stratum + sphaira

Stratosphere combines Latin stratum ('a spreading' or 'a layer,' plural strata) with Greek sphaira ('sphere'). Stratum comes from the verb sternere ('to spread'). Geological layers — sedimentary strata laid one atop another — gave the term weight. The Greeks had sphaira for the heavens: the spheres of Aristotle, concentric and perfect. Combining them: a layer-sphere, a stratified dome above the Earth.

Léon Teisserenc de Bort, a French meteorologist, made the discovery from his observatory at Trappes, a village 30 kilometers west of Paris. Between 1898 and 1902, he launched unmanned balloons fitted with thermometers and barometers. They rose through the troposphere — the layer where weather happens. Then they passed into a new region where the temperature stopped falling. The air was stable, layered, almost still. In 1902, he named it the stratosphere.

The term caught immediately. The troposphere ('turning sphere') was where weather turned and mixed and fought. The stratosphere was where the air settled into strata, where the chaos above simplified into order. The discovery was elegant: a layer exactly 10 to 50 kilometers up, colder at the bottom, warmer at the top (due to ozone), where jet planes would eventually cruise in silence. Teisserenc de Bort had mapped a boundary invisible to the naked eye.

Today the stratosphere is famous chiefly for its ozone layer, which protects us from ultraviolet radiation. The irony: a layer Teisserenc de Bort discovered by watching balloons rise has become a symbol of human fragility. We punched a hole in it with chlorofluorocarbons. The very stability he marveled at is what gives that hole such dangerous consequence — when something changes there, it stays changed.