Thursday 9 January 2014

Red sky at night...

An article in The Daily Mail featuring photographs of some polar bears watching the sky turn red as the sun sets reminds me of something else which has been bugging me recently.

We know that the explanation for red sunsets is down to the scattering of light by molecules and particles in the atmosphere, blue and UV light is scattered more and red and yellow light is scattered less, see e.g. Science Daily.

One of those TV scientists (Coxy, I think) demonstrated this by filling a long fish tank with very diluted milk and shining a white light in at one end. The milk solution at that end appeared blue-ish and at the other end appeared orangey-yellow.

So that's all perfectly plausible - but this explanation suggests that the sun/sky would be the same red-orange colour at sunrise as it is at sunset.

I'm no early riser, and have seen far fewer sunrises than sunsets in my life, but by and large, I have seldom noticed that red-orange effect in the morning, it's much more noticeable at sunset.

Have I missed something or does everybody else have the same impression?

(I can think up a perfectly plausible explanation for this, which might or might not be nonsense - during the day, more particles are emitted, more dust is thrown up etc, and as the sun warms the atmosphere, it creates convection currents which stir it all up, so at sunset, the sun is shining through the muck it has spent all day churning up.

During the night, the atmosphere cools and settles, and most of the particles and dust sinks back to the ground, so in the morning the sun is shining through relatively clean air again, so there is less scattering of the blue and UV light.)

10 comments:

View from the Solent said...

Mark, you've missed nothing, sunrises tend to be redder. In line with your explanation.

Dinero said...

yes warm air would be a candidate, but what about people on the west coast viewing the sunset over the sea.

PJH said...

This Yahoo question would indicate that someone else agrees with Mark's hypothesis.

On the other hand this Physics forum post rather amusingly suggests it has something to do with red-shift and blue-shift due to the speed of the rotation of the earth wrt the sun. Highly giggle-worthy.

Dinero said...

not completely a giggle the observever is rotating away from the sun at sunset and vise versa, not that the speed would be large enough.
GPS calculations use the observers rotataion speed in relation to source of the GPS satellite signals for the highest accuracy.

Mark Wadsworth said...

Thanks all.

OT1H, I'm chuffed that I observed and guessed the answer correctly for once, OTOH I'm a bit disappointed that I haven't learned something new and counter-intuitive today which I can bore people with in future.

PJH said...

You could always mess with their minds and use the blue/red-shift explanation Mark...

Robin Smith said...

Imagine you are standing on the moon during an eclipse of the sun, the earth just about blocking the disk of the sun.

What do you think you would see?

A black disk surrounded by a thin edge of red atmosphere?

Also google NASA ISS photos of space walks etc and see the same kind of thing when the ISS or Shuttle is on the dark side... not of the moon!

Mark Wadsworth said...

RS, nope, seen from the Moon, Earth is ten times as big as the Sun (or whatever ratio, but it is a lot bigger). The Sun would disappear behind Earth completely.

PJH said...

Lunar eclipse as seen from the moon. Another link (with more pictures).

Mark Wadsworth said...

PJH, thanks, I was right as to relative size (I didn't have to look that one up) and in some photos the atmosphere lights up with and in some it lights up red-orange. Hmm.