Why insolation varies: tilt, angle of rays, day length and aspect

Detail

1. Five factors that change insolation

• Insolation (incoming solar radiation) is the energy from the Sun that reaches the Earth as short waves. It gives the Earth both heat and light [3].
• The amount that reaches the ground is not the same everywhere. It changes over a day, a season and a year.
• NCERT Class 11 lists five factors:

#	Factor	What it controls	How much it matters
1	Rotation of the Earth on its axis	Day and night. Insolation is zero at night and highest near local noon.	Major
2	Angle of inclination of the Sun's rays	How concentrated the energy is on each unit of ground	Major
3	Length of the day	How many hours the ground receives energy	Major
4	Transparency of the atmosphere	Clouds, dust and water vapour absorb, reflect and scatter sunlight	Minor
5	Configuration of land (aspect)	Which slopes face the Sun	Minor (local)

• Trap: NCERT says factors 4 and 5 matter least. Clear skies still have a visible effect, though. Subtropical deserts get more insolation than the cloudy equator, even though the equator has a higher noon Sun.
• Factors 2 and 3 both come from one root cause: the tilt of the Earth's axis.

2. Inclination of the Earth's axis

• Inclination of the Earth's axis: the Earth's axis is not upright compared with the plane of its orbit.
• It makes 66.5° with the plane of the orbit.

• This equals 23.5° from the perpendicular to that plane (NASA: "about 23.5 degrees with the perpendicular to the ecliptic", the ecliptic being the plane of the Earth's orbit) [3].

• Parallelism of the axis: the axis keeps pointing the same way in space, towards the Pole Star, all year round.

• What follows from this, step by step:
• The Earth goes round the Sun, but the axis keeps its direction.
• So for half the year the Northern Hemisphere leans towards the Sun, and for the other half it leans away.
• The latitude where the noon Sun is directly overhead therefore moves between 23.5°N and 23.5°S.

• Result: the seasons. They happen because of the 23.5° tilt [2][3].

• Distance is NOT the cause of the seasons. The Earth is closest to the Sun (perihelion) around 3–5 January. That falls in the northern winter [3].

• The two Tropics and the two Polar Circles are set by the tilt:
• Tropics (23.5°N and S): the furthest the overhead noon Sun ever reaches from the equator.
• Arctic and Antarctic Circles (66.5°N and S, which is 90° − 23.5°): the lowest latitudes that get at least one full day of 24-hour daylight or 24-hour darkness.

3. Solstices and equinoxes — the yearly path of the overhead Sun

• Solstice: the day when the Sun's noon path is furthest north or furthest south of the equator [2].
• Equinox ("equal night"): the day when the Sun is exactly above the equator, so day and night are equal [2].

Event	Date (approx.)	Noon Sun overhead at	Northern Hemisphere	Polar regions
Summer solstice	21 June	Tropic of Cancer (23.5°N)	Longest day	24-hour daylight north of the Arctic Circle (NCERT Fig. 8.1); 24-hour night south of the Antarctic Circle
Autumnal equinox	23 September (NASA: ~22 September [3])	Equator	Day = night everywhere	Sun on the horizon at both poles
Winter solstice	22 December	Tropic of Capricorn (23.5°S)	Shortest day	24-hour night north of the Arctic Circle; 24-hour day south of the Antarctic Circle
Vernal (spring) equinox	21 March	Equator	Day = night everywhere	Sun on the horizon at both poles

• In polar winter the Sun never rises. In polar summer it never sets [3].
• The Tropic of Cancer crosses 8 Indian states, from west to east: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. On 21 June the noon Sun is overhead along this line.

4. Length of the day

• Rule: in summer, days get longer as you go away from the equator. In winter, they get shorter as you go away from it.
• At the equator, day length stays close to 12 hours all year.

• At the equinoxes, every place has about 12 hours of day and 12 hours of night [2][3].

• Why: on 21 June the North Pole leans towards the Sun.

• So more of each northern circle of latitude lies on the lit side of the circle of illumination (the line that divides day from night).

• The further north the place, the bigger the lit share. At the Arctic Circle, the whole circle is lit.

• NCERT exercise — Chandigarh vs Thiruvananthapuram:

• Chandigarh (~30.7°N) has longer June days than Thiruvananthapuram (~8.5°N), because it lies further from the equator.
• On 21 June, Chandigarh gets about 14 hours of daylight. Thiruvananthapuram gets only about 12.5 hours.

• In December the order flips: Chandigarh then has the shorter day.

• Effect on insolation: longer days add more hours of heating. This is why high-latitude summers (for example in Siberia and Canada) can get a lot of insolation in a day, even though the Sun stays low in the sky.

5. Angle of incidence of the Sun's rays

• Angle of incidence: the angle at which the Sun's rays strike the ground. It depends mainly on latitude, and it changes with the season as the overhead Sun moves.
• Torch-on-a-ball diagram:
• Shine a torch straight down on a ball. You get a small, bright circle.
• Tilt the torch. The same light now spreads into a large, pale oval.

• The Earth is round, so near the equator rays fall almost vertically. Towards the poles they fall more and more slantingly.

