INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 63 



PART V. CLOUDS 



In general, a cloud differs from fog only in its location. Both 

 result from the cooling of air to a temperature below its dew point, 

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

 movement of the air, and hence the cloud is nearly always separated 

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

 is induced by cooling of the air at and near the surface, and com- 

 monly 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. 



Classification of clouds. — The classification of clouds is based upon 

 their form and appearance. Ho^vever, there is a general relationship 

 between the form of clouds and their height, as shown by actual 

 measurements, so that a classification according to form and appear- 

 ance is also, in effect, one according to height. 



The classification and descriptions which follow are taken from the 

 abridged edition of the International Cloud Atlas, published in 1930. 



The heights given are for temperate latitudes, and refer not to sea 

 level but to the general level of the land in the region. It should 

 be noted that, in certain cases, there may be large departures from 

 the given mean heights, especially as regards cirrus, which may be 

 found as low as 3,000 meters (10,000 feet) in temperate latitudes, 

 and in polar regions even almost as low as the surface. 



CLASSIFICATION OF CLOUDS * 



HIGH CLOUDS 



(Mean lower level, 6,000 meters, 20,000 feet.) 

 Cirrus. 



Cirrocumulus. 

 Cirrostratiis. 



MIDDLE CLOUDS 



(Mean upper level, 6,000 meters, 20,000 feet; mean lower level, 2,000 meters, 

 6,500 feet.) 



Altocumulus. 

 Altostratus. 



■* Abbreviations for entry of cloud names on weather reports will be found in the 

 condensed instructions on the inside of the front cover of Form 1210A. 



