In climatology, which reference level is commonly treated as the standard zero for altitude measurements?

Concept overview / definitionIn basic climatology, a common reference level is needed to describe height (altitude) consistently. Sea level is widely used as the standard reference, treated as “zero level,” and heights are measured above it.

Why the correct option is correctThe idea is to anchor altitude to a uniform baseline. Sea level provides a practical global reference, so when we say a place is at higher altitude, it means it is above this standard zero level.

Why the other options are incorrectGround level varies from place to place and cannot serve as a single standard reference. Cloud base, tropopause, or any atmospheric layer height is not fixed globally and also changes with latitude, season, and weather, so they are unreliable as a universal zero.

UPSC exam tip / common confusionUPSC often mixes “altitude” and “height.” Remember: altitude is usually referenced to mean sea level, while local height can be relative to nearby terrain. Fix the reference first, then apply logic.