1 Clouds occur in a wide variety of forms because they are shaped by many processes operating in the atmosphere. The basic requirements for cloud formation are moist air and a lifting process. When humid air is lifted by rising currents, the water vapor eventually reaches its dew point, the temperature at which it condenses into a collection of water droplets. From the ground, we see these tiny particles as a cloud. If the droplets continue to acquire moisture and grow large enough, they fall from the cloud as rain or snow.
2 Clouds are classified according to their form, shape, and altitude, and by whether or not they produce precipitation. Based on form and shape, the simplest distinction is between clouds that have a heaped or a layered look. Based on altitude, the most common clouds are grouped into four families: low clouds, middle clouds, high clouds, and clouds exhibiting vertical development. Low, middle, and high clouds are produced by gentle uplift of air over broad areas. In contrast, those with vertical development generally cover smaller areas and are associated with much more vigorous uplift.
3 Among the first scientists to identify the importance of clouds in meteorology was British naturalist Luke Howard, who was also the first to devise a system for grouping clouds. Howard classified clouds by their appearance—heaped or layered—and named the three principal categories of clouds cumulus, stratus, and cirrus. He also described several intermediate and compound modifications in order to accommodate the transitions occurring between the forms. Developed in 1803, the essentials of Howard"s classification scheme are still in use today. Contemporary weather forecasters continue to divide clouds into two main groups: heaped clouds, resulting from rising unstable air currents; and layered clouds, resulting from stable air currents.
4 Most heaped clouds are cumulus: puffy white clouds capped with a cauliflower-like dome created by convection, the transfer of heat by moving air. Cumulus clouds form at low or middle altitudes, with bases ranging from 1,000 to 10,000 feet (303 to 3,033 meters) above the earth. Low-level cumulus clouds are detached from one another and generally have well-defined bases. Their outlines are sharp, and they often develop vertically in the form of rising puffs, mounds, domes, or towers. The sunlit parts are brilliant white; the base is relatively dark and roughly horizontal. Cumulus clouds are commonly known as fair-weather clouds because they usually do not produce precipitation. However, a variation known as cumulonimbus can reach great heights and have dense, glaciated tops that cause heavy rain, hail, or thunderstorms.
5 Stratus, or layered, clouds appear as large sheets, or strata, with minimal vertical and extended horizontal dimensions. The air around them is stable, with little or no convection present. Stratus clouds spread laterally to form layers that sometimes cover the entire sky, to the horizon and beyond, like a formless blanket. Stratus clouds have a relatively low base. They can be a uniform gray sheet, or a layer of patchy, shapeless, low gray clouds. The layer may be thin enough for the sun to shine through, but stratus clouds may also carry drizzle or snow, in which case they are called nimbostratus.
6 While cumulus and stratus clouds generally form at low or middle altitudes, cirrus clouds form at high altitudes. Cirrus clouds are detached clouds that take the form of delicate white filaments, strands, or hooks. These clouds can be seen at close hand from the window of a jet plane flying above 25,000 feet (7,600 meters). When viewed from the ground, bands of threadlike cirrus clouds often seem to emerge from a single point on the western horizon and spread across the entire sky. Cirrus clouds are composed almost exclusively of ice crystals. Their fibrous appearance results from the wind "stretching" streamers of falling ice particles into feathery strands called "mares" tails." Snow crystals may fall from thicker, darker cirrus clouds, but they usually evaporate in the drier air below the cloud.
7 Among the several compound cloud types described by Luke Howard are cirrocumulus and cirrostratus clouds. Cirrocumulus clouds appear as small, well-defined white flakes or cotton patches in close horizontal arrangement. Cirrostratus clouds are a thin, white layer resembling a veil. Because of their ice content, cirrostratus clouds are associated with the halos appearing around the sun or moon.

单选题 According to paragraph 1, what happens at the dew point?

单选题 What is the main purpose of paragraph 2?

单选题 The word vigorous in paragraph 2 is closest in meaning to

单选题 The author discusses Luke Howard in paragraph 3 in order to

单选题 According to paragraph 3, Luke Howard"s system of cloud classification acknowledged that

单选题 Cumulus clouds are characterized by all of the following EXCEPT

单选题 The word sharp in paragraph 4 is closest in meaning to

单选题 The word laterally in paragraph 5 is closest in meaning to

单选题 It can be inferred from paragraph 5 that stratus clouds

单选题 The word fibrous in paragraph 6 is closest in meaning to

单选题 According to the passage, in what way are stratus and cirrostratus clouds similar?

填空题 Look at the four squares, A, B, C, and D, which indicate where the following sentence could be added to the passage. Where would the sentence best fit?
These strands often warn of the approach of a warm front signaling the advance of a storm system.
While cumulus and stratus clouds generally form at low or middle altitudes, cirrus clouds form at high altitudes. A Cirrus clouds are detached clouds that take the form of delicate white filaments, strands, or hooks. These clouds can be seen at close hand from the window of a jet plane flying above 25,000 feet (7,600 meters). B When viewed from the ground, bands of threadlike cirrus clouds often seem to emerge from a single point on the western horizon and spread across the entire sky. Cirrus clouds are composed almost exclusively of ice crystals. C Their fibrous appearance results from the wind "stretching" streamers of falling ice particles into feathery strands called "mares" tails." D Snow crystals may fall from thicker, darker cirrus clouds, but they usually evaporate in the drier air below the cloud.

填空题 Select the appropriate sentences from the answer choices and match them to the type of cloud that they describe. TWO of the answer choices will NOT be used. This question is worth 3 points.

Heaped Clouds

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Layered Clouds

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A. They form where air is stable and no convection occurs.
B. They fall when water droplets in the air grow very large and heavy.
C. They develop vertically in the shape of domes, mounds, or towers.
D. They have extended vertical and minimal horizontal dimensions.
E. They are associated with rising unstable air currents.
F. They can spread out like a blanket covering the whole sky.
G. They have a fluffy white top and a flatter, darker bottom.