Sunlight in the atmosphere and its spatial distribution

Detail

1. Passage of sunlight through the atmosphere

• Insolation (incoming solar radiation) means sunlight that reaches the Earth. It comes mostly as short waves: ultraviolet, visible light and near-infrared.
• The air lets most short waves through. It is largely transparent to them, so most of this energy reaches the ground or the cloud tops.
• Some of it is absorbed on the way down. Different gases take out different parts:

Part of sunlight	Absorbed by	Where
Ultraviolet (UV)	Ozone	Higher up, in the ozone layer of the stratosphere
Near-infrared	Water vapour, ozone and other gases	Troposphere (the lowest layer of the atmosphere, where weather happens)
Visible light	Very little is absorbed	Mostly passes through, but some is scattered

• Why this matters:
• Little sunlight is absorbed on the way down, so the atmosphere is heated mainly from below, by the warm ground.
• Humid air takes out more near-infrared than dry air. So humid tropical air lets less sunlight reach the ground than dry desert air.

2. Scattering of sunlight and the colour of the sky

• Scattering of sunlight means that tiny gas molecules and particles in the air bounce light off in all directions instead of letting it go straight through. Some scattered light goes back to space and some comes down to the ground as diffuse light (soft light from the whole sky, not straight from the Sun).
• Rayleigh scattering is scattering by particles much smaller than the wavelength of light, such as nitrogen and oxygen molecules.
• How strongly light is scattered goes up as the inverse fourth power of the wavelength. So shorter waves are scattered far more [6].
• Blue light has short, high-frequency waves, so air molecules scatter it much more than red light [4].

• Result: blue light comes at us from every part of the sky, so the daytime sky looks blue.

• Red and orange at sunrise and sunset:

• When the Sun is low, its light passes through much more air before it reaches us [4].
• On that long path, most of the blue is scattered away to the sides.

• Mostly red, orange and yellow light is left to reach the eye [4][5].

• Mie scattering is scattering by larger particles such as haze, dust, smoke and cloud droplets.

• It scatters all colours about equally.
• Result: polluted, dusty or cloudy skies look white or grey, not deep blue. Examples are winter haze over the Indo-Gangetic Plain and pre-monsoon dust over north-west India.

Feature	Rayleigh scattering	Mie scattering
What scatters the light	Gas molecules (much smaller than the wavelength)	Haze, dust, smoke, cloud droplets (about as large as the wavelength or larger)
Effect on colours	Scatters blue far more than red	Scatters all colours about equally
What you see	Blue day sky; red sunrise and sunset	White or grey hazy sky; white clouds

3. Spatial distribution of insolation at the surface

• Spatial distribution of insolation means how much sunlight reaches the ground at different places on the Earth.
• Range: about 320 W/sq m in the tropics down to about 70 W/sq m at the poles. These are yearly averages.
• The highest values are over the subtropical deserts, not at the equator.
• Subtropical high-pressure belts lie near 20°–30° N and S. Air sinks there, so skies are mostly clear.
• The deserts in these belts get the most sunlight. Examples are the Sahara and Arabian deserts, the Thar (Rajasthan, India), the Kalahari, the Australian deserts and the Atacama.

• Result: the Northern Hemisphere's highest temperatures are found in the subtropics (NCERT exercise).

• Why the equator gets less than the tropics:

• At the equator, warm moist air rises every day.
• It forms thick cumulonimbus clouds and heavy rain, as over the Amazon, the Congo basin and Indonesia.

• These clouds reflect much of the sunlight back to space. So less reaches the ground, even though the Sun is high all year.

• Continents get more than oceans at the same latitude. Skies over land are usually clearer, while oceans have more cloud and water vapour above them.

• Middle and high latitudes get far less in winter than in summer.
• The Sun is low in the sky, so its rays are slanting and spread over a larger area.
• Days are short, and near the poles there may be no sunlight at all.

Region	Approximate insolation	Main reason
Subtropical deserts	Highest (about 320 W/sq m in the tropical belt)	Clear skies under sinking air
Equatorial belt	Lower than the subtropics	Persistent clouds
Mid-latitude oceans	Lower than land at the same latitude	More cloud
Poles	About 70 W/sq m	Low Sun angle, long polar night

4. Why India's hottest month is May, not June

• The Sun is highest over India in June. At the summer solstice (21 June), the overhead Sun is at the Tropic of Cancer (23.5° N), which passes through Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram.
• But monsoon clouds cut the sunlight from June onwards:
• The south-west monsoon reaches Kerala in early June and covers most of India by July.
• Thick monsoon clouds and rain reflect and block sunlight.

• Less sunlight reaches the ground, and rain and wet soil cool the surface.

• Result: May, the month just before the monsoon, has the clearest skies and the most heating. It is the hottest month over most of India.

