Why Does the Sun Appear to Be Yellow in the Daytime and Orange at Sunset?

A brilliant sunset.

When people look up at the sky with the expectation of a daytime or evening view, they are really harboring expectations for the refraction of light in the atmosphere. This affects the color of the sky and sun at different times of the day. The main factor is the position of the sun in relation to the observer's point of view.

Temperature and Color

The sun is a temperate star.

The reason why our sun appears yellow or orange is due to the surface temperature. The sun is about medium temperature for a star at around 5,505 degrees Celsius, and so its peak frequency coincides with the part of the electromagnetic spectrum that allows light to appear yellow.

Rayleigh Scattering

At its highest point, the light of the sun travels through the least amount of atmosphere.

Ordinarily, the sun maintains its yellow appearance during the day. The atmosphere through which light travels scatters short wavelengths of visible light like blue and violet. This is due to the fact that air is good at Rayleigh scattering, or selective scattering, which is a term used to describe the way in which the wavelength of light is scattered by particles. These particles must be small compared to the wavelength, and air molecules are around a thousand times smaller. From the observer's point of view, a blue sky and white or yellow sun means that the light travels a shorter distance through the atmosphere, so less of the shorter wavelength is scattered.

Sunset

Shorter wavelengths are much more likely to be scattered.

During the evening, light has to travel a much longer distance to the observer, so the blue and violet contributing to the color of the sky throughout the day become scattered to a much more amplified degree, causing brilliant orange and red hues in the sunset---colors on the other end of visible light. Therefore, light from the sun can appear different colors in separate locations.

Dust and Pollution

Colors can become subdued or enhanced by large particles.

A lot of confusion surrounds whether things like dust particles and aerosols subdue or enhance the intensity of a sunset. The answer is that it can depend upon the size or the ubiquity of the particles. Volcanic ash, dust and salt from the sea can sometimes impart more brilliant colors. This has a greater likelihood to occur during sunset rather than sunrise because later in the day these particles are more likely to be scattered in the air. But pollution, especially if the particles are not uniform in size, can often mute or wash out colors, which is why a natural sunset can often look more intense than a smoggy one.

Clouds

Clouds can catch the light from the sun.

Clouds can work to enhance sunsets by catching the light of a setting or rising sun. However, they usually have to be high up so they intercept purer color and not the kind that is often subdued at lower altitudes.

References

• National Oceanic and Atmospheric Administration: The Colors of Twilight and Sunset