 1.1 27 1000 21.9 19.3 2.6 2 WSW 20.0 1.9 27 1045 21.4 19.3 2.1 3 WSW 19.6 1.8 27 1140 21.6 19.2 2.4 3 SW 18.8 2.8 27 1330 23.4 19.6 3.8 3 SW 11.3 12.1 27 1525 23.8 19.7 4.1 2 SW 10.7 13.1

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FIG. 3. A graph of the Table 1 data from the 27 June 1980 fog period. The numerals beside the connected points represent air and dew point temperatures.

The excess of 7bp over water Tat 0220,27 June 1980 (Table second series of observations mentioned previously, as a 1), was enough to produce supersaturation (S) and to initiate further test of Emmons' explanation of the low fogs at the fog by mixing. This process is revealed more completely by a canal. second series of observations made before the arrival of the Table 2 presents these observations made over a period of coldest water, and during a fog development as the cold about two hours (1918-2113), on 25 June 1981, before and water arrived at site A. A dip-bucket was used to sample during the time when fog formation on the canal again coin- water for temperature measurements. As will be seen later, cided with the arrival of the coldest water (column 6) near the this is probably not a realistic way to determine the actual end of the west-going ebb tide. It is interesting, however, that surface water temperature. its formation did not appear to follow completely the pattern delineated by the T, w (Taylor) diagram, as will be seen later. b. Temperature (T) and mixing ratio (w) diagrams This diagram is used here to demonstrate that the canal fogs and supersaturation probably result from mixing of moist air masses of different A mechanism for producing S and fog, through the process temperatures. of mixing moist air masses having different temperatures, Figure 4 shows a Taylor diagram on which the curved line, has been explained in detail using 7"and w diagrams (Taylor, dividing the graph in two parts, represents saturation w over 1917, p. 247; Rodhe, 1962, p. 53). It is useful here to give an pure water as a function of air temperature Td at a surface air example of the application of this mixing theory, using the pressure of 103 mb. Above the curve, the air is supersaturated

TABLE 2. Observations made on a pier over canal, 25 June, 1981, at site A (see Fig. 1). All air temperature and wind speed measurements made at 4 m height, except that at 2050 h at 0.5 m. Note that the low fog began forming when the air and dew point temperatures were the same and the canal temperatures much lower. EST = 75th meridian civil time; Td = dry-bulb temperature; 7DP = dew point temperature; rcw = temperature canal water; w = mixing ratio of air. Thermometer error corrections applied; sensitivity 0.2°C.

1 2 3 4 5 6 7 8

Time Td 7DP w Td-D P Tew Td Tew Wind EST °C °C gkg"1 cc °C °C m/s SW

1918 18.9 18.6 13.69 0.3 13.9 5.0 4.0 1937 18.8 18.5 13.60 0.3 10.9 7.9 4.8 1953 19.2 18.9 13.95 0.3 10.1 9.1 2.9 2007 19.4 19.0 14.02 0.4 9.4 10.0 1.7 2038 19.4 19.4 14.40 0.011 9.1 10.3 3.8 2050 16.2 16.2 11.72 0.0 fog* 9.0 7.2 — 2113 19.6 19.6 14.58 O.OJ1 8.9 10.7 4.3 * Low fog began to form over canal at —2005, but did not build up to the 4 m level on the pier until 2012.

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FIG. 4. A T, w diagram illustrating the role of mixing of warm moist air over cold sea water in the formation of supersaturation (S) and fog over the canal at site A (Fig. 1), 1918-2038 h, 25 June 1981. See text.

