There are few things which illustrate so clearly the illusion of appearances as a study of the clouds. The commonest of sights, they still contain the greatest mysteries.
We all know how dew condenses on the ground. In the case of fog, when the temperature falls below dew-point, the surplus of water vapor which the cool air can no longer hold condenses about innumerable specks of dust floating in the air, to form a cloud or fog. These minute particles are all similarly electrified and so repel each other, which is one reason why a fog never rains; although the moisture will often collect in abundance on the leaves and trees and blades of grass. Indeed, under some trees condensations and dripping is often so rapid it may appear at times to be raining.
Science calls these land fogs "radiation fogs," and explains that they occur when the night is clear, with sufficient water vapor in the air, a high dew-point, and a gentle breeze. As the lower air is chilled below the dew-point, it condenses into a mass of visible cloud, which seems to move and spread as more and more air is cooled.
Once the process of fog formation has begun, it continues through the hours of darkness, and as more and more air is cooled, fog lakes and fog seas are formed, often hundreds of feet in depth.
With the coming of morning, the sun's rays warm the ground, and the lower air, which can again reabsorb the surplus water vapor, now clears, thus giving the appearance of the fog lifting. Then, as the warm air begins to rise, more and more space is cleared of fog, until finally the remainder is broken into fragments and carried upwards on the rising air currents.
If one climbs a hill through hundreds of feet of cloud to the sun-drenched summit, and notes the temperature as he climbs, he will find a continual cooling of the air as he rises, until at the top of the sea of fog the air becomes increasingly warmer as he continues to ascend. This elevation at which the air ceases to become cooler, and instead becomes warmer, is known as a temperature inversion, and determines the upper limit to which the fog rises.
Those who have studied the upper atmosphere tell us that the troposphere, or region of storms and clouds in which we live, extends to about seven miles above the surface of the earth. The air steadily decreases in temperature throughout this belt, as one ascends, to some eighty degrees below zero Fahrenheit. Above the troposphere lies the stratosphere, in which clouds do not appear and in which the temperature not only ceases to become colder but, as we ascend, actually becomes warmer, reaching at a certain high level 750 degrees.
The belt or line of division between the troposphere and the stratosphere is called the tropopause, and corresponds on the large scale to the temperature inversion on top of the fog sea in the small. We may, if we wish, think of our fog-swept hillside as a small division repeating many of the characteristics of the troposphere, and as we look at far distant clouds high in the sky we may know that they, too, are located on similar invisible temperature inversions, and understand why so many are at the same height. Often we may see several levels of clouds in the sky at the same time.
One hot dry summer day I climbed a 5,000-foot peak in the Oregon Coast Range, camping on the summit over night. The next morning a gentle rain was falling, and I found the peak covered with a cloud. Glimpses of the sunlit valley were seen, while far away other peaks, each with its own moisture-giving cloud-cap, appeared, marking the Cascade Range for a distance of 150 miles.
The wind was blowing hard, and the cloud was not composed of the same particles for any two successive minutes. Moreover, the air over the mountains as well as that over the plain must have contained the same amount of moisture. At times, the cloud even moved for a short distance against the wind.
Out of the visible into the invisible for a brief instant; then, 75 miles away, into the visible again it moved for another brief instant of time. Then once again into the All.
Look beneath each cloud-cap and you will find pointed alpine firs, mountain- and moisture-loving pines, ferns, and mosses. "Arboreal islands" some have called the mountaintops, these areas of northern life, within a warmer, dryer climate — life which could not exist without these messengers from the sea, these "angels of the sea," as John Ruskin called the clouds.
Is it any wonder that the ancients personified the winds and clouds and looked upon them as agents of divine beings? Nay, as divine beings themselves. Vayu, the Vedic god of the air — the Indra of a later day — with his "children" the Maruts, who bring rain and hail, the tornado and the summer breeze, the life-giving fog or mist — angels indeed these are, making "the solitary place glad" and the desert to "blossom like a rose."
Let those who wish speak of temperature, rising air-currents, etc. But these do not explain all, for nature is a wonderland of mystery in which the simplest event is often the least understood.
All trees, flowers, clouds, and the droplets of rain that compose them, have their analogies in the invisible world, and if we will observe, reflect, and compare, we shall know the Reality behind the Appearance that we call nature.