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Re: (meteorobs) streetlights in the cold....



Perhaps you can explain something to me.  If there is no condensation in the air,
does the air become less transparent?  After all moist air is less dense than dry
air, i.e. less mass per unit volume.

Robert Goler wrote:

> > 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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