a guide to the sky 1A. Cirrocumulus. When this high cloud forms, it can give the sky the appearance of 1B. Cirrus (uncinus and floccus). These clouds have fine trails of ice emanating from a 1C. Cirrus (spissatus and uncinus). These clouds (thick patch above mountain) are the 1D. Cirrus (fibratus). These are patchy ice crystal clouds with gently curved or straight wind blowing on a pond of white water. This cloud is often seen on the fringes of storms, and small tuft or a hook. The bend reveals a sudden change in wind direction and/or speed. The densest of all Cirrus ice clouds. They are the only Cirrus that, by definition, can have shading filaments. They are older versions of Cirrus clouds. By definition they are not thick enough to after a spell of fine weather, signals a change. Boston, Massachusetts bright white specs are “floccus”, compact tufts that eventually lead to more trails of ice. underneath in the midday hours. Some strands of uncinus are above the patch of spissatus. produce gray shading except when the sun is low in the sky. Near Horseshoe Reservoir, Arizona Kingman, Arizona Near Red Rock Canyon, Nevada 2A. Cirrostratus (fibratus). This ice cloud layer is one of the more reliable predictors of 2B Altostratus (opacus). This cloud, more like “Altonimbostratus” is mainly filled with 2C. Altocumulus (perlucidus). Layer clouds like these often indicate that large areas 2D. Altocumulus (opacus). These denser layer clouds are the middle-level equivalent of precipitation in the offing. Before the satellite era, studies indicated that in winter rain or snow ice crystals and snowflakes as are Nimbostratus clouds (4D). This cloud always covers all or most (thousands of square km) are undergoing a gradual ascent, associated with a deterioration in Stratocumulus clouds in structure, depth, and appearance except that their bases are higher. followed the appearance of Cirrostratus about 70 percent of the time. Sonoma, California of the sky, and precipitation is usually imminent. Western Kansas the weather. Westlake Village, California “Opacus” means the sun’s disk cannot be seen through them while “translucidus” means that it can. Chincoteague Island, Virginia 3A. Altocumulus (castellanus and floccus). These clouds indicate that the mid-level of 3B. Altocumulus (castellanus and floccus). Strands of falling snow extend below cloud 3C. Altocumulus (lenticularis). The presence of these clouds, hovering over the same 3D. Altocumulus (lenticularis), “up close and personal.” This lenticularis cloud was the atmosphere is primed for deep convection. “Floccus” is used when the rounded turrets bases, but no precipitation reaches the ground (called virga). Virga is composed of ice crystals location downwind of a mountain range or peak, indicates strong winds aloft, and an atmosphere downwind of Mount Rainier, Washington. The laminated profile of another one can be seen have a ragged, irregular base rather than a flat one. Sonoma, California and snowflakes. These clouds are likely colder than –10° C. Westlake Village, California that is resisting lift. Both factors suggest that a trough and low are upstream and approaching. that was downwind of another major Cascade volcanic peak, Mount Adams. Mount Rainier, Near Panamint Springs, California Washington 4A. Stratus (nebulosus). When Stratus is overhead, the sky is low, the air damp, and hilltops 4B. Stratocumulus (stratiformis). Stratus clouds can become Stratocumulus clouds as 4C. Stratocumulus (stratiformis). Many widespread Stratocumulus clouds, such as 4D. Nimbostratus. This steadily precipitating cloud covers the entire sky and appears are hidden. The lack of precipitation other than perhaps fine drizzle, signals that it is a thin cloud, the air rises over coastal hills, up and downdrafts enhance and dissipate the “nebulous” Stratus pictured here, are associated with storms, either on their peripheries, or under the rainy clouds diffuse because cloud detail is muted by falling precipitation. The appearance of more cloud and not Nimbostratus. Duncan’s Landing, California layer, producing the look of Stratocumulus. Coastal hills between Bodega Bay and Petaluma, themselves. If they are moving from the northwest and their bases are rising, it is a good sign detail usually means that the deep cloud tops have lowered and the storm is coming to an end. California that the storm is departing. Chincoteague Island, Virginia Cannon Beach, Oregon 5A. Cumulus (humilis). The flattened profile of these clouds is a sign of fair weather. 