Etymology
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obnubilate (v.)

"to darken, cloud, overcloud," 1580s, from Latin obnibulatus, past participle of obnubilare "to cover with clouds or fog," from ob "in front of, against" (see ob-) + verb from Latin nubes "cloud," from PIE *sneudh- "fog" (see nuance). Related: Obnubilated; obnubilating. Middle English had obnubilous "obscure, indistinct" (early 15c.).

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rack (n.3)

[clouds driven before the wind], c. 1300, rak, "movement, rapid movement," also "rush of wind, collision, crash," originally a northern word, possibly from Old English racu "cloud, storm" (or an unrecorded Scandinavian cognate of it), reinforced by Old Norse rek, *rak "wreckage, jetsam," or Old English wræc "something driven," both of which would be from Proto-Germanic *wrakaz, from PIE root *wreg- "to push, shove, drive" (see urge (v.)).

From late 14c. as "rain cloud." Often confused with wrack (n.) "destruction," especially in the phrase rack and ruin (1590s), which perhaps is encouraged in that case by the visual alliteration. Rack is "fragments of raggy clouds;" wrack is, in its secondary sense, "seaweed cast up on shore." Both probably come, ultimately, from the same PIE root, as does wreak.

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drizzle (v.)

1540s, transitive, "shed in small drops;" 1560s, intransitive, "fall in very fine particles, as water from the clouds," of uncertain origin. Perhaps it is an alteration of drysning "a falling of dew" (c. 1400), from Old English -drysnian, which is related to dreosan "to fall" (see dreary). Or perhaps it is a frequentative of Middle English dresen "to fall," from Old English dreosan. Related: Drizzled; drizzling.

As a noun, "a light rain, mist," from 1550s.

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disperse (v.)

late 14c., dispersen, "to scatter, separate and send off or drive in different directions," from Latin dispersus, past participle of dispergere "to scatter," from dis- "apart, in every direction" (see dis-) + spargere "to scatter" (see sparse). The Latin word is glossed in Old English by tostregdan. Intransitive sense of "to separate and move apart in different directions without regularity" is from 1520s. Of clouds, fears, etc., "to dissipate," 1560s (transitive), 1590s (intransitive). Related: Dispersed; dispersing.

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cloud (n.)

Old English clud "mass of rock, hill," related to clod.

The modern sense "rain-cloud, mass of evaporated water visible and suspended in the sky" is a metaphoric extension that begins to appear c. 1300 in southern texts, based on similarity of cumulus clouds and rock masses. The usual Old English word for "cloud" was weolcan (see welkin). In Middle English, skie also originally meant "cloud." The last entry for cloud in the original rock mass sense in Middle English Compendium is from c. 1475.

The four fundamental types of cloud classification (cirrus, cumulus, stratus, nimbus) were proposed by British amateur meteorologist Luke Howard (1772-1864) in 1802.

Meaning "cloud-like mass of smoke or dust" is from late 14c. Figuratively, as something that obscures, darkens, threatens, or casts a shadow, from c. 1300; hence under a cloud (c. 1500). In the clouds "removed from earthly things; obscure, fanciful, unreal" is from 1640s. Cloud-compeller translates (poetically) Greek nephelegereta, a Homeric epithet of Zeus.

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swart (adj.)

Old English sweart "black, dark," of night, clouds, also figurative, "wicked, infamous," from Proto-Germanic *swarta- (source also of Old Frisian, Old Saxon, and Middle Dutch swart, Dutch zwart, Old Norse svartr, German schwarz, Gothic swarts "dark-colored, black"), from PIE root *swordo- "dirty, dark, black" (source of sordid). The true Germanic word, surviving in the Continental languages but displaced in English by black. Of skin color of persons from late 14c. Related: Swartest.

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condense (v.)

early 15c., "thicken, make more dense or compact" (implied in condensed), from Old French condenser (14c.) or directly from Latin condensare "to make dense," from assimilated form of com-, here perhaps an intensive prefix (see com-), + densare "make thick," from densus "dense, thick, crowded," a word used of crowds, darkness, clouds, etc. (see dense).

Sense in chemistry and physics, "to reduce to another and denser form" (as a gas or vapor to a liquid) is from 1660s. Intransitive sense "become denser" is from 1650s. Related: Condensed; condensing.

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clear (v.)

mid-14c., "make clear (an obscure subject) in the mind, explain, elucidate;" late 14c., "make clean, cleanse, purify; clarify (a liquid), remove what clouds or diminishes brightness or transparency;" also "prove innocent, vindicate;" of the weather, sea, sky, clouds, etc., "clear up, become fair or calm;" from clear (adj.).  Related: Cleared; clearing.

Intransitive sense of "become free from murkiness" is from 1580s. Meaning "to free from obstructions" is from 1520s; that of "to free from entanglement" is from 1590s; that of "pass (an obstacle) without entanglement or collision" is from 1630s. Sense of "to remove (something) out of the way" is from 1670s; that of "to clear land of trees and underbrush" is from 1690s. Meaning "to leap clear over" is first attested 1791. Meaning "to gain (a sum of money) in clear profit" is from 1719. Meaning "get approval for (a proposal, etc.) from authority" is from 1944; meaning "establish as suitable for national security work" is from 1948.

To clear (one's) throat is from 1881; earlier clear (one's) voice (1701). To clear out "depart, leave" (1825), perhaps is from the notion of ships satisfying customs, harbor regulations, etc., then setting sail. To get clear of is from 1590s. To clear up is from 1620s of weather, 1690s as "make clear to the mind." Clear the deck (1802) is from sailing ships. Clear the air in the figurative sense is from late 14c. To clear the coast (1520s) was to make it suitable for landing.

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nebula (n.)

mid-15c., nebule "a cloud, mist," from Latin nebula, plural nebulae, "mist, vapor, fog, smoke, exhalation," figuratively "darkness, obscurity," from PIE root *nebh- "cloud."

Re-borrowed from Latin 1660s in sense of "cataracts in the eye;" astronomical meaning "luminous cloud-like patch in the heavens" is from c. 1730. As early as Herschel (1802) astronomers realized that some nebulae were star clusters, but the certain distinction of relatively nearby cosmic gas clouds from distant galaxies (as these are now properly called) was not made until the 1920s, when the latter were resolved into individual stars (and nebulae) using the new 100-inch Mt. Wilson telescope.

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geology (n.)

1795 as "science of the past and present condition of the Earth's crust," from Modern Latin geologia "the study of the earth," from geo- "earth" + logia (see -logy). German Geologie is attested by 1785. In Medieval Latin, geologia (14c.) meant "study of earthly things," i.e. law, as distinguished from arts and sciences, which concern the works of God. Darwin used geologize as a verb.

There rolls the deep where grew the tree.
      O earth, what changes hast thou seen!
      There where the long street roars, hath been
The stillness of the central sea.
The hills are shadows, and they flow
      From form to form, and nothing stands;
      They melt like mist, the solid lands,
Like clouds they shape themselves and go. 
[from "In Memoriam," 1850]
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