218 HEAT. 



very soon equal to their weight, and then the velocity of fall is uniform. 

 Stokes (Carrib. Phil. Trans., vol. ix. p. 94) showed that this limiting 

 velocity is proportional to the square of the radius of the drop, and his 

 formula, with the now accepted coefficient of viscosity of air, gives the 

 velocity for drops '001 inch radius as about - 8 inch per second. From 

 the observed radii of coronas round the sun and moon, we know that 

 the drops in clouds are often much less than this, so that the velocity 

 of fall is very much less than 1 inch per second. 



The change of the cloud-drops and crystals into rain-drops and snow- 

 flakes has not been made out in full detail. It appears probable that 

 rain-drops are formed by coalescence of cloud-drops. Defant (Sitz d. 

 Wiener Akad., May 1905 ; Sci. Abst., 1905, p. 634) found drops of various 

 sizes in the same rainfall, but those most frequent had weights in the ratios 

 1:2:4:8, apparently formed by the coalescence of drops of equal size. 



In thunderstorms the clouds are usually of very great depth. A 

 drop therefore tends to grow, perhaps by coalescence, through a very 

 considerable range of fall, and this may account for the largeness of the 

 drops as they reach the ground. 



Hail. The genesis of hail is much more complicated than that of 

 the ordinary raindrop. The fall of hail accompanies very great atmos- 

 pheric disturbances, in which there is ascent of large masses of damp air 

 to very great heights. The hailstones usually show a structure of several 

 coatings of ice and snow round a core of snow, and it is supposed that 

 they may begin as snow in the upper regions, then descend some distance 

 where the air is above the freezing-point. Here they condense on them- 

 selves a coat of ice. Then they may be carried up again into the snow 

 region and acquire a coat of snow. Descending again they will receive 

 another coat of ice, and the number of coatings is supposed to show the 

 number of alternations.* 



Fog is cloud formed on the surface of the earth. The condition for 

 its formation appears to be that the upper air is warmer than the surface 

 layer and nearly saturated. The lower layers cooling below the satura- 

 tion point, the excess is deposited as fog. As the number of dust 

 particles, especially in town air, is very great near the ground, the drops 

 may be very minute and very numerous, and the fog may be much more 

 opaque than ordinary cloud. When the upper air is warmer than the 

 lower air, the smoke rising from chimneys does not tend to rise so high, 

 as its excess of temperature above its surroundings is less. Hence in 

 a town fog the smoke hangs near the ground and mixes with the fog 

 instead of rising away from the surface as when the upper air is colder. 

 In confirmation of this account of fog, it may be noticed that often a fog 

 comes on just before the break up of a frost. A warm damp wind has 

 already come on overhead, and the vapour diffusing from this into the 

 still, cold surface layers, supersaturates them and produces deposition of 

 the excess of vapour as fog. 



Dew. The formation of dew was first successfully explained by 

 Wells in a celebrated " Essay on Dew," in which he pointed out that 

 when the surface of the earth radiates out its heat at night so far as to 



* An account of the atmosphere and of the mode of determining the condition 

 of the upper regions by means of kites is given in Rotch's Sounding the Ocean 

 of Air. 



