Clouds

Clouds-those thin white wisps, high in the sky. These are delicate fibrous cirrus, tiny ice crystals in the subfreezing zone, marking the paths of moisture tongues through the sky. As the moisture supply becomes more plentiful, they increase, making chaotic patterns as the wild gales of the heavens sweep them in senseless lines, this way and that way. Maybe a jet plane adds its contribution like a giant finger painter sweeping across the sky. Finally all is covered. The blue sky is gone and only a white sheet, cirrostratus, remains. Now come darker fingers underneath the white cirrostratus. Outdoor men, wise in the ways of the weather, say the sky is thickening, that there is a possibility of rain. Others, who learned their cloud knowledge from books, say the moisture supply is more plentiful and and that an altostratus sheet is now invading the sky, with its under surface at middle levels. The sun which shone so well through the cirrus and cirrostratus is now dimmed and shines "as through ground glass." Dark, sharply outlined patches and lines of low cloud now begin to float across the sky-dark because they are shaded from the sun, sharply outlined because they are of liquid droplets and lower than the uniform cloud mass above. The cloud sheet thickens still more and reaches down to lower levels. This is nimbostratus, the rain bringer. Its presence does not always mean rain will fall, but the probability has been greatly increased.

High wind in the night sky

After a period of time, a line of blue sky shows again, now in the west. Cracks and seams appear in the gray overcast. The clouds become more rounded as if their centers were bulging upward. Drier air is pouring over the desert up to the cloud levels. The low, broken clouds of bad weather have gone. The other clouds are breaking up. The sheet of nimbostratus is gone, or disintegrated into altocumulus clouds, each element detached and separated by a space of sky from its neighbor. These form in regular patterns across the sky. The sun, low in the west, peeps out under the cloud level, and the whole sky is lit to a flaming red by the last rays of the dying day.

As we reflect on the beauty of the sunset colors, let us turn aside briefly to examine clouds from a more materialistic viewpoint. Clouds are composed of minute particles of moisture which have condensed into visible form in the atmosphere. These particles are water droplets or ice crystals, depending on the temperature and state of the air. The high, white, windswept cirrus are ice-crystal clouds, while the fluffy cumulus-type clouds and stratus sheets which blot out the sun's disk are water-droplet clouds.

Cloud droplets are exceedingly small. One particle is, On the average, 1/1200 inch in diameter. It would take approximately 8,000,000 cloud droplets to supply the moisture to form one rain drop 1% inch in diameter. Cloud droplets, small as they are, however, feel the pull of gravity. When in still air, cloud particles fall slightly due to this pull of gravity. As these particles reach drier air beneath the general cloud level, they evaporate, hence the visible cloud itself does not seem to settle in the atmosphere. Wind currents at the cloud level, however, usually are of such speed as to make the pull of gravity of minor importance.While cloud formation is a good indication of the presence of moisture in the air, there are a few prerequisites to the formation of clouds even when the moisture is present. The air must be cooled to the condensation point before clouds can form. This cooling is usually accomplished by expanding air, when currents rise from the ground; or when they are forced upward over a colder, more dense air mass, or over a natural geographic barrier. Occasionally the air is cooled and clouds are formed by mixing with a colder air mass. Another necessary factor in the formation of clouds is the presence of "condensation nuclei." These are small particles in the atmosphere around which the new cloud droplet forms. Experiments have revealed that these are mostly composed of dust particles, smoke particles, and certain chemi-

cals. The most prevalent of the chemicals are salt particles left in the atmosphere from evaporated sea spray.

There are four primary cloud types. The cirrus are the high fibrous white clouds composed of ice crystals. Stratus are the sheet clouds and may occur at any level. Cumulus are the clouds formed by localized rising air currents. These form with flat bases and show vertical development. Then finally there is the nimbostratus, an amorphous rainy-looking cloud which is usually at the lowest level when present in a cloud system. These four names, cirrus, stratus, cumulus and nimbostratus, are freely combined to give the various cloud forms to be seen in nature. Other Latin-type names are also used in combination with these to subdivide the cloud species further.

Some of these names are not only highly descriptive, but quite picturesque. For example, altocumulus castellatus refers to lines or groups of small cumuliform clouds whose vertical development is in the form of turrets such as would be seen on a medieval castle, hence castellatus. A translucent cloud such as altostratus translucidus is semi-transparent, while altostratus opacus is dense enough in some part to hide the sun's disk. Fractostratus refers to a low sheet (stratus) cloud badly broken up, usually whipped by wind. Cirrus filosus are filaments or strands of cirrus, while cirrostratus nebulosus is a very uniform veil covering all or most of the sky, sometimes very thin and hardly visible, sometimes relatively dense, but always nebulous, without definite details.* One of the most interesting types of cloud is the cumulus. These clouds, built on on rising air currents, start as small puffy cumulus humilis and may grow through stages from the swelling cumulus congestus, to the towering cumulus with caps or hoods (pileus), to the full-grown thunder-head, or cumulonimbus.

The isolated, growing, cumulus clouds can often be seen in all their glory from top to bottom. These clouds often rise to great heights. In continental United States a height of 35,000 feet is not uncommon for a full-grown cloud. When a cumulus cloud pushes into the freezing zone due to its height in the atmosphere, feathery or indistinct edges appear at the top. These are of minute ice crystals. When a towering or swelling cumulus has reached this stage, it is said to have become a cumulonimbus. If convection-or the rising air currents-remains strong enough, the ice crystal top may spread out into an "anvil." Lightning usually occurs within such a cloud or from the cloud to the ground. Rain may or may not fall.

Under certain conditions of convection and wind, off-shoots may form from the parent cloud, either at the top or at various lower levels. A fibrous cloud sometimes flows out horizontally from the top of the cumulonimbus and spreads over the sky like a cirrostratus sheet. This cirrus mass, high in the sky, may mean that the top of the cloud column has hit an unusually strong wind current which is blowing off the top as it rises, or that the cloud has reached the tropopause, that level, high in the atmosphere, which acts as a warm "ceiling" beyond which convection cannot take place; and at which all rising air currents must flatten out and take a horizontal course, or return to lower elevations. Occasionally offshoots can be seen spreading out from the cumulonimbus in the middle or lower levels. These

levels may be well down in the zone where the temperature of the air is above freezing. Offshoots at these levels are altocumulus or stratocumulus layers, the name depending on their height. These clouds may be formed by a slight discontinuity in the temperature field at that level or a particularly strong horizontal wind current which catches some of the rising air and swirls it outward. If the day's convective forces are not particularly aggressive, the cumulus and cumulonimbus clouds begin to dissipate towards late afternoon. The rising air column is broken up, and the clouds lose their towering shape. The stratocumulus clouds which have flattened out at a middle level often persist longer than the main column. Even newclouds are added at their level as the main convection currents become too weak to rise further.

These stratocumulus clouds can be seen in bands or strips across the sky, one behind the other, like echelons or stair steps. They themselves decay in strips, often about sunset. These are the clouds that make the richest sunset colors. The atmosphere, acting as a selective absorber, filters out the shorter blues and greens in the sun's rays when they shine through a long path of air at and after sunset. The remaining reds and yellows are caught and reflected by the under portions of the clouds, giving a colorful benediction to the day which only the Almighty, working through nature, can paint.