Evening sun is not as hot as the mid day sun. What is the reason?

1. Introduction to Insolation and Solar Constant (basic)

To understand how our planet stays warm, we must start with Insolation — a portmanteau of 'Incoming Solar Radiation.' At its simplest, this is the solar energy that reaches the Earth. The Sun radiates energy in the form of short-wave electromagnetic radiation, primarily in the ultraviolet and visible spectrums Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282. While the Sun emits a staggering amount of energy, the Earth, being a tiny speck 150 million kilometers away, intercepts only a minute fraction of it. This energy is the primary driver of our weather, climate, and life itself. To measure this energy consistently, scientists use a benchmark called the Solar Constant. This is the amount of solar energy received per unit area (per square centimeter) per minute at the top of the atmosphere. It is valued at approximately 1.94 calories per square centimeter per minute (or roughly 1361 Watts/m²). We measure it at the top of the atmosphere because once the rays enter our air, they are scattered by molecules (Rayleigh scattering) and absorbed by water vapor and ozone, which would skew the baseline reading FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.68. The amount of insolation actually reaching the surface is not uniform across the globe. It varies based on several factors, most notably the angle of inclination of the sun's rays. When the sun is directly overhead (midday), the rays strike at a 90° angle, concentrating energy over a small area and traveling a shorter path through the atmosphere. Conversely, at the poles or during sunset, the rays strike at a slant angle. This 'slanted' delivery spreads the same amount of energy over a much larger surface area and forces the radiation to pass through a thicker layer of the atmosphere, leading to greater loss of energy through reflection and absorption FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.67. Interestingly, the maximum insolation is not recorded at the Equator, but over subtropical deserts. This is because the Equator experiences frequent cloud cover which reflects sunlight away, whereas the clear skies of the subtropics allow more direct radiation to hit the ground FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.68. Generally, land surfaces also heat up more quickly and receive more insolation than oceans at the same latitude due to differences in transparency and specific heat.

Sources: Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.67-68

2. Atmospheric Composition and Radiation Interaction (basic)

To understand why the sun feels different at different times of the day, we must first look at what the atmosphere is made of. Our atmosphere is a complex mixture of gases—primarily Nitrogen (78%) and Oxygen (21%)—along with trace gases like Argon (0.934%) and Carbon Dioxide (0.03%) Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6. Beyond these gases, it contains "variable" components like water vapour, dust particles, and ozone, which play the most critical role in interacting with solar radiation.

When solar radiation enters this atmosphere, it doesn't just pass through cleanly; it undergoes attenuation (weakening) through three main processes:

• Absorption: Molecules like Ozone (O₃) are specialists, absorbing dangerous ultraviolet radiation in the 0.1 to 0.3-micron range Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.11. Water vapour and CO₂ also absorb infrared radiation, contributing to the greenhouse effect Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283.
• Scattering: This happens when light hits small gas molecules. If the wavelength of light is larger than the particle, it scatters—a process called Rayleigh scattering, which gives us our blue sky Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283.
• Reflection: Larger particles like dust, soot, and clouds (aerosols) act like tiny mirrors, reflecting energy back into space, which generally has a cooling effect on the Earth Environment, Shankar IAS Academy (ed 10th), Climate Change, p.259.

The intensity of heat you feel depends on the angle of inclination of the sun's rays. At midday, the sun is directly overhead, and the rays take the shortest, most direct path through the atmosphere. However, during the evening, the sun is near the horizon, and its rays must travel through a much thicker layer of the atmosphere at a slant FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8: Solar Radiation, Heat Balance and Temperature, p.67. This increased path length means there are far more molecules and particles in the way to scatter and absorb the energy before it reaches you, making the evening sun feel much milder.

Feature	Midday Sun	Evening Sun
Angle of Rays	Vertical / Overhead	Slanting / Low Angle
Atmospheric Path	Shortest distance	Longer distance
Energy Loss	Minimal scattering/absorption	High attenuation (energy loss)
Area Concentration	Concentrated on a small area	Spread over a large area

Sources: Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.11; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283; Environment, Shankar IAS Academy (ed 10th), Climate Change, p.259; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8: Solar Radiation, Heat Balance and Temperature, p.67-68

3. The Earth's Heat Budget and Albedo (intermediate)

To understand the Earth's Heat Budget, imagine the planet as a household managing its finances. The "income" is the solar radiation (insolation) arriving from the Sun, while the "expenditure" is the heat the Earth radiates back into space. For the Earth to maintain a stable average temperature over time, its total income must exactly match its total expenditure. If it kept more than it gave away, the planet would heat up indefinitely; if it lost more, it would freeze. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Chapter 8, p.69.

