Definition
The fourth major layer of Earth's atmosphere, lying above the mesosphere and beginning at roughly 50 miles (80 km) above the surface. In this layer, temperature rises sharply with increasing altitude due to absorption of high-energy solar radiation by the very thin air.
Plain English
The high-altitude layer of the atmosphere where the air gets hotter the higher you go, because it absorbs strong radiation from the sun.
Context Anchor
Seen in atmosphere-layer discussions in the Instrument Flying Handbook, especially when separating normal flying weather from the much higher layers of the atmosphere.
Derivation
From Greek 'thermos' meaning 'hot' and 'sphaira' meaning 'sphere' or 'ball.' Literally the 'hot sphere' — fitting, because temperatures in this layer can climb to thousands of degrees, even though the air is so thin you wouldn't feel the heat.
Why Pilots Care
Pilots do not operate airplanes in the thermosphere, but knowing where it is helps keep the layers of the atmosphere straight and makes clear that clouds, turbulence, icing, and normal flight weather occur much lower.
Grounding Statement
Picture the thermosphere as the thin upper edge of the atmosphere, close to space, where sunlight heats the few air particles that are still present.
Intuition Check
Do not assume higher always means colder. In the thermosphere, temperature increases with altitude because the thin air absorbs strong energy from the Sun.
Example Sentence 1
The thermosphere sits well above the altitudes used by civil aviation, beginning around 50 miles up.
Example Sentence 2
Solar activity that heats the thermosphere can slightly change the density of the air at the very top of the atmosphere.