Posted by: Rob Viens | May 26, 2012

Misty Blue Mountains

On May 26th Darwin was contemplating the blue mountains in the distance:

“During to day & yesterday there has been a strong breeze from the SW; the amount of evaporation which a current of air produces in these countries is very great & in consequence the comparative state of dryness of the road has been today very remarkable. After dinner I walked to the Bay & had a good view of the Organ mountains; I was much struck by the justness of one of Humboldts observations, that hills in a Tropical country seen from a distance are of a uniform blue tint, but that contrary to what generally is the case the outline is defined with the clearest edge.” (May 26)

He elaborates a little further in Voyage, writing:

“During this day I was particularly struck with a remark of Humboldt’s, who often alludes to “the thin vapour which, without changing the transparency of the air, renders its tints more harmonious, and softens its effects.” This is an appearance which I have never observed in the temperate zones. The atmosphere, seen through a short space of half or three quarters of a mile, was perfectly lucid, but at a greater distance all colours were blended into a most beautiful haze, of a pale French grey, mingled with a little blue. The condition of the atmosphere between the morning and about noon, when the effect was most evident, had undergone little change, excepting in its dryness. In the interval, the difference between the dew point and temperature had increased from 7°.5 to 17°.” (Voyage of the Beagle)

What Darwin is describing in his blue (or French grey) mountains is the effects of Rayleigh scattering. Light coming from above hits molecules (oxygen, nitrogen, etc.) and particles in the atmosphere. The smallest wavelengths of light (the blue) are more likely to interact with these particles and be scattered in all directions (while the higher wavelengths, such as red, pass on through). This is because the wavelength of blue light is similar to the size of the particles (while the redder wavelengths are much longer). The more distance between you and the mountains, the more likely blue light from the sun will get scattered towards your eyes, giving them a blue tint. (This is the same reason the sky is blue, too.) On the other hand, when you are looking at a sunset, the blue light gets scattered off in all directions, so it is primarily just the red wavelengths that make it to your eye.

The blue tint of the North Cascades Mountains here in Washington State (USGS photo):

North Cascades Mountains

Darwin wraps up his journal with a brief (and poetic) praise of the written word (referring to the Personal Narrative of his hero Alexander Humboldt):

“Few things give me so much pleasure as reading the Personal Narrative; I know not the reason why a thought which has passed through the mind, when we see it embodied in words, immediately assumes a more substantial & true air. — In the same manner as when we meet in dramatick writings a character which we have known in life, it never fails to give pleasure.” (May 26)

For more on Humboldt be sure to see Darwin on Humboldt. (RJV)

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