Insolation

Also called: Incoming solar radiation · Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"

Meaning

Insolation (short for incoming solar radiation) is the energy from the Sun that reaches the Earth. It arrives mostly as short waves: visible light, plus some ultraviolet and near-infrared.

It matters because the Earth gets almost all its energy from the Sun. How much insolation a place receives, and at what angle, sets how hot that place gets. The way the ground and air handle this energy also explains the greenhouse effect and the Earth's energy balance.

Explanation

How insolation reaches the Earth

• 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 almost everywhere.

• 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.

• A slanting ray spreads the same energy over a bigger area, so each square metre gets less.

• Insolation changes from place to place and over time. In settled textbook terms, it depends on:

• latitude (how far a place is from the equator)
• season
• time of day
• cloudiness

• the angle of the Sun's rays

• Radiative equilibrium:

• Over a long period, the Earth sends the same amount of energy back to space as it receives. So the planet as a whole neither warms up nor cools down.
• NASA calls this radiative equilibrium: energy coming in from the Sun equals heat going out, so the global temperature stays fairly steady [3].
• This balance is now slightly upset, because greenhouse gases are trapping a little extra heat.

Solar constant and the global average

• 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.

Source	Value
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].
• The global average is only about 340 W/sq m, one quarter of the solar constant [2][3]. Here is why:
• Only half the Earth is lit at any moment, and the night side gets nothing. This halves the average [3].
• On the lit half, the rays come in more and more at a slant towards the poles. This halves it again [3].

• In geometry terms, the Earth catches sunlight on a flat disc (area πr²) but spreads it over a sphere (area 4πr²). That gives one quarter.

• What happens to those 340 W/sq m [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 albedo is about 0.29 (29%) [3].
• The "constant" is not truly constant:
• Sunspots are dark, cooler patches on the Sun. Their number rises and falls over about 11 years. Over one such cycle, the solar constant changes by only about 0.1% [2].
• Earth-Sun distance changes the energy received by about 7% between perihelion and aphelion. This follows the inverse-square law (energy falls with the square of the distance): (152 ÷ 147)² ≈ 1.07.
• So distance changes insolation far more through the year than the sunspot cycle does.

Shortwave in, longwave out (Planck's law)

• 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.
• Stefan-Boltzmann law: the energy a body gives off rises with the fourth power of its absolute temperature (E = σT⁴).
• Absolute temperature is temperature in kelvin (K), counted from absolute zero (−273 °C).

• The Sun's surface is about 20 times hotter than the Earth's. Since 20⁴ = 160,000, each square metre of the Sun gives off about 160,000 times more energy.

• Wien's displacement law: as temperature rises, the peak wavelength gets shorter (λmax ≈ 2,898 µm·K ÷ T). A µm (micrometre) is one-millionth of a metre.

	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 [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	Absorbs much of it (water vapour, CO₂)

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

Direct and diffuse radiation

• Diffuse radiation: sunlight scattered by air, dust and clouds. It reaches the ground from all over the sky, not straight from the Sun's disc.
• Global radiation: direct plus diffuse radiation falling on a flat surface.
• Dust and smoke scatter incoming short waves, which changes how much direct sunlight reaches the ground.

Where it occurs

• Top of the atmosphere (whole globe): the solar constant of about 1,361 W/sq m [2] is measured here. The global day-and-night average here is about 340 W/sq m [2][3].
• Where the Sun is overhead vs towards the poles:
• Rays strike straight down only where the Sun is directly overhead.

• Towards the poles the rays come in at a growing slant, so each square metre gets less energy [3].

• Earth's orbit:

• Perihelion (nearest point to the Sun) is about 3 January, at about 147 million km. Insolation is slightly more.
• Aphelion (farthest point) is about 4 July, at about 152 million km. Insolation is slightly less.
• The Northern Hemisphere has winter at perihelion and summer at aphelion.

• 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.

• India (IMD network):

• The India Meteorological Department (IMD) 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].

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

Don't confuse with

• Insolation vs solar constant: insolation is the sunlight actually reaching the Earth, and it changes with place and time. The solar constant is one fixed reference value: energy on a surface facing the Sun straight on, at the top of the atmosphere, at the Earth's average distance from the Sun (about 1,361 W/sq m) [2].
• Insolation vs terrestrial radiation: insolation is shortwave (peak about 0.5 µm) and mostly passes through the air. Terrestrial radiation is longwave (peak about 10 µm), given off by the warm Earth and absorbed by water vapour and CO₂.
• Direct vs diffuse radiation: direct radiation comes straight from the Sun's disc. Diffuse radiation is scattered by air, dust and clouds and arrives from the whole sky. Its share is almost twice as high in polluted Indian cities [4].
• Insolation vs albedo: insolation is the energy coming in. Albedo is the share sent back out by reflection, about 0.29 (29%) for the Earth [3].

Prelims Hooks

• Solar constant: NCERT gives 1.94 cal/sq cm/min (about 1,353 W/sq m). The modern value is about 1,361 W/sq m (SORCE; Kopp and Lean, 2011), and the older estimate was 1,366 W/sq m [2].
• Global average insolation at the top of the atmosphere is about 340 W/sq m, one quarter of the solar constant [2][3]. Trap: the answer is a quarter, not a half.
• Energy budget: 29% is reflected, 23% is absorbed by the atmosphere and 48% is absorbed by the surface [3]. So the surface absorbs more than the atmosphere.
• Perihelion is about 3 January (about 147 million km). Aphelion is about 4 July (about 152 million km). Trap: "The Earth is nearest the Sun during the Northern Hemisphere summer" is wrong.
• The solar constant varies by about 0.1% over the 11-year sunspot cycle [2] and by about 7% between perihelion and aphelion.
• Wien's law: the Sun (5,800 K) peaks near 0.5 µm (visible light). The Earth (288 K) peaks near 10 µm (infrared). So the atmosphere is heated mainly from below.

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 insolation arriving at the top of the atmosphere [3].
• The warm ground gives off long waves, which water vapour and CO₂ trap.

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

• Distance vs tilt (GS-I):

• Insolation changes 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.

• So seasons and the spread of insolation across latitudes are controlled by the tilt of the Earth's axis (about 23.5°), which sets the angle of the Sun's rays and the length of the day.

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

• The Sun's output changes only about 0.1% over a sunspot cycle, and satellites have tracked it steadily since 1978 [2]. This helps scientists separate natural solar change from warming caused by greenhouse gases.
• IMD records since 1958 show polluted Indian cities get nearly twice the share of diffuse radiation of clean areas [4]. Aerosols (fine dust and smoke particles) cut direct sunlight, which affects local heating and how much power solar panels produce.

Related concepts

• Solar constant
• Shortwave and longwave radiation
• Planck's law
• Aphelion
• Perihelion

Read more

• Solar radiation and insolation