METEOKOLOGICAL OBSERVATIONS. 303 



srr. " or " low cir.-s." It often happens that the observer looks upon the edge of a layer of 

 stratus or cirro-stratus, which appears like a straight line, as in the case of the horizontal 

 bands across the cumulo-stratus. 



The distinction between stratus and cirro-stratus seems to depend on the relative 

 levels above the earth's surface at which they are observed. Clouds are seen at all levels 

 between the highest cirrus and the lowest stratus, so that it is often difficult to determine 

 whether a particular sheet or layer of cloud is a high stratus or a low cirro-stratus. 



The same remark is applicable to cir.-c. and the smaller forms of cumulus, for there 

 is a stage in which they seem, as it were, to run into each other. 



LOWEK CLOUDS. 



Fig. 4. Stratus. This is a sheet or layer of cloud, of uniform thickness. It has 

 but little variety of light and shade, and belongs to the lower regions of the atmosphere, 

 so much so that Luke Howard speaks of it as " Ground Fog," the cloudy formation 

 which spreads over low grounds in the evening. All low detached clouds, which look 

 like a piece of lifted fog, and are not in any way consolidated into a definite form, are 

 strati, and may be called " detached " stratus. 



When a stratus is at a high level, it may pass into a cirro-stratus. 



Figs. 5 and 6. Cumulus. This class of clouds comprises all those which have 

 a globular or rounded form, from the small white cloud represented in fig. 5, to the 

 heavy mass represented in fig. 6, which is almost a cumulo-stratus. The cumulus some- 

 times takes a cylindrical shape, forming itself into long horizontal rolls, between which 

 gleams of light are seen, but which are often so closely packed as to hide the blue sky. 

 These are called by us Roll-cumulus. (See fig. 5.) 



Fig. 7. Cumulo-stratus. This is the cumulus, as it were, changing into a nimbus. 

 It is dark and flat at its base, and is traversed by horizontal lines of dark cloud, 



Fig. 8. Ximliu-s. This is the rain-cloud. Whilst on the horizon, or as it advances 

 towards the observer, its front frequently presents a marked outline like that of 

 a very heavy cumulus-stratus with rain falling from it, and with some cirrus above, so 

 that Howard has called it the cumulo-cirro-stratus. When it has overspread the whole 

 sky, it is usually so mixed up with, or concealed by, the falling rain, that it generally 

 assumes an uniform dark appearance. 



Fig. 8 represents a nimbus before it has overspread the sky ; there are also smaller 

 clouds of the same kind which produce a passing shower. They are easily distinguished 

 by the fact that rain or snow, &c,, is falling from them. 



It is believed that the foregoing description is sufficient to explain the ordinary forms 

 of clouds, but the appearances are often very much intermixed. Thus, before rain we 



