In meteorology, how does absolute humidity (mass of water vapour per unit volume of air) vary with altitude?

Introduction:
Absolute humidity (AH) quantifies vapour mass per air volume and, unlike relative humidity, is not normalized by saturation capacity. Understanding AH variation with height is important for flight operations, cloud physics, and rainfall formation.

Given Data / Assumptions:

• Altitude increase implies lower pressure and temperature on average.
• Most atmospheric water vapour resides in the lower troposphere.

Concept / Approach:
As pressure falls with height, a fixed vapour mass occupies a larger volume, and cooler temperatures reduce evaporation sources. Consequently, the actual vapour mass within a given volume typically declines with altitude, so AH decreases.

Step-by-Step Solution:
Recognize vertical gradients: pressure ↓, temperature ↓ with altitude.Lower temperatures curb evaporation and support condensation.Therefore, average AH diminishes upward.

Verification / Alternative check:
Profiles from radiosondes show sharp AH decreases above ~3–5 km; most precipitable water is concentrated near the surface.

Why Other Options Are Wrong:

• Increase/Constant: Contradict observed vertical humidity profiles.
• None of these: Not applicable, as decrease is the correct trend.

Common Pitfalls:

• Confusing absolute humidity (per volume) with specific humidity (per mass of air), which varies differently.

Final Answer:
It decreases at higher altitudes.