• Very high temperatures occur over north-west and central India, for example in Rajasthan and Vidarbha.
• This links to the topic indian-climate-monsoon.

5. Measuring sunlight in India

• India Meteorological Department (IMD) began measuring solar radiation in 1879 at Kolkata [3].
• 1957: IMD opened four radiation stations at Pune, New Delhi, Kolkata and Chennai [3].
• 1985–86: 14 more stations were added, and the network grew to 45 stations [3].
• Jammu was added in 2020 and Agartala in 2022. IMD now runs a network of 47 solar radiation stations (2025) [3].
• 2009–2014: IMD started measuring long-wave radiation, UV-A and UV-B [3]. From 2014–15, data loggers send real-time data by GPRS to IMD's server in Pune [3].
• Gap: IMD says that about 800 district-level stations are needed for full coverage. The Himalayan region, the deserts and the north-eastern states have too few stations (2025) [3].

6. Global dimming and brightening (beyond NCERT)

• Global dimming is a fall in the amount of sunlight reaching the Earth's surface.
• Measuring stations around the world saw a widespread fall from the 1950s to the 1980s, of a few per cent.

• The IPCC AR6 (2021) says, with high confidence, that this fall was real. It was not a local effect and not a fault in the instruments [2].

• Cause, step by step:

• Factories, power plants and burning released sulphur dioxide (SO₂) and black carbon (soot), which form aerosols (tiny particles in the air).
• Aerosols reflect and absorb sunlight directly.
• They also act as seeds for cloud droplets. This gives clouds more but smaller droplets, so the clouds become brighter and reflect more sunlight.
• Result: less sunlight reaches the ground.

• IPCC rates aerosols as a major cause with medium confidence. Natural changes in cloud cover may also have played a part [2].

• Brightening is the partial recovery of surface sunlight after the dimming.

• Around the world, dimming turned into brightening in about 1990 [2][8].
• The change in surface sunlight between 1980 and 2000 follows the change in emissions of SO₂ and black carbon [2].

• Cleaner-air laws in Europe and North America reduced aerosols, so more sunlight reached the ground.

• The masking effect: dimming cooled the surface. So it partly hid greenhouse warming, and cleaning the air uncovers some of that hidden warming.

• After 2000, the pattern differs by region [2]:
• Brightening continued in parts of Europe and the USA.
• Sunlight became more or less stable in China and India.
• Dimming continued in some other areas.

• (NCERT-linked note: dimming went on longer over polluted South and East Asia, including India — Wild, 2009.)

• In India: sunshine hours have kept falling in all seasons over 1988–2018. Only north-east India shows a mild levelling-off in some seasons [7].

• This links to black carbon and aerosols in the topic atmosphere-composition-weather.

Prelims Hooks

• Ozone absorbs ultraviolet; water vapour absorbs most of the near-infrared in the troposphere. Visible light mostly passes through.
• Blue sky = Rayleigh scattering by gas molecules. Scattering goes up as the inverse fourth power of the wavelength [6]. White or grey hazy sky = Mie scattering by dust, haze and droplets.
• Red sunsets happen because sunlight has a longer path through the air when the Sun is low, so the blue is scattered out [4].
• Highest surface insolation is over the subtropical deserts, not the equator. This is a common trap. The equator gets less because of cloud.
• Yearly average insolation is about 320 W/sq m in the tropics and about 70 W/sq m at the poles.
• At the same latitude, continents get more insolation than oceans.
• India's hottest month is May. The summer solstice is in June, but monsoon cloud cuts insolation from June.
• IMD's first solar radiation measurement was at Kolkata in 1879. It now has 47 solar radiation stations [3].
• Global dimming happened from the 1950s to the 1980s. It turned into brightening around 1990 [2].
• After 2000, surface sunlight has been about stable in India and China and still brightening in Europe and the USA [2].

Mains Points

• Cloud cover, not latitude alone, controls surface insolation.
• This is why subtropical deserts get more sunlight than the equator, and why India peaks in May rather than June.

• It shows that the monsoon shapes India's temperature through the year (GS-I: climatology, Indian climate).

• Aerosols have two effects, one bad and one good.

• They harm health and reduce the sunlight reaching crops and solar panels.
• But they also hide part of greenhouse warming. Cleaning the air under clean-air programmes may uncover more of that warming over South Asia.

• So air-quality and climate policy must be planned together (GS-III: environment).

• Falling sunshine hours affect solar energy planning in India.

• Sunshine hours have kept falling over 1988–2018 [7].
• IMD still has only 47 stations against a need of about 800 at district level [3].

• This matters for solar-power site choice, crop modelling and climate monitoring (GS-III: infrastructure, energy).

• Regional patterns of brightening and dimming show different stages of pollution control.

• The West has been brightening since about 1990, while India has been about stable after 2000 [2].
• These patterns can be used to illustrate climate justice and the differing responsibilities of countries in climate talks.