proceedings: Washington academy of sciences 505 



Condensation on the nuclei of the atmosphere is divided primarily 

 into fog and cloud, but a sharp distinction between them that would 

 enable one always to say which is which is not possible. In general, how- 

 ever, a fog differs from a cloud only in its location. Both are owing, as 

 explained, to a cooling of the atmosphere to a temperature below its 

 dew point, but in the case of the cloud this cooling usually results from 

 vertical convection, and hence the cloud is nearly always separated 

 from the earth, except on mountain tops. Fog, on the other hand, is 

 induced by relatively low temperatures at and near the surface, and 

 commonly itself extends quite to the surface, at least during the stage 

 of its development. In short, fog consists of water droplets or ice 

 spicules condensed from and floating in the air near the surface; cloud, of 

 water droplets or ice spicules condensed from and floating in the air 

 well above the surface. Fog is a cloud on the earth, cloud a fog in the 

 sky. 



Fog may be divided into two types according to the process by 

 which the necessary cooling of the air is produced. These are radia- 

 tion fog and advection fog. The first, or radiation fog, occurs during 

 still clear nights when the atmosphere is rather humid. On such occa- 

 sions the surface of the earth and the lower atmosphere are cooled by 

 radiation to a temperature sufficiently low to induce both surface and 

 volume condensation. Hence the name "radiation fog." The other 

 type, to which the name "advection fog" is given, is the result of the 

 horizontal flow of warm humid air to a colder region or of cold air to a 

 relatively warm humid region. 



Clouds cannot be so simply and logically classified as fogs. Their 

 exact mode of production, whether by convection, mixing, or radiation, 

 is not alwaj^s obvious. Hence thej^ generally are classified according 

 to appearance and position. The fundamental types are : cirrus, stratus, 

 cumulus, and nimbus. To these are added several alto, fracto, and 

 combination types, such as "alto-stratus," "fracto-stratus," and 

 "cirro-stratus." In addition to all these various special and more or 

 less unusual forms are recognised, such as "billow cloud," "lenticu- 

 lar cloud," "banner cloud," "scarf cloud," "mammato-cumulus," et 

 cetera. 



Not all altitudes are equally frequented by clouds. There are five 

 elevations of maximum and five of minimum cloudiness, each of which 

 may be simply explained. 



Discussion: Messrs. Bauer and Ford discussed this paper. 



The second paper on Notes on dip-of-horizon measurements made on 

 the "Galilee" and "Carnegie" was presented by Mr. W. J. Peters, 

 and was illustrated by lantern sHdes. 



This paper states that while rehable observations have shown the 

 sea-horizon to have been elevated occasionally 10' to 15' above its 

 normal position by refraction, such large values are probably confined 

 to the borders of equatorial and polar currents and to small areas of 

 water swept by breezes blowing directly off heated lands or office fields. 



In over 3000 observations taken during ten years work at sea by the 

 Department of Terrestrial Magnetism, the refraction at the horizon 



