Why does sunlight change color during the day?
Air scatters short wavelengths of light much more than long ones. Rayleigh scattering is roughly proportional to 1 over wavelength to the fourth power, so blue scatters far more than red. That's why the sky is blue. When the sun is low, its light travels through much more air to reach you, more blue is scattered out of the direct beam, and what's left is warmer.
How warm does it get?
We ran a clear-air model of direct sunlight for Heliograph. Lower sun, warmer light:
| Sun height above the horizon | Modeled color temperature |
|---|---|
| 90°, straight overhead | about 5100K |
| 30° | about 4600K |
| 10° | about 3400K |
| 5° | about 2650K |
Modeled estimate for direct sun in clear air, from our own Rayleigh-plus-haze model. Hazy air runs warmer. Shade lit by blue sky runs much cooler.
Altitude matters too. Less air above you means less scattering, so the same sun height reads cooler in Denver than in Nashville in the same model.
What can't place and time tell you?
- WeatherClouds, haze, and smoke change the light.
- ShadeA subject in shade is lit by blue sky, not the direct sun.
- Reflected colorGrass, walls, and water bounce their color onto your subject.
- Other lightStreetlights, windows, and flash.
The sun gives you the light you should expect, not the light you got.
Why does auto white balance get golden hour wrong?
Auto white balance tries to make the scene look neutral. At midday that's close to right, because direct sunlight is close to neutral. At golden hour, the warmth is the point, and auto white balance can push it back toward neutral. Knowing the sun was 5° above the horizon is a reason to keep the warmth instead.
How did the question turn into an app?
It became Heliograph, a lab on this site. Drop in a photo, and it uses the time and place to work out the sun's height, models the light, and builds a Lightroom white balance preset as a starting point. It works best on raw files, because phone photos usually have their white balance baked in.
Heliograph is at stuartsingleton.com/heliograph.
Color depends on the light, and the light changes all day. That's part of why chasing film's color is a moving target, and why depth matters more.
Frequently asked questions
Why is sunlight warmer at sunrise and sunset?
Low sun light passes through more air, and air scatters blue light much more than red (Rayleigh scattering), so the direct beam that reaches you is warmer.
Can white balance be set from location and time?
Partly. Place and time predict the color of direct sunlight in clear air, but not clouds, haze, shade, or reflected color. It works best as a starting point.
What is Heliograph?
A lab on stuartsingleton.com that uses a photo's time and place to model the sunlight and build a Lightroom white balance starting point.