May 9, 1901] 



NA TURE 



43 



FOG FORMA TIONS. 



T3RIEF reference has already been made (vol. Ixiii. p. l6l, 

 December 13, 1900) to some interesting observations and 

 photographs of fog made by Mr. A. CI. McAdie on Mount 

 Tamalpais, a little to the north of San Francisco. Several articles 

 upon the subject have been contributed by Mr. McAdie to the 

 U.S. Monthly Weather Review, and the particulars given below 

 have been derived from one in the is.sue of November, 1900. 

 We are fortunate in being able to reproduce one of Mr. McAdie's 

 striking photographs of fog, through the courtesy of Prof. Cleve- j 

 land Abbe. | 



Fog is very prevalent on the central coast of California, ' 

 especially in the vicinity of the Bay of San Francisco. The 

 topography of the district is remarkable, because of the 

 close juxtaposition of ocean, bay, mountain and foothill. A 

 valley, level as a table, 450 miles long and 50 miles wide, 

 having afternoon temperatures of 100° or over, is connected by 

 a narrow water passage with the Pacific Ocean, the mean tem- 

 perature of the water in this locality being 55°. Thus within a 

 distance of 50 miles in a horizontal direction there is frequently 

 a difiference of 50° in temperature, while in a vertical direction 

 there is often a difTerence of 30° in an elevation of half a mile. 

 High bluff's, ridges and headlands are at such an angle to the 

 prevailing strong westerly surface air currents that an air 

 stream is forced with increased velocity through the (lolden 

 Gate, and there must of necessity be considerable piling up of 



An attempt has been made at the Mount Tamalpais station to 

 correlate the surface pressure conditions with fog. There are, 

 however, many different types of fog. The conditions prevailing 

 in winter, when tule fog, formed in the great valleys, drifts slowly 

 seaward, are very different from those prevailing in summer, 

 when the sea fog is carried inland. A typical pressure distribu- 

 tion accompanying sea fogs has been recognised. In general, a 

 movement southward along the coast of an area of high pressure 

 in summer means fresh northerly winds and high temperatures in 

 the interior of the State, with brisk, westerly winds, laden with 

 fog, on the coast. 



Direct cooling by contact or radiation is shown by von Bezold 

 to be more efficient as a cause of rainfall than cooling by mixture, 

 but in the production of fog it is probable that cooling by 

 mixture (except in the case of ground fogs) is the most im- 

 portant factor to be considered. It is to be noted that reverse 

 pressures should also be studied, for perhaps a close watch upon 

 the conditions prevailing when fog is rapidly dissipating might 

 conversely throw light upon the order and relative importance 

 of the three ways of cooling, viz., mixture, expansion and 

 radiation. 



Von Bezold's deductions may be thus summarised : More 

 vapour condenses when a stream of air and vapour at low tem- 

 perature impinges on a ma.ss of warmer air than with reversed 

 conditions. Ocean fogs, as a rule, form when cool air flows over 

 warm, moist surfaces, but in the case under discussion, where 

 the ocean surface temperature is 13° C. (55° F.) and the air tem- 

 perature may reach 27° C. (80° F.), it is evident that the above 



Fig. I.— Lifted fog. Height above the ground, about 500 metres. View from U.S. Weather Bureau Observatory, Mount Tan 



ilpi 



both air and water vapour at this point. The locality may 

 indeed be considered as a natural laboratory, in which experi- 

 ments connected with cloudy condensation of water vapour are 

 daily wrought, and it is therefore of more than passing interest 

 to the meteorologist. 



Much faithful work has been done in physical laboratories on 

 the behaviour of water vapour at varying volumes, pressures and 

 temperatures. Regnault, Thomson, Broch, Ailken, Kiessling, li. 

 von Helmholtz, Hertz, Rayleigh, von Bezold, Barus, Marvin 

 and others have wtirked upon the change of state from vapour 

 to liquid and from liquid to solid, and while many irregularities 

 are noted in the behaviour of water vapour, the general problems 

 ol decreasing volumes and increasing pressures until condensa- 

 tion points are reached have been solved ; and it is well under- 

 stood that ihe vapour-liquid and liquid-solid condensations are in 

 themselves but two phases in a chain of condensation phenomena. 

 The problem of fog is therefore a limited one. It may be con- 

 sidered as a special case of cloud development, occurring in the 

 first and second .stages of Hertz, viz., the unsaturated and 

 saturated stages. Condensation in the free air, as in these fog 

 formations, takes place under conditions different from those 

 obtaining in the laboratory. There are no fixed restraining 

 walls, though the strongly stratified outlines suggest sharply 

 limited air streams. Again, saturation as it occurs in free, 

 constantly changing air and true adiabalic saturation are not 

 identical. Saturation in the free air must be studied in dis- 

 advantageous circumstances, for the work must be done at a 

 distance, with instruments neither sufficiently delicate nor 

 accurate, and there is no control of conditions possible. 



does not hold. It is more probable that condensation is the 

 result of the sharp temperature contrasts at the boundaries of 

 certain air currents having different temperatures, humidities and 

 velocities, and that the contours of the land play an important 

 part in the originating and directing these air currents. The 

 summer afternoon fogs of the San Francisco Bay region, then, are 

 probably due to mixture more than radiation or expansion. The 

 winter tule fogs of the Sacramento and San Joaquin valleys are 

 probably pure types of radiation fog, where the process of cloud 

 building is from the cooled ground upward. Occasionally in 

 summer, when the w.irm air has been pumped out of the valleys 

 and there is rapid radiation, ground fog forms. An illustration 

 of this is given in the accompanying figure, where fog covers a 

 number of valleys. 



UNIVERSITY AND EDUCA TIONAL 

 INTELLIGENCE. 



OxfORii. — Prof. Townsend, the new Wykeham professor of 

 physics, delivered an inaugural lecture at the Clarendon Labora- 

 tory on Friday, April 26, upjn the recent developments of 

 electro-optics. 



Mr. N. V. Sidgwick, of Christ Church, has been elected to 

 an official Fellowship in Natural Science (Chemistry) at Lincoln 

 College. 



Mr. A S. Hunt has been elected to a research fellowship at 

 Lincoln College in order to enable him to prosecute his re- 

 searches upon Egyptian p.rpyri. 



NO. 1645, V(JL. 64] 



