When millions of vapor particles unite, they form droplets of moisture. As these increase in size, they finally become heavy enough to fall to earth as precipitation in such varied forms as rain, snow, sleet, hail and dew.It is estimated that 16 million tons of precipitation in any of these forms falls earthward each second. Through the process of evaporation it is then drawn back into the atmosphere. In nature's balanced operations, evaporation equals precipitation.
As water falls to earth in this never-ceasing moisture circulating system, it serves to cleanse both the air and the ground. No doubt you have many times noted the fresh, clean smell of the air after a heavy rain. This is because the rain has absorbed suspended solid matter (dust, dirt and soot), gases, odors and other impurities, polluting the air over the area. While precipitation may remove large quantities of impurities, it never succeeds in wholly eliminating them.
When precipitation continues for some time, the first amounts to fall are apt to contain a great deal more suspended particles and dissolved solids than that which falls later. An analysis of the mineral content of rainwater in a large city after four hours of precipitation and again after 22 hours shows the following variations (expressed as parts per million as calcium carbonate):
| After 4 hours | After 22 hours | |
| Calcium | 42 | 8 |
| Magnesium | 1 | - |
| Sodium | 11 | - |
| NH,(Ammonia) | 3 | 5 |
| Bicarbonate | 19 | 5 |
| Chloride | 10 | 5 |
| Sulfate | 27 | 3 |
| Nitrate | 1 | - |
As you can see, even this sweeping of the atmosphere did not remove all the dissolved solids in 22 hours of rainfall.
Of all forms of precipitation, the snow falling high in the mountains contains the least amount of mineral content. This is due, however, to the smaller amount of dust in the atmosphere at high altitudes. As a result, many mountain streams deriving their water from high fallen snow have extremely low dissolved mineral content.
Rainwater is also saturated with dissolved air (about 20 to 29 milliliters per liter from 60 ° to 32°F). The amount of free carbon dioxide in rain varies from 2 to 6 parts per million. Any amount of free carbon dioxide above 1 or 2 ppm comes not from the atmosphere itself, but from other sources such as chimneys or industrial fumes. Rainwater also encounters sulfuric acid from the gases in burning coal over cities. In addition, it may pick up bacteria and the spores of microorganisms.
How much water falls in a day in the United States? The United States Geological Survey has estimated that approximately 4,300 billion gallons of water fall within the continental limits of this country each day.