On such a day as this, the air near the ground is so damp that it gives up vapor easily, as you can prove by allowing a glass of ice water to stand on a table and watching the drops of water gather there, causing the glass to "sweat" (Fig. 9). The sun beats down upon the heated ground and the surface becomes like a furnace, so that the air near the ground is warmed. [Illustration: _Fig. 15. A "thunder head," or cumulus cloud._] Air that is warm is lighter than cool air, and, being lighter, will rise, for the heavy cool air will settle and push it up, as a chip of wood will rise in a pail of water, because it is lighter than the water which pushes it to the top. This is why the warm air rises from a furnace, or a stove, or a lamp. It is the reason why the hot air rises through a house chimney; undoubtedly you can find other illustrations, as ventilation, and can find abundant opportunity to prove that warm air will rise. The warm, moist air near the ground becomes so light that the heavy air above settles down and pushes it up, so that an uprising current of air is formed above the heated ground, much as an uprising current of hot air rises through the chimney when the stove is lighted. Rising thousands of feet into the sky the warm air reaches such a height, and finally comes to a place so cool, that some of the vapor must be condensed, forming fog particles, which in turn form a cloud. On such a day, if you will watch a cloud, you will notice that its base is flat (Fig. 15); and this flat base marks the height above ground where the temperature of the atmosphere is low enough to change the vapor to fog particles. Of course the air still rises somewhat above this base and continues to get cooler, and to have more and more vapor condensed. This makes a pile of clouds resting on a level base, but with rounded tops (Fig. 15). Sometimes the base of these summer clouds, called cumulus clouds, is a mile above the ground and their tops fully a mile higher than this. [Illustration: _Fig. 16. Photograph of a lightning flash._] Just as on the mountain side, where the drops grow larger until they must fall, so here, fog particles grow to drops of such a size that they are too heavy to float. This growth is often aided by the violent currents of air, which sometimes tumble and toss the clouds about so that you can see the commotion from the ground. These currents blow one particle against another, forming a single drop from the collision of two; then still others are added until the rain drop is so heavy that it must fall. But sometimes the air currents are so rapid that the drops are carried on up, higher and higher, notwithstanding the fact that they are heavy. Then they may be carried so high, and into air so cold, that they are frozen, forming hail. These "hailstones" cannot sink to the ground until they are thrown out of the violent currents, when they fall to the ground, often near the edge of the storm.