The first crucial concept in this budget is Albedo. Before the Sun's energy can even begin to warm the ground, a significant portion is reflected directly back into space. Albedo is the measure of a surface's reflectivity, expressed as a percentage or a fraction. For instance, fresh snow has a very high albedo (reflecting up to 70-90% of light), while dark oceans have a very low albedo, absorbing most of the heat. Physical Geography by PMF IAS, Chapter 19, p.283. On average, the Earth's albedo is about 35%. This means out of 100 units of incoming solar energy, 35 units are reflected back by clouds, ice, and the atmosphere before they can contribute to the planet's temperature.

The remaining 65 units are absorbed: 14 units by the atmosphere and 51 units by the Earth’s surface. However, the Earth does not keep this energy. It eventually radiates all 65 units back into space as longwave terrestrial radiation. This complex exchange—where incoming shortwave radiation is balanced by outgoing longwave radiation—is what we call the Heat Balance of the Earth-atmosphere system. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Chapter 8, p.74.

Surface Type	Albedo Characteristic	Effect on Heat Budget
Fresh Snow / Ice	High (70-90%)	Reflects most energy; keeps the surface cool.
Thick Clouds	High (70-80%)	Major contributor to the Earth's 35% reflection.
Dark Soil / Forests	Low (5-15%)	Absorbs most energy; contributes to warming.
Water Bodies	Very Low (2-10%)	Absorbs high amounts of solar radiation.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Chapter 8: Solar Radiation, Heat Balance and Temperature, p.69; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.283; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Chapter 8: Solar Radiation, Heat Balance and Temperature, p.74

4. Latitudinal Heat Balance and Seasonal Variations (intermediate)

At its core, the Earth is a giant heat engine that constantly seeks balance. Because our planet is a sphere tilted at an angle of 23.5° (or 66½° to its orbital plane), solar energy—known as insolation—is not distributed equally. In the tropics, the sun's rays hit the surface almost vertically, traveling a short distance through the atmosphere. Conversely, near the poles, the rays strike at a slant, passing through a much thicker layer of the atmosphere where they are scattered by molecules (Rayleigh scattering) and absorbed by water vapor and ozone FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8, p. 67. This creates a fundamental imbalance: the Earth receives far more energy at the equator than at the poles.

This imbalance leads to the concept of the Latitudinal Heat Balance. If we look at the Earth as a whole, the regions between 40° North and 40° South receive more solar radiation than they lose back to space, creating a net radiation surplus. Regions beyond 40° latitude toward the poles lose more heat through terrestrial radiation than they gain from the sun, resulting in a net radiation deficit Physical Geography by PMF IAS, Chapter 19, p. 293. Without a mechanism to fix this, the tropics would boil and the poles would freeze solid. However, our planet uses atmospheric circulation (winds) and ocean currents as a global conveyor belt to transfer excess heat from the surplus zones to the deficit zones, maintaining the habitable temperatures we enjoy today FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8, p. 70.

Region	Radiation Status	Primary Characteristic
0° to 40° N/S	Heat Surplus	Insolation gained > Heat lost to space.
40° to 90° N/S	Heat Deficit	Heat lost to space > Insolation gained.

Seasonality further complicates this picture. As the Earth orbits the sun, the sub-solar point (where the sun is directly overhead) shifts between the Tropics of Cancer and Capricorn. This shift changes the angle of inclination and the length of the day throughout the year. For instance, during summer in the Northern Hemisphere, the middle and higher latitudes receive significantly more radiation due to longer day-lengths and a more direct solar angle compared to the winter months FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8, p. 68.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8: Solar Radiation, Heat Balance and Temperature, p.67; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8: Solar Radiation, Heat Balance and Temperature, p.68; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8: Solar Radiation, Heat Balance and Temperature, p.70; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.293

5. Atmospheric Optics: Rayleigh Scattering (intermediate)

To understand Rayleigh Scattering, we must first look at the nature of light and the 'obstacles' it encounters in our atmosphere. Sunlight is composed of a spectrum of colors, each with a different wavelength. When this light enters the atmosphere, it interacts with gas molecules (like Nitrogen and Oxygen) and fine particles that are significantly smaller than the wavelength of visible light. According to the principles of atmospheric optics, these tiny particles are much more effective at scattering shorter wavelengths (the blue end of the spectrum) than longer wavelengths (the red end) Science, Class X (NCERT 2025 ed.), Chapter 10, p.169. In fact, red light has a wavelength about 1.8 times greater than blue light, meaning blue light is scattered with much greater intensity in all directions, which is why the clear sky appears blue to our eyes.

