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Fog: Its Causes and Forecasting with Special Reference to Eastern and Southern United States (III)* J

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Fog: Its Causes and Forecasting with Special Reference to Eastern and Southern United States (III)* J Fog: Its Causes and Forecasting with Special Reference to Eastern and Southern United States (III)* J. J. GEORGE Chief Meteorologist, Eastern Air Lines, Inc., Hapeville, Ga. (2) Quantitative Studies of Fog Causss and Forecasting at Certain Airports (continued). JACK SON'VILLE, FLORIDA T JACKSONVILLE, where the air- (high-pressure ridge) forms down the port is located only 14 miles coastal plain with the center of the A from the shore of the Atlantic HIGH to the northeast. Air is di- ocean, the fog problem is predom- rected over the water close to the inantly one of advection rather than shore as indicated in Figure 6. The of radiation. This is also true of all stations examined which were within the limits of sea-breeze effects. In such situations advection fog forms with a slight or moderate pressure gradient directed from water towards land. With such a flow, the surface wind decreases slowly at night under the combined influences of increasing stability caused by surface cooling over the land and the tendency toward production of a land-breeze which counteracts the general pressure gradient to some extent. The result is a mass of moist, almost stagnant air over the adjacent land surfaces which is easily cooled by nocturnal radiation, producing fog. When the pressure gradient is oriented any direction except from sea to land the land breeze is additive to the surface wind produced by the pressure gradi- ent and usually no fog results because of excessive mixing of the lower lay- ers caused by the abnormally high surface wind. Advection Types:- FIG. 6. TRAJECTORIES FOR NE STRATUS AT JACKSONVILLE, FLA. (Line of circles is warm "Northeast Stratus" is the name front; cross-section below.) given a peculiar formation which temperature of the cold air is in- forms occasionally in winter along the creased rapidly by passage over the coast from Charleston, S. C., to Vero comparatively warm waters near the Beach, Fla., and which resembles in coast. (Mean surface-water isotherms essentials the stratus of the Califor- are entered in the figure according to nia littoral. A wedge of cold air Slocum). The flow under such a pres- ^Concluded from April and June BULLETINS, sure distribution normally takes place 1940. almost exactly parallel to the north- Unauthenticated | Downloaded 10/07/21 11:24 AM UTC 286 BULLETIN AMERICAN METEOROLOGICAL SOCIETY [Vol. 21 ern limit of the Gulf Stream. Con- a process of continual change. How- version of cold polar air to warm, ever, if the pressure gradient sud- moist, tropical maritime air takes denly becomes flat along the coast and place rapidly over the warmer waters, allows radiation from the land sur- while the air flowing over the cooler face to act on stagnant air for some shore waters has only a comparatively time, stabilization is rapid and the fog short path over water and is much will remain all night. less affected. This flow rapidly pro- duces frontogenesis which seems to occur almost invariably near the northern limit of the Gulf Stream. In Figure 6, the region of frontogene- sis is indicated by means of small cir- cles between the converging lines. The showery type of weather such a flow (cold air over warm water) usually produces is easily transformed to a low-stratus type if a limit to con- vection develops at a low level. The limit in this instance is simply the warm front produced as just de- scribed along the Gulf Stream. This front is oriented north-south with the FIG. 7. FOG GRADIENT-WIND-ROSE FOR JACK- SONVILLE, FLA. cold air to the west and is illustrated in cross section along the line marked Figure 7 shows the distribution of AB in Figure 6. The wind arrows in fog according to the direction of the the cross section do not represent the gradient wind. The types of fog are air flow in the vertical as customary, remarkably consistent with the dia- but horizontal directions assuming gram. Only the stratus types from north at the top of the illustration. the southwest quadrant and occasional The front is usually shallow vertically ground fogs from the NW may be and for that reason is frequently over- considered as pure radiation types, looked in analysis. The stratus usu- and these are a small minority of the ally forms with ample ceilings and total. slowly lowers as the front approaches The large majority of fog occurs in the shore or as radiational cooling winds from some easterly direction takes place from the top of the