Follow the blue
Start with clear air and a high Sun. Compare blue and red transmission, then lower the Sun without changing the air.
FIELD NOTE 04 / SUNSET LAB
The long way through the light.
Why does the same Sun make a blue sky and a red sunset?
A SKY MADE BY SCATTERING
Follow the sunlight.Lower the Sun. Watch the blue light leave the direct beam.
Three-band sky model- Path through molecular air
- 3.2 vertical columns
- Blue light remaining
- — at 450 nm
- Red light remaining
- — at 650 nm
Lower the Sun, change the amount of air and haze, and watch scattering reshape the sky and the sunlight that reaches you.
Light left in the direct solar beam
Transmission · 0–100% · wavelength in nmThrough your atmosphereBefore entering the air
Try a starting point
Ready to explore.
Make a prediction. Change one thing. See what happens.
Model notes & references ↗FOLLOW THE SCIENCE
One source. Two different journeys.
White sunlight contains a range of wavelengths. Air molecules scatter shorter visible wavelengths more strongly than longer ones. When you look away from the Sun, some of that scattered light reaches your eyes and gives the sky its blue colour.
Looking toward a low Sun tells the other half of the story. The direct beam has travelled through more air. Much of its blue light has been scattered away, so the remaining light is richer in reds and oranges. NASA: why the sky is blue ↗
A FEW QUESTIONS TO TRY
Let’s see what happens.
Take the longer route
Start near the horizon. Switch to Light’s path. Why does the path grow even though you have not added air?
Take the air away
Remove both molecules and haze. The Sun still shines. What happens to the sky around it?
Scattered away is not destroyed.
THE DIRECT BEAM
Light that stayed on course.
The graph measures the fraction of each wavelength that reaches you without being scattered. A value of 20% means four fifths has left that particular beam.
THE SKY AROUND IT
Light with a new direction.
The sky view adds sunlight scattered toward your line of sight. The loss from the direct beam helps explain why there is light elsewhere in the sky.
Rayleigh scattering varies approximately as 1/λ⁴. In this model, 450 nm blue light has about 4.4 times the molecular scattering coefficient of 650 nm red light. Aerosols follow a gentler wavelength dependence and scatter strongly forward, making the area around the Sun look hazier.
Haze does more than make a sunset red.
It also removes light from the direct beam and changes where scattered light goes. A very hazy horizon can be dim. Clouds, multiple scattering and the particular mix of particles make real skies more varied than this model.
The mathematics, assumptions & references
From density to transmission
Earth is a sphere of radius 6,371 km. The observer is 2 m above its surface. Molecular density falls as exp(−h/8 km); the aerosol scale height is 1.2 km. The integration ends at an altitude of 100 km.
τ(λ) = ∫ [βR(λ, h) + βa(λ, h)] ds
T(λ) = exp[−τ(λ)]
T(λ) = exp[−τ(λ)]
The reference vertical molecular optical depth at 550 nm is 0.10. The air slider multiplies it. βR varies as (550 nm / λ)⁴; the aerosol coefficient varies as (550 nm / λ)1.3. Its vertical optical depth is the haze setting. These are illustrative atmospheric parameters, not a fitted local atmosphere.
The path readout integrates molecular density along the Sun’s direction and divides by an 8 km vertical reference column. It describes geometry, so changing the air multiplier does not change this readout. Spherical geometry keeps the horizon path finite.
How the sky is drawn
L(λ) ∝ ∫ TSun→sample Tsample→eye [βRPR(θ) + βaPa(θ)] ds
The renderer samples a single scattering event, with the Rayleigh phase function 3(1 + cos²θ)/(16π) and a Henyey–Greenstein aerosol phase function with g = 0.76. Red, green and blue are represented by 650, 550 and 450 nm, followed by fixed exposure and display gamma. This is a three-band approximation, not full spectral colour matching.
It omits refraction, ozone absorption, clouds, ground reflection and repeated scattering. The sky has a fixed exposure; the Sun’s displayed size and brightness are adjusted for visibility. The spectrum graph is the numerical transmission model and is independent of that display exposure. A simpler colour preview is used if WebGL is unavailable.