6 
HINTS TO TRAVELLERS. 
rounded forms. They represent the condensation of moisture in ascending 
columns of heated air. Stratus clouds are low-lying sheets of condensed 
moisture, which, being usually seen at a low angle, appear like thin 
layers parallel to the horizon. The transitional type Cirro-Stratus is 
usually seen in the form of great feather-like clouds stretching across 
nearly the whole sky. Nimbus is a rather low-lying cloud from which 
rain is falling even if the rain is re-evaporated before reaching the 
ground. The lowest clouds of all, those resting on the surface of the 
ground and enveloping the observer, are called [mist and fog . The two 
are distinguished by the fact that a mist wets objects immersed in it, 
while a fog does not. All varieties of cloud are physically the same, 
consisting of minute globules of liquid water falling through a portion 
of air saturated with moisture. The globules being small offer a 
relatively great surface to friction, and so fall very slowly, and in the 
higher clouds they evaporate on the lower surface before they have 
time to agglomerate into raindrops. In the highest of all clouds, the 
cirrus type, the particles are probably spicules of ice and not globules of 
water. It is a common error to suppose that black clouds differ from 
white clouds. All clouds are white when they reflect the light of the 
sun, and all are black when they come between the eye and the sun in 
sufficient thickness to cut off a considerable portion of its light. 
The sudden appearance of a particular kind of cloud is important as 
a weather sign. It shows that changes are going on in the vertical 
circulation of the atmosphere. Hence if cirrus or cumulus cloud should 
be observed to be increasing the fact should be noted, and the direction 
in which the clouds are moving should be noted also. 
In observing cloud-motion attention should be given only to the sky 
overhead; at any lower angle the parallax due to viewing the clouds 
obliquely deprives the observation of value. It is also necessary to 
distinguish between the movement of the upper and of the lower clouds, 
as these are floating in very different parts of the atmosphere. It is 
comparatively rarely that the motion, say of nimbus and cirrus, is in the 
same direction. On a lofty mountain, strata of cloud which from below 
were seen to be cumulus may be passed through as layers of mist, and 
on emerging from them their upper surface may be seen below one. In 
many mountains the cloud-belt is as sharply defined as the snow-line, 
and its variations should be carefully observed. 
