Wednesday, February 27, 2013

sentimental sunsets-

Walks in the evening, next to the setting sun. Well, evening, thats almost an overstatement. The sun dips below the horizon at barely 18:00... Thankfully, the hours of daylight are growing ever plentier.

So I went for a stroll around the block. The sky was beautiful, the clouds akin bloody courtains, set out to dry against the bluish-turquoise, shifting to orange, backdrop. Ive written about that, how the sky changes colour depending on the time of day, Rayleigh scattering and all. What I didnt mention, is why the clouds are so devilishly splashed with vibrant red and orange.

In clouds theres another kind of scattering occuring, Mie scattering. Like Rayleigh scattering, both occur because particles in the air are of comparable size to the wavelengths of light which pass through them. That causes warping, scattering of light, to occur. Lossless in nature, electromagnetic forces bend the light around the particles, around Oxygen molecules, for example. That kind of selective warping is called Rayleigh scattering.

Mie scattering is different, because that happens when electromagnetic radiation (Light) warps around much larger particles, like the H2O molecules found in clouds. That means clouds bend wavelengths much outside the visible spectrum, leaving all the spectral colours to be scattered in a roughly equal manner.

So summarize what this means, lets sketch out - verbally - the lifespan of a lightwave. Having been sent out from the sun (story for another day), it travels a huge distance, basically unaltered throughout, and meets with our atmosphere. For every 100 meters it travels through our atmosphere, lets say 5% of the energy of shorter wavelenghts are scattered, down to your eyes below. Thats what makes the sky look blue, because the scattered short wavelenghts are blue. After one kilometre, only 59.9% of the energy of the shorter wavelengths is remained. (I expect ALL of you to know why its not 50%.)

Analysing this bunch of light at this stage, it would appear reddish, because the blue components have been scattered, but the red ones remain untouched (im simplifying this to red, long wavelength, and blue, short wavelength. I can do things like this, deal with it.). This, is when it hits a cloud. Mie scattering occurs, and the whole ray of light is broken down and scattered. The scattering will be the same colour that the light was, because the scattering is basically lossless. Since its at a more than tangential angle to the surface of the earth, this light hits the underside of clouds. Awesome, but might be hard to visualize. Anyway, this means that red light, bounced off the cloud, will hit your retina.


Keep in mind now, nothing I wrote is even approximate. All the numbers, except for the calculations I made using them, are made up. 5% isnt scattered every 100m. I made that up. But its synologous to how Rayleigh scattering kinda works, while keeping it "simple".

No art for you today, for once. I went skiing the other day, though. I didnt have any photos of a beautifully red sunset, but this one from the skiing trip shows both Rayleigh and Mie scattering. Wonderful. These are rather low clouds, and not alot of rayleigh scattering has occured, as theyre not that red at all, and neither is the sky. But because they are such low clouds, you can see the shadow of the horizon, which is also pretty mindblowing. The clouds nearer the horizon are illuminated from underneath by the sun, but the clouds nearer the camera and me arent, and appear darker. This is because from the perspective of the sun, those fall on the other side of the planet, and thus, are blocked from the light emanating from the sun.  The same way that we, surface dwellers, are in the shadow of the earth during nighttime.

I really do get too excited about the most trivial things, dont I?

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