The top of the tree canopy, out across Rock Creek Park and beyond, is beginning to turn color. Above, the sky is deep, pure blue, shading to pale - nearly white - toward the southern horizon. In the sky are two layers of clouds. The higher one hangs, motionless, as near as I can tell. Beneath it, there is another layer - long bands of white and gray, sliding quickly along from west to east.
I perceive the clouds as moving through the air, since I can see the clouds, but not the air around. In reality, the clouds are part of the air, and it is the entire atmosphere that is moving. The cloud isn't a separate thing in the atmosphere. It isn't like an airplane - a solid object that moves through the air.
The difference between a cloud and a similar volume of air that you can't see is just the amount of moisture present. Air with enough moisture forms larger droplets that reflect light. What looks like a completely different phenomenon from the invisible air is simply the result of a threshold being crossed. When there's enough water in the atmosphere, we can see it. There's no difference in kind, just in degree.
Clouds are visible atmosphere. Energy from the sun can heat the air so that the moisture moves back to the invisible state. Indeed, in the course of 15 minutes or so, the upper layer of clouds slowly, almost imperceptibly, melts away into the blue sky, while the clouds below continue to stream from one horizon to the other.
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