Solar radiation and insolation

Detail

1. The Earth's energy comes from the Sun

• The Earth gets almost all its energy from the Sun.
• Over a long period, the Earth sends the same amount of energy back to space. So, taken as a whole, it neither warms up nor cools down.
• NASA calls this state radiative equilibrium: the energy coming in from the Sun equals the heat going out to space, so the global temperature stays fairly steady [3].
• This balance is now slightly upset. Greenhouse gases are trapping a little extra heat. The later section on the heat budget covers this.

2. Insolation

• Insolation (incoming solar radiation): the energy from the Sun that reaches the Earth.
• It arrives mostly as short waves: visible light, plus some ultraviolet and near-infrared.
• The Earth catches only a tiny share of the Sun's output:
• The Earth is small and very far from the Sun.

• It catches only about one two-billionth of all the energy the Sun gives out.

• The rays strike at a slant:

• The Earth is nearly a sphere (a geoid).
• So the Sun's rays hit the top of the atmosphere at a slant almost everywhere. They hit straight down only where the Sun is directly overhead.
• Slanting rays spread the same energy over a larger area, so each square metre gets less.

3. Solar constant

• Solar constant: the energy that falls on a surface facing the Sun's rays straight on (at a right angle), at the top of the atmosphere, when the Earth is at its average distance from the Sun.
• Values:

Source	Value	Note
NCERT	1.94 calories per sq cm per minute	about 1,353 W/sq m
Older satellite estimate (before SORCE)	1,366 W/sq m	later revised downward [2]
SORCE satellite (Kopp and Lean, 2011)	about 1,361 W/sq m	the accepted modern value [2]
TSIS-1 instrument, 2019 solar minimum	1,361.6 ± 0.3 W/sq m	the latest measurement [2]

• Today scientists usually call it Total Solar Irradiance (TSI): all the Sun's energy, across all wavelengths, falling on one square metre at the top of the atmosphere [2].
• Steady satellite measurements of TSI began in 1978 [2].

Why the global average is only about 340 W/sq m

• Averaged over the whole globe, day and night, the energy arriving is about 340 W/sq m. This is one quarter of the solar constant [2][3].
• Why it is one quarter:
• Only half the Earth is lit at any moment. The night side gets nothing. This halves the average [3].
• On the lit half, the rays come in at a slant that grows towards the poles. This halves it again [3].

• Put simply in geometry: the Earth catches sunlight on a flat disc of area πr², but spreads it over a sphere of area 4πr². That is one quarter.

• What happens to those 340 W/sq m (NASA Earth Observatory) [3]:

Fate of incoming sunlight	Share
Reflected back to space (mainly by clouds, also bright surfaces and the air itself)	29%
Absorbed by the atmosphere (water vapour, dust, ozone)	23%
Absorbed by the surface (land and ocean)	48%
Total absorbed by the Earth system	71%, about 240 W/sq m

• Albedo (the share of sunlight a surface or planet reflects): the Earth's is about 0.29 (29%) [3].

The "constant" is not truly constant

• Sunspot cycle:
• Sunspots are dark, cooler patches on the Sun. Their number rises and falls over about 11 years.

• The solar constant changes by only about 0.1% over one 11-year cycle [2].

• Earth-Sun distance:

• The energy received changes by about 7% between perihelion (nearest point) and aphelion (farthest point).

• Energy falls with the square of the distance (the inverse-square law): (152 ÷ 147)² ≈ 1.07, which is about 7%.

• So distance causes a far bigger change through the year than the sunspot cycle does.

4. Planck's law: shortwave and longwave radiation

• Planck's law (NCERT Class 11 box): every body gives off radiation. The hotter the body, the more energy it gives off, and the shorter its wavelength.
• Two simpler laws come out of this idea (beyond NCERT):

Law	What it says	Formula	Meaning for Sun vs Earth
Stefan-Boltzmann law	Energy given off rises with the fourth power of the absolute temperature (in kelvin)	E = σT⁴	The Sun's surface is about 20 times hotter than the Earth (5,800 K vs 288 K). 20⁴ = 160,000, so each square metre of the Sun gives off about 160,000 times more energy
Wien's displacement law	The peak wavelength gets shorter as temperature rises	λmax ≈ 2,898 µm·K ÷ T	Sun: 2,898 ÷ 5,800 ≈ 0.5 µm. Earth: 2,898 ÷ 288 ≈ 10 µm

• Absolute temperature: temperature measured in kelvin (K), counted from absolute zero (−273 °C). 288 K = 15 °C, the Earth's average surface temperature.
• µm (micrometre): one-millionth of a metre.