• Two reasons why slanting rays heat less:

• Bigger area: the same beam covers more ground, so each square metre gets less energy.
  • At a Sun height of 90°, the beam covers 1 unit of area.
  • At 30°, it covers 2 units of area, so each unit gets half the energy.

• Longer path through the air: a slanting ray passes through a thicker layer of atmosphere.

  • On the way, more energy is lost to absorption (gases and dust take up the energy), scattering (air molecules send light off in all directions) and diffusion (light spread out by particles).
  • Less energy is left by the time the ray reaches the ground.

• The summer hemisphere gets the Sun's rays more vertically than the winter hemisphere [3]. So the angle of rays and the day length work together: summer has both higher rays and longer days.

• India's tropical position:
• About half of India lies south of the Tropic of Cancer, between 8°4'N and 23.5°N.
• Every place there has the noon Sun directly overhead twice a year.
  • The first time is when the Sun moves north (between 21 March and 21 June).
  • The second time is when it moves back south (between 21 June and 23 September).
• So the noon Sun is high all year, and insolation stays high.
• Example: on 21 June, the noon Sun at Thiruvananthapuram is actually slightly to the north of overhead.

6. Transparency of the atmosphere (minor factor)

• Clouds reflect sunlight back to space. Dust and smoke scatter it. Water vapour absorbs it.
• Result:
• Cloudy places like the equatorial belt and monsoon India in July–August get less insolation at the ground.
• Clear-sky subtropical hot deserts get the most. Examples are the Sahara and the Thar Desert in Rajasthan.

7. Aspect (configuration of land)

• Aspect: the compass direction a slope faces. It controls how directly, and for how long, the slope receives the Sun.
• Northern Hemisphere: the noon Sun stands to the south for any place north of the Tropic of Cancer.
• South-facing slopes (adret) get the Sun's rays more directly and for more hours. They are warmer and drier.
• North-facing slopes (ubac) stay in shade for longer. They are cooler and moister.

• The words adret and ubac come from the French Alps.

• Southern Hemisphere: the pattern is reversed. North-facing slopes are the sunny ones.

Feature	Adret (south-facing, N. Hemisphere)	Ubac (north-facing, N. Hemisphere)
Sunshine	More direct, longer hours	Less, often shaded
Soil	Warmer, drier, more evaporation	Cooler, wetter
Vegetation	Thinner cover, grassland	Denser forest
Snowline	Higher (snow melts earlier)	Lower (snow stays longer)
Human use	Villages, terraced fields, orchards	Forest, little settlement

• In India: in Himalayan valleys, villages, terraced fields and orchards are built on south-facing slopes. North-facing slopes keep thicker forest and a lower snowline.
• Why it matters for the exam: aspect is a local factor. It does not change the global pattern, but it makes two sides of the same valley clearly different in temperature, vegetation and land use.

Prelims Hooks

• The Earth's axis makes 66.5° with the plane of its orbit, which is 23.5° from the perpendicular. Watch for options that swap these two figures.
• Perihelion (Earth closest to the Sun) comes in early January, during the northern winter. So distance from the Sun does not cause the seasons; the tilt does.
• 21 June: Sun overhead at the Tropic of Cancer. This is the longest day in the Northern Hemisphere, and there is 24-hour daylight north of the Arctic Circle (66.5°N).
• 22 December: Sun overhead at the Tropic of Capricorn. 21 March and 23 September: Sun overhead at the equator, with equal day and night everywhere.
• In June, day length increases from south to north. Order: Thiruvananthapuram < Chennai < Nagpur < Delhi < Chandigarh < Srinagar.
• NCERT's two least important factors of insolation are transparency of the atmosphere and configuration of land (aspect).
• The highest insolation falls over subtropical deserts, not the equator, because the equator is cloudy.
• Adret = sunny south-facing slope; ubac = shady north-facing slope (Northern Hemisphere). Ubac slopes have the lower snowline and denser forest.
• Places south of the Tropic of Cancer in India have the noon Sun overhead twice a year. Places north of it never do.
• The Tropic of Cancer passes through 8 states. It does not pass through Odisha, Bihar or Uttar Pradesh, which are common traps.

Mains Points

• One cause, two effects:
• The 23.5° tilt, with an axis that always points the same way, moves the overhead Sun between the Tropics.
• This changes the angle of the rays and the length of the day together.
• Together these produce the seasons and the fall in insolation from the equator to the poles.

• That fall drives the global heat balance, with surplus heat at low latitudes and deficit at high latitudes, and so the general circulation of the atmosphere.

• Slanting rays heat less for two reasons:

• The energy is spread over a larger area.
• The ray takes a longer path through the atmosphere, so more energy is lost to absorption and scattering.

• This explains cold high latitudes, and why winter is weaker even where the days are not much shorter.

• India's tropical location:

• The noon Sun is high all year, and overhead twice a year south of 23.5°N.
• This gives high insolation and long growing seasons.

• It also brings strong pre-monsoon heating of north-west India (May–June), which helps draw in the south-west monsoon. Clear-sky Rajasthan gets the most solar energy.

• Aspect shapes land use in the mountains (GS-I settlement geography):

• In the Himalaya, villages, terraces and orchards are on sunny adret slopes, while ubac slopes stay forested.
• Aspect also matters for siting solar plants, forest and snow management, and avalanche risk on shaded slopes.