The correct option is Lower density of air at higher elevations..
Explanation
The phenomenon of temperature decreasing with an increase in altitude is known as the Normal Lapse Rate. In the troposphere, the temperature generally drops at a rate of approximately 6.5°C per 1,000 meters of ascent.
Detailed Analysis:
- Atmospheric Heating Source: The atmosphere is primarily heated from below by terrestrial radiation (longwave radiation emitted by the Earth's surface) rather than directly by incoming solar radiation. Therefore, the layers of air closest to the Earth's surface are heated the most.
- Air Density and Composition: As altitude increases, the air becomes less dense (rarified). The lower layers of the atmosphere contain a higher concentration of dust particles, water vapor, and greenhouse gases (like carbon dioxide), which are effective at absorbing and retaining heat. At higher elevations, the air is thinner and contains fewer of these heat-absorbing agents, leading to lower temperatures.
- Analysis of Incorrect Options:
- High humidity at higher elevations. High humidity: Higher elevations generally have lower humidity because cold air has a lower moisture-holding capacity.
- Proximity to the sun at higher elevations. Proximity to the sun: The variation in distance to the sun between sea level and mountain peaks is negligible compared to the total distance from the sun, making it irrelevant to local temperature differences.
- Stronger winds at higher elevations. Stronger winds: While winds may be stronger at altitude, they are a dynamic feature of weather, not the fundamental thermodynamic cause of the vertical temperature gradient.
Key Takeaway:
Temperature decreases with altitude primarily because the atmosphere is heated by the Earth's surface (terrestrial radiation) and the density of air (and its heat-retention capacity) diminishes with height.