(5); below it is unsaturated. When two air masses differing in water at that temperature. The diagram shows that the fog Tare mixed, S may result. We use as an example of supersat- should actually have formed between 1918 and 1937 h. uration by mixing the data of 2007 h, 25 June 1981 (see Table Following 2007 h, the fog deepened until, at 2030 h, it was 2). At this time, the fog was just beginning to form over the over 7 m high. During the whole time, I was only ~500 m well-illuminated canal, but had not yet reached the 4 m level. downwind from the south edge of the canal, where the fog The air temperature at 4 m was 19.4°C and w was 14.02 g kg-1, was first forming (see Fig. 1). The average age of the fog at while at the canal surface, the air was saturated over sea the observation site was therefore ~ 150 s (see winds, Table 2, water at 9.4°C. When these T and w values are plotted on column 8). Fig. 4 (see points marked 2007, at upper right and lower left), note that a straight line connecting them falls largely on the 4. Discussion upper S side of the saturation curve over pure water. This line represents all possible mixtures of the two 2007 air masses Thus, all of the relevant observations I made indicate that the (see proof by Taylor, 1917, p. 267). canal fogs are probably formed by the mixing process, as On the other hand, at 1918 h (Table 2), before the fog suggested by Emmons. However, as stated above, their initi- started forming, these Tand w values were 18.9°C and 13.69 g ation appears to require much higher temperature differ- kg-1, and a line joining them and the values for saturated air ences between the mixing air masses (i.e., differences between at the sea water temperature (points 1918, Fig. 4), falls en- air and water temperature), than the theory indicates. I sug- tirely on the unsaturated side of the saturation curve. Later, gest that this greater temperature difference is only apparent, at 2038 h (Table 2), when the fog had developed up to and and that actually the surface of the water (e.g., the upper few above the 4 m level, these 7" and u> values were 19.4°C and millimeters) that was in direct contact with the air was con- 14.40 g kg"1 at 4 m, and 9.1°C at the surface. When plotted on siderably warmer. Fig. 4, a straight line joining points 2038 falls almost entirely Unpublished experimental evidence has revealed that in on the S side of the saturation curve over pure water. low winds the average temperature of the upper few millime- Thus this series of observations also supports, in part at ters of a sea water surface can be several degrees C warmer least, the idea that the canal fogs result from supersaturation than the bulk water, when the bulk water is many degrees due to mixing. However, it is puzzling to note on Fig. 4 that below the dew-point temperature of the overlying air. This mixing lines connecting the Tand w values at the intervening warming of the surface layer is probably due to latent heat hours 1937 and 1953 (Table 2), also fall largely on the S side released by condensation of water vapor on the cold water of the pure water saturation curve. The reader can readily surface. demonstrate this on the figure using a straight edge. (These Before and during the early stage of fog formation, when lines have been deleted to avoid confusion.) Fog did not form, the largest w gradient occurs between the water and the over- however, despite the apparent attendent supersaturation of lying air, condensation should be most rapid. Condensation the air, until 2007 h, when the connecting mixing line (line A) would both warm and freshen the sea water surface, thus showed a maximum possible transient S of ~2.7%. Note, for markedly influencing the effective saturation w of the air in instance, the S or excess of w where the mixing line A inter- contact with this warmer fresher surface. It is perhaps not sects the 14.0°C point, above the saturation u> over pure surprising therefore that measurements of the temperature of

Unauthenticated | Downloaded 10/07/21 07:07 PM UTC 166 Vol. 63, No. 2, February 1982 the subsurface waters, sampled by dip bucket (see Table 2), The Buzzards Bay end of the Cape Cod Canal is a good fail to account for the time of formation of the fog on the area for the study of the physics of fog formation by mixing. canal.

Acknowledgments. I thank Gardner Emmons for making available to me his early stimulating and perceptive observations, and Duncan 5. Conclusions Blanchard for his review of the manuscript. The U.S. Army Corps of Engineers, New England Division, contributed to this study by Study of several fogs at the west end of the Cape Cod Canal allowing use of their facilities at strategic locations on CC Canal. indicate that Emmons was correct in his judgment that the Financial support by the Office of Naval Research is appreciated. low canal fogs are associated with the arrival of cold water during the final stages of the west-flowing ebb tide, and that References they result from mixing of moist air masses of different temperatures. Emmons, G., 1947: Vertical distributions of temperature and humid- A Taylor diagram is used to show that the fogs form by ity over the ocean between Nantucket and New Jersey. Pap. Phys. mixing, but at greater apparent initial temperature differ- Oceanogr. Meteorol., 10, 25 pp. ences between the mixing air masses than the theory requires. Rodhe, B., 1962: The effect of turbulence on fog formation. Tellus, This apparent excess temperature difference is probably due XIV, 49-86. to condensational warming of the upper few millimeters of Taylor, G. I., 1917: The formation of fog and mist. Quart. J. Roy. the water surface. Meteorol. Soc., XLIII, 241-268. •

