Any water molecule that strays beyond the edge of the cloud is immediately zapped by the dry air beyond, allowing the cloud to keep its fine edge. Much higher cirrus clouds are composed of ice, and the zone between the edge of the cloud and the air beyond is not so clearly delineated, which is why they tend to be blurry at the edges.

For all the heft and fury of the occasional anvil-headed storm cloud, the average cloud is actually a benign and surprisingly insubstantial thing. A fluffy summer cumulus several hundred yards to a side may contain no more than twenty-five or thirty gallons of water— "about enough to fill a bathtub," as James Trefil has noted. You can get some sense of the immaterial quality of clouds by strolling through fog—which is, after all, nothing more than a cloud that lacks the will to fly. To quote Trefil again: "If you walk 100 yards through a typical fog, you will come into contact with only about half a cubic inch of water—not enough to give you a decent drink." In consequence, clouds are not great reservoirs of water. Only about 0.035 percent of the Earth's fresh water is floating around above us at any moment.