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Re: (meteorobs) streetlights in the cold....
Thank-you for your response. I have noticed that the light seems to travel
much further on colder nights. However, I still have a problem: On those
cold nights, why does the light appear to travel directly upward any more
than it does in any other direction? (It doesn't appear to depend on the
amount of fallen snow.) By your explanation, it seems to me that the light
from the lamps ought to appear brighter overall, rather than simply in one
direction.
> I'm wondering whether anyone has a scientific explanation for why the
light
> from streetlamps appears to travel upward on very cold nights....
>
It should be noted that even on warm nights, the light from streetlamps
also travels upwards. The key thing here is that on cold nights, there
tends to be more moisture in the air, which provides more 'particles' in
the air to scatter light. It is this additional scatter which allows
us to see the light visible, a larger distance away from the streetlamp,
then what would occur under a warmer night.
Such an effect could be emphasized perhaps if there is snow on the ground.
But since I've never seen snow, I'm not too sure on this one.
As to why there is more moisture on colder nights, this stems from the
fact that colder air has a reduced capacity for holding water vapour. That
is, the air temperature is close to its dew point temperature, which is
the temperature at which the air would become 100% saturated with water
vapour. Any additional vapour would condense out as very tiny water
drops, eg fog, and thus add to the scattering. However, even if the air
isn't completely 100% saturated, there will still exist these tiny water
droplets in smaller numbers though.
In fact, whenever you exhale on cold nights, that "smoke" coming out of
your mouth is caused by the moisture in your breath briefly saturating the
air, and is, for want of a better name, 'fog'. The rate at which it
disappears from view is determined by how close to saturation the air is.
The faster it disappears, the "drier" the air is.
I hope some of this helped.
Cheers
--
Robert A. Goler
E-mail robert@neumann.maths.monashdot edu.au
http://www.maths.monashdot edu.au/~robert/
Department of Mathematics and Statistics
Monash University
Clayton, Vic 3168
Australia
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