Autumn Light

[Clark_K]

I’ve always thought the sunlight had a different character during autumn. More of a white tone than yellow. On clear, sunny days it’s almost like looking through water.

I convinced myself that it was all in my mind. How could the sunlight be any different in October than in June or February?

But lately, I’ve wondered if the angle of the sun actually has some effect on the color of the light. If the sun is lower in the sky and, therefore, sunlight cuts across more atmosphere before it reaches my eyes, could the additional atmosphere be filtering out some of the yellows?

Does this theory have any validity at all?

[Laughing_Lagomorph]

There is validity in that the angle of sunlight changes throughout the year (unless you live on the Equator which I see by your location you don’t). In the Northern Hemisphere the sun is lowest in the sky around Dec. 21st.

[Walloon]

The sun being at a lower angle in autumn would, if anything, make the sunlight less white. It’s going through more suspended particulate matter.

[MikeS]

@Walloon

The sun being at a lower angle in autumn would, if anything, make the sunlight less white. It’s going through more suspended particulate matter.

That’s assuming, of course, that there’s the same amount of suspended particulate matter & other associated haze in the atmosphere in the fall as there is in the summer, which probably isn’t a good assumption.

[KneadToKnow]

There is no question in my mind that the sky is a different shade of blue at different times of year, with the deep azure of October and November my personal favorite, and I think this would be related. Perhaps there is something about the change from a warmer, more humid air mass to a cooler, usually drier air mass (keep my location in mind here) has something to do with it.

[Clark_K]

Good points about particulate matter and moisture. Maybe they have an effect.

It certainly seems like something is happening and it’s not just one person’s perception.

I guess Walloon’s right that, generally, light gets more red-orange as it takes a longer path through the atmosphere. That’s why we have a red sunset, correct? So my initial thought is probably all wrong.

[Walloon]

Reply from a meteorologist friend:

At a lower angle, the sun is shining through a longer column of air due to the Earth’s curvature. As there is a longer transit time through the column, there is an opportunity for more light scattering by particulate matter, both natural (i.e. nitrogen and oxygen) and human-generated (smoke particles, etc). This explains why the sky is blue during the day and red at sunset and sunrise. The nitrogen and oxygen in the atmosphere scatter the blue-violet wavelengths but as the column length increases, the blue and violet wavelengths have been scattered out before reaching your eye, leaving the red-orange part of the spectrum to see. Addition of particulate matter can result in especially vivid sunsets (ex. after Mount Saint Helens or Mount Pinatubo) due to additional particulate matter that scatters the red-orange part of the spectrum.

In North America and most of Eurasia, winter winds are primarily from the north, originating in less-populated, less-polluted areas resulting in less human-generated particles in the column that would scatter wavelengths other than blue and violet, resulting in deep blue skies typically seen in winter or in the summer after a cold front passage in which the air has a Canadian origin. Additionally, cold air is denser and would have more particles in the column, resulting in additional opportunities for wavelength scattering.

[cantara]

Could it also have anything to do with the decreasing amount of green? The trees shed their leaves and other plants go dormant and lose their bright green hues. Also the amount of shade decreases due to the bare trees making things seem brighter.

I would suspect that while the atmospheric influences are dominant in the changing colour of the seasons, your perception is changed due to the difference in the way familiar places look.