stratosphere (n.)

a re-Latinized spelling, attested beginning mid-15c., of Middle English spere (c. 1300) "cosmos; space, conceived as a hollow globe about the world," from Anglo-French espiere, Old French espere (13c., Modern French sphère), from Latin sphaera "globe, ball, celestial sphere" (Medieval Latin spera), from Greek sphaira "globe, ball, playing ball, terrestrial globe," a word of unknown origin.

According to Beekes there are no certain cognates outside Greek, but the Greek word also has been borrowed into Syrian (espero), Ethiopian (spir), Armenian (sp'er), and (non-Indo-European) Georgian (spero).

From late 14c. in reference to any of the supposed concentric, transparent, hollow, crystalline globes of the cosmos believed to revolve around the earth and contain the planets and the fixed stars; the supposed harmonious sound they made rubbing against one another was the music of the spheres (late 14c.), Milton's sphery chime.

Also from late 14c. in the general sense of "a globe; object of spherical form, a ball," and in the geometric sense of "solid figure with all points equidistant from the center." The meaning "range of something, place or scene" of activity, knowledge, etc. is recorded c. 1600 (as in sphere of influence, 1885, originally in reference to Anglo-German colonial rivalry in Africa).

*sterə-, also *ster-, Proto-Indo-European root meaning "to spread."

It forms all or part of: consternate; consternation; construct; construction; destroy; destruction; industry; instruct; instruction; instrument; obstruct; obstruction; perestroika; prostrate; sternum; sternocleidomastoid; strain (n.2) "race, stock, line;" stratagem; strategy; strath; strato-; stratocracy; stratography; stratosphere; stratum; stratus; straw; stray; street; strew; stroma; structure; substrate; substratum; substructure.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit strnoti "strews, throws down;" Avestan star- "to spread out, stretch out;" Greek stronymi "strew," stroma "bedding, mattress," sternon "breast, breastbone;" Latin sternere "to stretch, extend;" Old Church Slavonic stira, streti "spread," strana "area, region, country;" Russian stroji "order;" Gothic straujan, Old High German strouwen, Old English streowian "to sprinkle, strew;" Old English streon "strain," streaw "straw, that which is scattered;" Old High German stirna "forehead," strala "arrow, lightning bolt;" Old Irish fo-sernaim "spread out," srath "a wide river valley;" Welsh srat "plain."

before vowels strat-, word-forming element of Latin origin referring to layers or layering, also stratus clouds, from combining form of Latin stratus "a spreading" (from PIE root *stere- "to spread"). In later use often short for stratosphere, in reference to high altitudes.