Winter sun and dust

[principled]

A pretty basic question for your Monday evening entertainment!
I am sure we have all noticed that in the winter the low angle of the sun's rays appears to show up every stain/mark on a pane of glass, and when it's light enters a room, one always sees the dust that is floating in the air.
Now, is this effect because:
a) The light rays of the low sun are hitting vertical bodies such as windows nearer a 90 deg angle than in they do in summer (this would seem logical)
or
b) The low winter sun is hitting the window at a more acute angle than in summer?

and the supplementary question...Why does it have this effect?
P

[abacus9900]

b), I think you'll find. Because of the tilt of the earth's axis, during winter it is inclined away from the sun's rays which means they strike the countries in the northern hemisphere at a more shallow angle than during the summer. Presumably, this makes shadows longer and makes things appear more 'spread.'

[Progenitor A]

Nov 15, 2010 19:00:15 GMT 1 principled said:

A pretty basic question for your Monday evening entertainment!
I am sure we have all noticed that in the winter the low angle of the sun's rays appears to show up every stain/mark on a pane of glass, and when it's light enters a room, one always sees the dust that is floating in the air.
Now, is this effect because:
a) The light rays of the low sun are hitting vertical bodies such as windows nearer a 90 deg angle than in they do in summer (this would seem logical)
or
b) The low winter sun is hitting the window at a more acute angle than in summer?

and the supplementary question...Why does it have this effect?
P

Both possibilities seem odd to me.
The angle that that the sun makes with objects on earth depends only upon its elevation.
The sun has the same elevation in winter as it does in summer except that some of the higher elevations are missing (and both of your possibilities are concerned with lower elevations)

It probably (if it is true) concerns the quality of the air - the air in winter tends to be clearer than it is in summer - (the water vapour content, for example is less in the cold winter days, and water vapour collects dust) and clearer air means less diffusion of the light so it is more intense. (So oddly the answer may reside in the thread title!)

[carnyx]

The sun spends far longer in the 'low zone' in the winter, and also more peope are awake at these low-zone times to see the effect.

At these times the spots on the window are very strongly backlit, so the local contrast ratio between the (black) spots and the surrounding bright glare is very high, and are obvious. If you look closely ( as the house-proud do) you will still see these spots even in lower-contrast conditions later in the day when the sun is high.

If you can recall film and slide projectors, you would remember the bright dust-motes and cigarette smoke hanging in the air in the projector beam, visible against the dark background ... and the dark dust on the projection lens which is visible agaist the bright glare. They become visible becase of the locally high contrast ratio.