The correct option is 6.5°C per 1,000 m.
Explanation
The atmosphere is primarily heated by terrestrial radiation emitted from the Earth's surface rather than directly by incoming solar radiation. Consequently, in the troposphere, temperature typically decreases as altitude increases. This vertical gradient of temperature is quantified as the Normal Lapse Rate.Detailed Analysis:
- Definition: The Normal Lapse Rate is the average rate at which the temperature of the atmosphere drops with an increase in altitude under normal conditions.
- Standard Value: According to standard physical geography, the Normal Lapse Rate is defined as 6.5°C per 1,000 meters (or 1 km).
- Comparison with 1°C per 165 m: While 1°C per 165 meters is a frequently cited approximation in general contexts (which equates to roughly 6.06°C per 1,000 meters), the scientifically standardized value used in meteorology and the International Standard Atmosphere (ISA) models is 6.5°C per 1,000 meters. Therefore, 6.5°C per 1,000 m is the precise correct answer.
- Distinction: This rate applies to the static environment and is distinct from the Adiabatic Lapse Rate (Dry or Wet), which applies to moving (rising or sinking) air parcels.
Key Takeaway:
The Normal Lapse Rate represents the average vertical temperature decrease in the troposphere, standardized at 6.5°C for every 1,000 meters of ascent.