The thickness of the atmosphere that light must penetrate—the path length—is the second critical factor. During midday, the sun is directly overhead, and the rays travel the shortest distance through the atmosphere. However, during sunrise or sunset, the sun is near the horizon, and its rays must travel a much longer, slanted path through the densest layers of the atmosphere Fundamentals of Physical Geography, Class XI (NCERT 2025 ed.), Chapter 8, p.67. By the time this light reaches your eyes, most of the shorter blue wavelengths have been scattered away and lost from your line of sight, leaving behind the longer-wavelength reds and oranges to dominate the horizon.

It is important to distinguish Rayleigh scattering from other atmospheric interactions. While Rayleigh scattering involves particles smaller than the wavelength of light, larger particles like dust or water droplets behave differently. If the obstructing particle is larger than the wavelength, reflection or Mie scattering (which is not wavelength-dependent, explaining why clouds look white) occurs instead Physical Geography by PMF IAS, Chapter 19, p.283. Additionally, certain gases like ozone and water vapor don't just scatter light; they absorb specific portions of solar radiation, further attenuating the energy that reaches the surface.

Interaction Type	Particle Size	Resulting Effect
Rayleigh Scattering	Smaller than wavelength	Blue sky, red sunsets (selective scattering)
Reflection/Mie	Larger than wavelength	White clouds, hazy/grey sky (non-selective)
Absorption	Varies (Molecules)	Energy loss, greenhouse effect (O₃, CO₂, H₂O)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.169; Fundamentals of Physical Geography, Class XI (NCERT 2025 ed.), Chapter 8: Solar Radiation, Heat Balance and Temperature, p.67-68; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.283

6. Solar Angle of Inclination and Path Length (exam-level)

When we talk about the heat we feel from the sun, we are discussing insolation (incoming solar radiation). The intensity of this energy is not uniform across the globe; it is primarily governed by the angle of inclination — the angle at which the sun's rays strike the Earth's surface. At the equator or during high noon, the sun is nearly vertical. These vertical rays are concentrated over a smaller surface area, delivering high energy per unit. Conversely, as we move toward the poles or toward sunset, the rays become "slant." These slant rays must spread their energy over a much larger area, which naturally reduces the net energy received at any single point FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8, p. 68.

Beyond surface area, there is a crucial atmospheric factor: the path length. Because the Earth is enveloped in a thick layer of gases, solar radiation must pass through the atmosphere before reaching us. When the sun is directly overhead, it takes the shortest possible path. However, when the sun is at a low angle (near the horizon), its rays must travel through a significantly greater depth of the atmosphere. This longer journey forces the radiation to interact more with air molecules, water vapor, and dust particles FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8, p. 68.

As this path length increases, the radiation undergoes more attenuation — a weakening of signal. This happens through three main processes: absorption by gases like ozone and water vapor, diffusion, and scattering. For instance, Rayleigh scattering is more pronounced over long path lengths, which is why the sky appears red at sunset but the sun feels significantly cooler than at midday Science, class X (NCERT 2025 ed.), Chapter 10, p. 169. Additionally, the atmosphere acts as an optical lens; at low angles, refraction bends the light, making the sun appear above the horizon even when it is technically below it, effectively lengthening our daylight hours Physical Geography by PMF IAS, Chapter 19, p. 255.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 8: Solar Radiation, Heat Balance and Temperature, p.67-68; Physical Geography by PMF IAS, Chapter 19: The Motions of The Earth and Their Effects, p.255; Science, class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.169

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes your recently learned concepts of insolation and the angle of inclination. As you have studied in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), the intensity of solar energy reaching the Earth's surface is primarily determined by the angle at which the sun's rays hit the atmosphere. When the sun is directly overhead at midday, the rays are vertical and take the most direct, shortest route to the surface. However, as the sun approaches the horizon in the evening, those same rays arrive at a slant, necessitating a much longer journey through the atmospheric layers.

To reach the correct answer, (D) In the evening, radiations travel larger distance through atmosphere, you must visualize the atmospheric column as a filter. Because the evening path is significantly longer, the solar radiation undergoes much more Rayleigh scattering and absorption by atmospheric gases and water vapor before it ever reaches you. As detailed in Science, class X (NCERT 2025 ed.), this scattering disperses energy in multiple directions, effectively "thinning out" the heat and light that actually makes it to the ground. Essentially, the atmosphere acts like a thicker blanket in the evening than it does at noon.

UPSC frequently includes distractors that sound plausible but violate basic physics. Option (A) is a trap because the speed of light (radiation) is constant and does not "slow down" in the evening. Option (B) is a common misconception; the Sun's actual surface temperature remains constant regardless of the time of day on Earth. While Option (C) correctly identifies that Ozone absorbs radiation, it is a partial truth used as a distractor; the primary physical reason for the drop in temperature is the geometric path length through the entire atmosphere, not just the action of the ozone layer alone.