5B. Cumulus (mediocris). This medium-sized cloud, forming on a still morning, is between 5C. Cumulus (congestus). These heavy looking clouds are tightly packed in convolutions 5D. Cumulonimbus (calvus). A strong rainshaft can be seen on the horizon under this These humble clouds indicate that the air is stable and probably very dry near cloud top; these about 1 and 2 km thick. Its further growth will be determined by the surface heating, the and turrets in which churning motions can be seen. The more obvious the churning motion, the towering cloud whose top has surpassed about 7-8 km ASL. “Calvus” means a fibrous texture is factors prevent further growth. Ballenhausen, Germany temperature and moisture profiles aloft, and whether the air is converging under it. stronger the updrafts, and the more rapidly such clouds will evolve into thunderstorms. not readily apparent in the upper portions of the cloud (e.g., on its right side). Kwajalein Atoll, Near Key West, Florida Near Durango, Mexico Marshall Islands 6A. Cumulonimbus (capillatus). On the West Coast after the passage of a cold front 6B. Cumulonimbus (capillatus incus). Blossoming like a flower, the growth and expansion 6C. Cumulonimbus (mammatus). Thunder grouses continuously among grotesque, 6D. Cumulonimbus. When strong localized shafts of precipitation are seen, the cloud is relatively shallow Cumulonimbus clouds like these often form. The “capillatus” term is used of this group of giant clouds unfolds aloft while hail the size of hen’s eggs and funnel clouds downward-moving bulges called “mammatus”. The weak rainshaft on the horizon at left for identified as a Cumulonimbus even if most of its structure is not visible. In general, the more when a fibrous appearance, usually due to ice, is present. Near Salem, Oregon are unleashed below. “Incus” means that an anvil is present. Parkston, South Dakota such a massive cloud system is a sign that large hail may be exiting through powerful updrafts pronounced the rainshaft, the taller and more bulging is the top that produced it. This sight was aloft. You’ll want to park your car in the garage. Parkston, South Dakota remarkable for its location. Death Valley, California When compared to the earth’s size, the layer of atmosphere where nearly all clouds “Middle-level” clouds, Altostratus and Altocumulus, are shown in rows two and three. Both Stratus and Stratocumulus clouds, except when their tops are colder than about “Congestus” (5C) are the largest Cumulus clouds. They are greater than 2 km (6,600 feet) exist is no thicker than the leather cover on a softball. Within this fragile layer is a myriad of These clouds have bases that range from 2 to 7 km (about 6,600 to 24,000 feet) AGL. –8°C (18°F), are composed solely of droplets. Heavy precipitation does not fall from them, thick and often appear as wide as they are tall. Occasionally these clouds undergo a transition continuously evolving forms of that most ubiquitous and amazing of all substances, water. Altostratus clouds (2B) almost always cover much or all of the sky when they are present and though drizzle (mist-like rain) or light snow can fall. Stratocumulus and Stratus clouds together from a highly detailed, cauliflower look (5C) at cloud top to a smoother, cotton-candy Water, unlike most substances on earth, exists in all three phases—gaseous, liquid, and solid— usually have gray shading throughout. They are primarily composed of ice crystals and cover more of the earth’s surface than any other type of cloud. appearance (5D). This change, which can occur within 10 minutes, is associated with the within the tiny range of atmospheric conditions found here on earth. Due to this versatility, snowflakes. The dominance of ice crystals and snowflakes causes most Altostratus clouds to When it is raining or snowing steadily from widespread dark and gray clouds, you are development of ice crystals or raindrops and can sometimes precede the fall of a sharp shower clouds can suddenly appear and disappear, and precipitate, pelting us with rain and snow. appear diffuse, with little detail (2A). This is because the concentrations of ice crystals and experiencing the underside of Nimbostratus clouds (4D). These clouds (technically considered from the cloud by a few minutes. Thus, a watchful eye on the tops of Cumulus congestus While these events are commonplace to us, in a cosmic sense, they are nothing less than snowflakes in clouds are only about a hundredth to a thousandth that of cloud droplets. “middle-level” clouds), are generally greater than 3 km (10,000 feet) thick, and their tops clouds can lead to the prediction of a shower just before it falls out of the bottom of the cloud! miraculous. And there is still considerable mystery. For example, it is not known how ice crystals Hence, droplet clouds look very sharp-edged while ice crystal clouds, like Cirrus and Altostratus often reach as high as those of Cirrus clouds. The tops of Nimbostratus, if you could strip away Cumulonimbus clouds are the “Tyrannosaurs” of the cloud world. The transition in the form in clouds, and in many clouds there are far more of them than scientists had expected. appear more fibrous or diffuse. Altostratus clouds are nearly always more than 2 km (6,600 feet) the bottom 90% of the cloud, often resembles the clouds shown in 1B, 2B, like solid layers of appearance of the tops of the cumulus clouds to a fibrous appearance, and in the appearance The sky itself, where we can most easily appreciate these miracles, can be a joy, revealing thick even when the sun can be seen through them.