Try to visualize the process. A stream of vapor has been warmed and is ascending. A mile up and it has cooled not only by the reason of altitude but also by the process itself. About each little dust-particle in the surrounding area vapor forms--vapor cannot form without something to form on, there being always enough dust from deserts and volcanoes to go round. If the cooling proceeds the tiny globules enlarge and as they increase in weight they settle and fall. Falling, they unite with others. If the air-strata are very warm and thick the drops may grow to a very considerable size. We see these in the middle of our great winter rains when the insweep of southern winds with all their warmth and moisture is very extensive. Also the first few drops that come from the thick, hot lips of the thundercloud are usually immense. The best way to measure the size of a raindrop is to have it fall in a box of dry sand. It rolls up the sand and measurements can be easily and accurately made. But the most interesting way is to let the first drops of the thunderstorm fall upon a sheet of blotting paper. If the same sort of blotting paper is used the measurements will be of just as much importance for comparison. Circles as big as teacups are formed sometimes. Heavy drops in winter mean a heavy fall, because they denote high temperatures which are uncommon and are bound to be followed by considerable condensation as the cooling proceeds back to normal temperatures. Small drops in summer mean either cooler weather, or sudden condensation. Small drops in winter are a sign of very thin moisture-bearing strata, or low temperatures, indicating that the rain will be light, protracted, and liable to change to snow.