Water vapour in the troposphere Depending on altitude and temperature, the concentration of water vapour in the troposphere varies from essentially zero up to about what percent (by volume)?

Introduction / Context:
Water vapour is the most variable greenhouse gas in the lower atmosphere. Its concentration controls humidity, cloud formation, and atmospheric stability. Environmental engineers need a sense of its typical bounds to reason about condensation, fogging, and the behaviour of pollutants and aerosols.

Given Data / Assumptions:

• Troposphere conditions (surface to tropopause).
• Cold, high-altitude air can be extremely dry; near-surface tropical air can be very humid.
• Values are approximate volumetric percentages.

Concept / Approach:
Near the surface in warm, humid climates, water vapour commonly reaches several percent by volume; over deserts and at high altitudes, it approaches zero. A widely cited envelope is 0–4% by volume in the troposphere. Hence, the upper bound asked in the question is about 4%.

Step-by-Step Solution:

Verification / Alternative check:
Meteorological references consistently quote 0–4% by volume for tropospheric water vapour, depending on weather and geography.

Why Other Options Are Wrong:

• 1% or 2%: Too low for humid tropical air.
• 8% or 12%: Exceed typical observed atmospheric limits.

Common Pitfalls:
Confusing relative humidity (%) with absolute volumetric fraction; these are related but not the same measure.

Final Answer:
4