announcements (continued from page 160) oil and gas, survey research and opinion polls, synthetic fibers, and X-rays for medical diagnosis. Each area is traced from its early content of such academic courses as biology, geography, and chemi- origins in basic research to its present advanced status. The report cal engineering. It also provides material for professional training in tells in narrative form about scientific research and describes how such subjects as architecture and town planning. Outside the aca- the results of such research affect and benefit society. demic framework, the journal should be helpful to the many institu- In a letter transmitting the report to President Reagan, NSB tions, field study centers, and teachers' centers where environmental Chairman Lewis M. Branscomb said, "We. ..believe that it is of issues are of primary importance. great importance for all Americans to appreciate how research, A large part of the new journal is devoted to an Information technological development and improvement in human welfare are section, which will assist those wishing to disseminate information inevitably and necessarily interrelated and intertwined." The report, about their activities. Typical issues will contain information on new he said, should help in achieving that understanding. legislation and policy developments; items from institutes, agencies, According to the 13th report, Science Indicators—1980, the Uni- and educational establishments on research projects, annual pro- ted States spends more money on research and development (R&D) grams, etc.; developments in educational technology; careers infor- and has more scientists and engineers engaged in those activities mation; book reviews and journal abstracts, and advance notice of than any other country except the Soviet Union. The report states, conferences and seminars. however, that other nations, particularly Japan and West Germany, The editors welcome the submission of papers for the journal and have greatly increased their technological capabilities. will supply instructions for authors on request. All papers are sub- The 368 page report is the fifth in a series devoted to the assess- mitted to referees. The editors of Information welcome news ment of U.S. science and technology through the analysis of quanti- releases, information and leaflets, journals for abstracting and tative indicators. In a letter transmitting the report to President books for review, and any other material pertinent to environmental Reagan, Lewis M. Branscomb, NSB Chairman, stated: "Through education. the Science Indicators reports, the Board hopes to contribute to a Environmental Education and Information is meant for all those broad understanding of the scientific and technological enterprise engaged in the practice, research, and teaching of environmental itself and of its impact on society." subjects, and should provide a cross-disciplinary medium for the Science Indicators—1980 reports that six out of every 1000 per- many specialists involved in environmental education. For further sons in the 1980 U.S. labor force worked as R&D scientists or information, write to Taylor & Francis Ltd., Account No. 04 810 engineers. This ratio is the largest of any nation except the Soviet 879, P.O. Box 9137, Church Street Station, New York, N. Y. 10049. Union. It also points out that the concentration of R&D scientists and engineers in the U.S. has declined from its high in 1968, and, although this ratio has increased slightly in the past few years, it has not regained its former level. In most other industrial countries, the ratio of R&D scientists and engineers in the labor force has increased NSB annual reports released steadily. Although the United States spends more on R&D than France, The 12th and 13th annual reports of the National Science Board Japan, and West Germany combined, the picture is somewhat differ- (NSB), the policy-making body of the National Science Foundation ent when the size of the economy is considered. In 1978, the U.S. (NSF), have recently been released. ratio of total R&D to Gross National Product (GNP) was 2.23%. The 12th report, titled Only One Science, examines six topics of This was higher than that of Japan (1.93%), but lower than that of current interest that represent a spectrum of scientific effort in the West Germany (2.37%). The R&D to GNP ratio peaked in the physical, biological, medical, and social sciences: computers and semiconductors, pesticides and pest control, seismic exploration for (icontinued on page 166)

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