216 HEAT. 



Convective Equilibrium. In this condition the temperature falls 

 as we rise at such a rate that any mass carried upwards will cool by 

 expansion so that it will always be at the same temperature, and of the 

 same density, as its new surroundings. This means that the fall of 

 temperature with decreasing pressure is that given by the adiabatic 

 relation, and near the earth's surface it is easily shown that the rate is 

 about 1 F. for each rise of 183 feet, or 1 C. for about 100 metres. If 

 the lower layers of air are cooled so that the rate of fall of temperature 

 upwards is less than this, or if the temperature rises as we ascend, the 

 equilibrium is stable. But if the lower layers are heated so that the fall 

 is more rapid, equilibrium is destroyed and circulation occurs. The lower 

 air rises, and the upper air falls to take its place. This circulation 

 frequently occurs on a hot summer day, when the surface of the earth, 

 heated by the sun, in turn heats the air in contact with it, and there are 

 small streams of ascending and descending air. These streams are 

 rendered evident by the quivering of objects seen through the surface 

 layers, the light being refracted by passage through a medium of density 

 varying from point to point. The quivering is often seen with the naked 

 eye, but is still more evident when a distant outline is looked at through 

 a telescope. Some of the haziness of outline is no doubt due to this 

 quivering. 



If a large mass becomes simultaneously heated, it may rise as a whole 

 and be thrust up through the upper layers. In rising it expands and cools, 

 and if it rises far enough and cools down enough, it may become super- 

 saturated at a certain height and begin to form cloud. Often such 

 formation may be watched in progress, a big piled-up mass of " cumulus 

 cloud " somewhat as in Fig. 127 being formed, the outline of the cumulus 

 marking the upper boundary of the rising column, the horizontal base 

 marking the cloud level or the point at which the air is cooled to 

 saturation point. When thunderstorms are gathering in hot weather, 

 the formation may be seen on a gigantic scale, the columns pushing far 

 up into the higher regions. The " strato-cumulus " clouds, which are 

 those with rather irregular globular or fleecy outlines, seen, say, in a 

 south-west wind with a broken sky, were probably formed originally as 

 cumulus clouds at the top of ascending columns, but they have drifted 

 far from their birthplace, as their form shows. The upper part is always 

 in advance of the lower, the upper wind travelling faster than the lower. 



The so-called " Ripple Clouds," almost like ripple marks in sand, have 

 been explained by Helmholtz &s due to alternate expansion and contrac- 

 tion of the air. When one stream of air moves over another, there will 

 be undulations in the surface of separation, and a mass of air at this 

 surface will alternately rise and fall as it travels on. If it is just at the 

 saturation point, when it rises to the top of an undulation it expands, 

 cools, and deposits cloud, which it dissolves again in falling into the 

 succeeding trough, where it contracts and heats. The parallel lines of 

 cloud then mark the summits of the undulations, and the air may be 

 travelling on at quite a different rate from the clouds. 



HaloS and Parhelia. If the temperature of the air is below the 

 freezing point, any excess of water-vapour is deposited as ice in the form 

 of minute crystals. The presence of these crystals, when forming only 

 a thin veil of cloud, is shown by halos round the sun and moon and by 