Shortwave vs longwave

	Sun	Earth
Surface temperature	about 5,800 K (about 5,500 °C) [3]	about 288 K (15 °C)
Peak wavelength	about 0.5 µm, visible light (with near-infrared) [3]	about 10 µm, thermal infrared (NASA gives a peak near 12.5 µm [3])
Name	Shortwave radiation	Longwave (terrestrial) radiation
What the atmosphere does	Mostly lets it pass (it is largely transparent)	Absorbs much of it: water vapour and CO₂ take it in
Result	Sunlight heats the ground first, not the air	The air is heated from below, by the ground

• Step by step:
• Short waves from the Sun pass through the air → the ground absorbs them and warms up.
• The warm ground gives off long waves (infrared) → water vapour, CO₂ and other gases absorb them.
• These gases send part of the heat back down → the lower air stays warm. This is the greenhouse effect.
• This is also why temperature normally falls with height in the lower atmosphere.

Indian measurement: diffuse radiation

• Diffuse radiation: sunlight scattered by air, dust and clouds, which reaches the ground from all over the sky rather than straight from the Sun's disc.
• Global radiation: direct plus diffuse radiation on a flat surface.
• What the IMD (India Meteorological Department) found:
• It has measured diffuse radiation at a network of 10 stations since 1958 [4].
• In heavily polluted cities, the share of diffuse radiation in global radiation is almost twice that in non-industrial areas [4].
• Diffuse radiation rose clearly in urban areas during the ten years up to the 1969 study [4].
• This shows that dust and smoke scatter incoming short waves. They change how much direct sunlight reaches the ground.

5. Earth-Sun distance: aphelion and perihelion

• The Earth's orbit is an ellipse (a slightly stretched circle), so its distance from the Sun changes through the year.

	Perihelion	Aphelion
Meaning	The point of the orbit nearest the Sun	The point of the orbit farthest from the Sun
Date	about 3 January	about 4 July
Distance	about 147 million km	about 152 million km
Insolation	Slightly more	Slightly less
Season in Northern Hemisphere	Winter	Summer

• Why this barely affects weather:
• On 3 January the Earth gets slightly more insolation than on 4 July.
• But the distribution of land and sea and the circulation of the atmosphere (winds) hide this small difference.

• So it has almost no effect on daily weather.

• Distance does not cause seasons. The tilt of the axis does:

• The Northern Hemisphere has winter at perihelion, when the Earth is nearest the Sun.
• If distance caused seasons, the whole Earth would have summer in January. It does not.

• The real cause is the tilt of the Earth's axis (about 23.5°). Tilt changes the angle of the Sun's rays and the length of the day (covered in the next section).

• Effect on tides:

• When perihelion falls on the same days as a new moon or full moon (when spring tides happen), the Sun's pull is a little stronger.
• This gives unusually large tides: very high high-tides and very low low-tides.

Prelims Hooks

• Solar constant: NCERT 1.94 cal/sq cm/min (about 1,353 W/sq m); modern value about 1,361 W/sq m (SORCE; Kopp and Lean, 2011). The older estimate was 1,366 W/sq m.
• Global average insolation at the top of the atmosphere = about 340 W/sq m = one quarter of the solar constant. It is a quarter, not a half: watch for this option trap.
• Earth's energy budget (NASA): 29% reflected, 23% absorbed by the atmosphere, 48% absorbed by the surface. So the surface absorbs more than the atmosphere.
• Solar constant changes: about 0.1% over the 11-year sunspot cycle; about 7% between perihelion and aphelion.
• Perihelion ≈ 3 January, ≈ 147 million km; aphelion ≈ 4 July, ≈ 152 million km. Trap: "The Earth is nearest the Sun during the Northern Hemisphere summer" is wrong.
• Wien's law: hotter body → shorter peak wavelength. Sun (5,800 K) peaks near 0.5 µm (visible); Earth (288 K) peaks near 10 µm (infrared).
• Stefan-Boltzmann law: energy given off ∝ T⁴ (absolute temperature).
• The atmosphere is heated mainly from below, by longwave radiation from the Earth, not directly by incoming shortwave sunlight.
• Seasons are caused by axial tilt, not by Earth-Sun distance.
• Perihelion together with a new or full moon → unusually large spring tides.

Mains Points

• Why the atmosphere is heated from below (GS-I):
• The air lets incoming short waves through, so the ground absorbs about 48% of the energy arriving at the top of the atmosphere.
• The ground then gives off long waves, which water vapour and CO₂ trap.

• This explains why temperature falls with height, why the greenhouse effect exists, and why a rise in CO₂ upsets the Earth's energy balance (the link to climate change).

• Distance vs tilt (GS-I):

• Insolation varies by about 7% between perihelion and aphelion. Yet the Northern Hemisphere has winter at perihelion, and land-sea distribution and air circulation hide the distance effect.

• Use this to show that the angle of the Sun's rays and the length of the day, both set by the tilt, control seasons and the spread of insolation across latitudes.

• Natural vs human forcing (GS-III, environment):

• The Sun's output changes only about 0.1% over a sunspot cycle, and satellites have tracked it steadily since 1978.

• This small range helps scientists separate natural solar change from warming caused by greenhouse gases.

• Pollution and sunlight (GS-III):

• IMD records since 1958 show that polluted Indian cities get nearly twice the share of diffuse (scattered) radiation of clean areas.
• Aerosols (fine dust and smoke particles) cut direct sunlight, which affects local heating and how much power solar panels produce.