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Clouds, Classification CLOUDS: KEYS TO UNDERSTANDING WEATHER, CLIMATE, AND THE HYDROLOGIC CYCLE -Jack R. Holt A MEMORIAL DAY TRIP understanding weather and the great Hydrologic If you don’t like the weather in Oklahoma, just Cycle of water from the oceans to the wait a minute. -Will Rogers atmosphere, and back, ultimately, to the oceans I recall a particular windless, sultry again. The description of the general features of Memorial Day in Tulsa. A group within my the great cycle had been known since before extended family had decided to go to a reservoir Greek Natural Philosophy, but an understanding outside of the city for a day of picnicking and of clouds, cloud formation and the physics of swimming. We secured a van for the purpose, clouds was absolutely necessary to connect the and spent the day trying to be comfortable under atmosphere to the ocean to help explain why a nearly cloudless sky. After a day in the parts of the earth were deserts and others were Oklahoma sun, we returned in late afternoon. I replete with rain. was elected to drive. From a distance I could see a large anvil-topped thunderhead building over A CLASSIFICATION OF CLOUDS the city, and the first thing that came to mind It is not in the least amiss for those who are was “maybe it will cool things off.” As we involved in meteorological research to give some approached, we saw that the cloud had grown to attention to the form of clouds. -Lamarck monstrous proportions and periodic lightening Jean Baptiste Pierre Antoine de Monet flashes punctuated its blackness. The whole chevalier de Lamarck (1744-1829; France) scene looked as though an angry god was about was one of the very first to attempt to use a to take vengeance on the city, a notion that was classification system as a way to support a theory not far from being wrong. of evolution. He had been made the curator of Rain began to fall in unrelenting sheets so worms (invertebrates) at the Musee Nationale in that the windshield wipers became completely Paris. In that position, he attempted to make ineffective. When I could see, the water had sense of the diversity of all animals without filled the street to above the curb and formed backbones and build on the classification system shallow streams that were growing. We made it of Linnaeus, many years earlier. He also tried to to my mother’s home, which stood on relatively apply a method of classification to clouds, other high ground and took shelter in the basement. natural chaotic structures. Lamarck saw that There, we sat, sometimes by candlelight, and clouds seemed to occur in five different types: told stories, ate popcorn, and waited. The storm • Hazy clouds (en forme de voile) raged throughout the evening and began to die • Massed clouds (attroupes) down in intensity only near midnight. However, • Dappled clouds (pommeles) the authorities urged listeners by radio to stay in • Broom-like clouds (en balayeurs) their homes, because most roads were flooded • Grouped clouds (groupes) and very dangerous. He published his classification system of clouds By morning, the rain had stopped, but Tulsa in 1802 and expanded it to twelve types in 1805. had become a shallow lake, and the small hill on Lamarck wrote that the purpose for such a which our house sat was one of many islands. classification system was to group them That morning, I went for a walk in the according to their causes. That is, he expected neighborhood and saw downed trees, cars in odd that their shapes were related to how they were places, and even one car sitting atop another. formed. Thus, his categories were linked to The news and weather programs that day (and potential explanations of their formation. for days after that) announced that we had In 1802, an Englishman named Luke received 11.5 inches of rain in a single storm. Howard (1772-1864) also saw a need to classify The morning began with pillows of fair weather clouds. He had been raised a Quaker, sent to a cumulus clouds and gave way to another hot day. Quaker school, and later in life became one of Weather and clouds are related to each the leading members of the Society of Friends. other. Furthermore, the discovery of clouds as Beyond that he did not have special education in physical entities that could be described, labeled, the sciences. Indeed, he was a successful and categorized, was a crucial link in pharmacist. Although he was only an amateur resembled the very successful Latin scientist, he had become a very prolific one. It classification system that Carolus Linnaeus had was at a meeting of amateur scientists that created for living things. When meteorologists Howard presented his classification system for all over the world had a means to categorize clouds. He categorized them into four basic types clouds by formal definitions, then they could and gave them descriptive Latin names: communicate aspects of clouds. • Cumulus (Latin, heap) • Stratus (Latin, layer) • Cirrus, (Latin, curl) • Nimbus (Latin, rain) The first three were named for their forms while the last one was named for its action, precipitation. He recognized that clouds by their nature change and can change from one form to another. While others had seen this as a problem with a descriptive taxonomy, Howard expanded his system to accommodate the transitional stages. For example, cumulus clouds could begin to bunch together. As they did so, they formed a kind of broken layer of bunched clouds, which he called cumulo stratus. FIGURE 2. High waves of cirrus and Other transitional clouds included: developing mid-level clouds as the leading edge • Cirro-Stratus (small, rounded clouds in of a warm front moves from south to north at the a layer) end of February 2005. • Cirro-cumulus FIGURE 3. This is a cumulus cloud that a cumulo-nimbus (note the snow shower falling from the left end of it). CLOUDS AND WEATHER The sky, too, belongs to the landscape. The FIGURE 1. Luke Howard. An image from the ocean of air in which we live and move, in which John Day website at . the bolt of heaven is forged, and the fructifying http://www.cloudman.com/luke/luke_howard.htm rain condensed, can never be to the zealous Naturalist a subject of tame and unfeeling His classification system was almost contemplation. -Luke Howard immediately successful and almost universally Clouds form when air becomes adopted. The reasons that the world adopted supersaturated with water, which then condenses. Howard’s system, as opposed to that of Lamarck There are several ways in which air can become likely are complex. However the consensus supersaturated: it can gain water, it can become seems to be that Lamarck used French phrases cooler, or it can change its ability to hold water while Howard used the more universal (and due to change in air pressure. Cloud formation neutral) Latin terms. Also, Howard’s system usually involves all three components. Fog is a special type of cloud whose formation is mountain (see Figure 5). As air is forced up and indipendent of air pressure because it develops over the mountain, water condenses so that the on the ground from air that is saturated and then windward side of the mountain has a wet climate cools off (usually through radiative cooling). with frequent rainfalls. As the air passes up and over the mountain, it descends on the other side. The warmer, denser air can hold much more water, but most of that has been wrung out of it. So, the other side is very dry, often desert, sitting as it does in the rain shadow of the mountain. FIGURE 5. A diagram of the influence that a barrier like a mountain can have in a on regional climates. The windward side of the mountain has frequent rainfalls while the leeward side is a desert in the rain shadow of the mountain. FIGURE 4. A Diagram of cloud formation from Of course, the global situation that thermal convection. The column on the left influences weather and cloud formation usually illustrates the relative density of air molecules is much more complex (go to Ice Ages for a with height due to pressure changes. The arrows more complete explanation). The atmosphere is indicate the movement of warmer, moisture- made of air masses that have different laden air into higher, less dense, cold levels temperatures and interact with each other. The where the moisture condenses to form clouds. line along which air masses collide is called a front. If a mass of warm air collides with a mass Most clouds develop as air rises. A simple of cold air, the warm air tends to override the case would be that of a thermal (see Figure 4). denser cool air (see Figure 6). The outcome is a Air near the ground is heated through the day, gradual change from high cirrus clouds followed and begins to rise displacing the cooler air above by midlevel stratus that gives way to low level it. It carries moisture that had dissolved at the stratus that frequently becomes a nimbus. temperature and pressure near the ground. As it Cold fronts have a different structure from rises, air carries moisture from near the ground at warm fronts. Because the approaching cold air is near sea-level atmospheric pressure. However, denser than the warm air that it is moving into, it air higher in the atmosphere has less air over it tends to form a bulge. Along the leading edge, pressing down; so, the air molecules are farther warm air is forced upwards.
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