cloud starting as ground fog, and various stratus. methods of forecasting were found A type of stratus somewhat similar necessary. During the summer months to this one occasionally is observed of June through September fog is al- during the fall and winter months most non-existent. In October it first when the flow from a cold HIGH is begins to form in appreciable quanti- just starting to have a trajectory ties and from October through Decem- across the water, and in this case ap- ber cooling curves seemed to be the parently the extremely stable lapse best available method of forecasting rate of the air mass is sufficient to the formation. Figures 8 and 9 are limit the convective layer. Such a representative composite-curves for fog, of course, can last only a few temperature variation for different hours, usually from dark to midnight, directions and velocities of the gradi- since the flow which produces it is in ent wind. To use them find the inter- Unauthenticated | Downloaded 10/07/21 11:24 AM UTC hours to time given by intersec- tion. Figure 9: Curve I. Add 3 hours to intersection. Curve II. Add 4 hours to intersection. FIG. 8. COMPOSITE CURVES OF TEMPERATURE VARIATION WITH WIND AT JACKSONVILLE in Oct.-Nov. section of the temperature curve with the dew point using the appropriate curve for the gradient wind taken from the 5 P.M. pilot-balloon run at sunset; and also the temperature FIG. 9. COMPOSITE CURVES FOR JACKSON curve should be started from its maxi- VILLE, December. mum value. Certain corrections are necessary since fog never forms im- TABLE 3. Time in hours to add to mediately at the intersection of tem- intersection of Temperature and Dew perature and dew point. These correc- Points for curves III and IV in Fig- tions follow:— ure 9. Figure 8: West-wind curve: Add 4 Gradient- hours to the time indicated by the Wind Direction Curve III Curve IV intersection and forecast only NE 4 7 ENE 5 6 light fog, visibility above 1 mile. E 4 4 East-wind curve: For due ESE 3 2 SE 2 3 east gradient winds only, add 5 SSE 3 4 hours to time found at intersec- tion. Visibility will usually re- For the remaining winter months main I to 1 mile for winds from other methods of forecasting proved this direction. more reliable and easier to use. For For all winds NNE to SSE inclu- the winter months of January and sive except due east, add 3 to 4 February, with gradient wind from Unauthenticated | Downloaded 10/07/21 11:24 AM UTC [Vol. 21 the East to SSE inclusive only, and used for each group of gradient-wind with polar air only (dew points less directions which may be treated as than 59°F) Table 4 is used: possessing fairly homogeneous prop- Separate tables of this type are erties of air. TABLE 4. Time of formation of moderate or dense fog, E-SSE gradient-wind. Dew-point Depression at Gradient-Wind Velocity (M. P. H.) Sunset (F°) 1 to 5 6 to 15 16 to 30 Over 30 0 to 5 (Use 1900 0000 (none 6 to 10 West- 1930 0139 recorded) 11 to 15 Wind Table, 2230 0230 erratic 16 to 20 Not Repro- 0100 0430 erratic 21 to 30 duced Here.) 0430 (none) erratic SAN ANT< no, TEXAS San Antonio lies on a steeply slop- during these months is so high that ing coastal plain 140 miles northwest sufficient data to produce quantitative of the coast of the Gulf of Mexico. rules for forecasting were not avail- Its elevation is 580 feet, but the able. The following remarks serve as ground rises to 1000 feet just 16 miles a guide however : to the northwest. The steep general 1. Fog forms with gradient wind be- slope of the terrain and its rather dry tween 12 and 40 mph. from the nature seem to preclude entirely all SSW to E inclusive, provided that normal cases of radiation fog in conti- a direct path of the air from the nental air. Only during January and Gulf has been established. February were cases of this kind 2. It forms with a dew point depres- found and they were few, brief, and sion at 1830 C.S.T. of 0 to 23°. occurred mostly when rain clouds dis- 3. The higher dew point depressions sipated during the night hours. For are always associated with higher this reason, only advection fogs in winds and ceilings. air from the Gulf will be discussed for this locality. 4. A ceiling of 300 feet or lower sel- It is worthwhile dividing the fore- dom or never forms before mid- casting problem into two seasons, win- night. ter and summer, and each of these 5. Ceilings 300 feet or lower are asso- into polar and tropical air masses. ciate with either:— Advection fog Summer, Tropical mari- (a) A rapid decrease in gradient time air:— wind after 1830. June, July, August: The cases in (b) Rapidly divergent isobars which ceilings below 400 feet are at- from coast to San Antonio.
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