What is the X in X-mas?
1909, from French stratosphère, literally "sphere of layers," coined by French meteorologist Léon-Philippe Teisserenc de Bort (1855-1913) from Latin stratus "a spreading out" (from past participle stem of sternere "to spread out;" see structure (n.)) + French -sphère, as in atmosphère. The region where the temperature increases or remains steady as you go higher. [An earlier stratosphere, attested in English 1908 and coined in German 1901, was a geological term for part of the Earth's crust. It is now obsolete.]
The region of the Earth's atmosphere extending from the tropopause to about 50 km (31 mi) above the Earth's surface. The stratosphere is characterized by the presence of ozone gas (in the ozone layer) and by temperatures which rise slightly with altitude, due to the absorption of ultraviolet radiation. See also exosphere, mesosphere, thermosphere, troposphere., See illustration at atmosphere.