Normally, the temperature decreases with the increase in height from the Earth’s surface, because

the atmosphere can be heated upwards only from the Earth’s surface

there is more moisture in the upper atmosphere

the air is less dense in the upper atmosphere.

Select the correct answer using the codes given below :

Question

Normally, the temperature decreases with the increase in height from the Earth’s surface, because

the atmosphere can be heated upwards only from the Earth’s surface
there is more moisture in the upper atmosphere
the air is less dense in the upper atmosphere.

Select the correct answer using the codes given below :

Accepted Answer

1 and 3 only Quick Verdict: In the troposphere, temperature normally decreases with altitude at an average rate of 6.5°C per kilometer (the Normal Lapse Rate). This occurs because the atmosphere is primarily heated from below by the Earth's surface, and the lower layers contain denser air that is far more capable of absorbing and retaining that heat. Thematic Discussion: The Mechanics of the Normal Lapse Rate When you ascend a mountain or travel upwards through the troposphere, the air becomes progressively colder, dropping at an average rate of 6.5°C for every kilometer of altitude gained. This phenomenon is geographically defined as the Normal Lapse Rate. To understand why this happens, we must look at how the atmosphere interacts with the Earth's surface and the forces of gravity. First, it is a common misconception that the atmosphere is heated directly by the Sun from above. In reality, the atmosphere is largely transparent to incoming shortwave solar radiation. It is the Earth's surface that absorbs this energy and re-emits it as longwave terrestrial radiation. Because the atmosphere can be heated upwards only from the Earth's surface through conduction, convection, and radiation, the air closest to this surface "radiator" absorbs the most heat. This explains why Statement 1 is correct. Furthermore, gravity compresses the atmosphere, pulling the vast majority of gas molecules down toward the surface. As a result, the air at sea level is highly dense and packed with heat-absorbing greenhouse gases. As you climb higher, the air is less dense in the upper atmosphere, and its heat-trapping capacity drastically diminishes, making Statement 3 correct. This concentration of mass near the surface also dictates the distribution of atmospheric moisture. Over 90% of the planet's water vapor—which itself acts as a powerful greenhouse gas—is concentrated in the lower troposphere, right where evaporation from oceans and lakes occurs. Therefore, the claim that there is more moisture in the upper atmosphere is a factual inversion. The sheer lack of moisture at higher altitudes contributes heavily to the freezing temperatures there, rendering Statement 2 incorrect. 💡 The Examiner’s Mindset: The "Factual Inversion" Trap UPSC examiners frequently create incorrect statements by simply flipping a geographical fact upside down. Statement 2 is a classic example. An unprepared student might vaguely remember that "clouds are up high" and assume there is more moisture in the upper atmosphere. The examiner rewards students who know that the hydrological cycle is overwhelmingly concentrated at the surface level. Mains Concept Builder: Normal Lapse Rate vs. Temperature Inversion While the Normal Lapse Rate dictates that temperature decreases with height, there are specific local conditions where this rule flips, known as Temperature Inversion (where temperature increases with height). This typically occurs during long, clear winter nights when the Earth's surface radiates heat away so rapidly that the ground becomes colder than the air immediately above it. Understanding inversion is critical for Mains, as it explains severe winter pollution in cities like Delhi—the warm inversion layer acts like a "lid," trapping cold, dense, smog-filled air near the surface and preventing it from dispersing. 📝 Mains Practice Question (GS Paper 1 - Physical Geography): Explain the concept of the Normal Lapse Rate. Under what geographical conditions does a Temperature Inversion occur, and how does it impact the air quality of urban centers in North India? (150 words, 10 marks) 📚 Standard Sources: Savindra Singh, Physical Geography, Chapter 34 (Temperature). NCERT Class 11 Fundamentals of Physical Geography, Chapter 9 (Solar Radiation, Heat Balance and Temperature).