Atmospheric layers and temperature lapse rate: in the troposphere (the layer closest to Earth where humans live), the average decrease of air temperature with height is approximately how many degrees Celsius per kilometre?

Introduction / Context:
The troposphere is the lowest atmospheric layer, extending from the surface to the tropopause. Weather phenomena occur here, and temperature typically decreases with altitude at a characteristic average rate known as the environmental lapse rate. Recognizing its approximate magnitude is fundamental for environmental and atmospheric engineering.

Given Data / Assumptions:

• Layer: troposphere.
• Quantity: average temperature decrease per kilometre.
• Global mean representative value is acceptable.

Concept / Approach:
The standard average environmental lapse rate in the troposphere is about 6.5 °C per km, though actual rates vary with humidity, stability, and weather conditions. Very small values like 0.05 °C/km or very large values like 15 °C/km are inconsistent with the typical mean used in engineering calculations.

Step-by-Step Solution:

Verification / Alternative check:
Meteorology references and standard-atmosphere tables use 6.5 °C/km for baseline calculations, including density, pressure, and temperature with height.

Why Other Options Are Wrong:
0.05 °C/km: Far too small; would imply nearly isothermal behavior.
1 °C/km: Too low for a representative mean.
15 °C/km: Unreasonably high for average conditions.

Common Pitfalls:
Confusing dry adiabatic lapse rate (about 9.8 °C/km) with the mean environmental lapse rate; not accounting for moisture and stability effects.

Final Answer:
6.5 °C/km