Fog and Tidal Current Connection at Cape Cod Canal—Early Recognition Alfred H
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Fog and Tidal Current Connection at Cape Cod Canal—Early Recognition Alfred H. Woodcock Department of Oceanography University of Hawaii and Recent Measurements' Honolulu, Hawaii 96822 Abstract Bay (in southern Massachusetts) 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 Buzzards Bay, 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(Emmons, the air temperature over the water at the head of Buzzards 1947), he sent me his notes summarizing the old, unpublished observations; these contained a suggested explanation of the cause of the Canal fogs.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. Bulletin American Meteorological Society 161 Unauthenticated | Downloaded 10/07/21 07:07 PM UTC 162 Vol. 63, No. 2, February 1982 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.