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Uplight of downtown Montreal

Uplight of downtown Montreal

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Top strip spectrum is of a new Sc-Na-Hg metal-halide lamp. Below is the spectrum off the clouds near the hot spot. The amount of "uplight" is obvious, and now we know what this uplight is made from; plenty of HPS & MH. There's also tri-phorphor fluorescent sources (611nm peak), which are contributing to the Hg lines as well. Some streets (10%-20%) have LED system; the continuum around the 440-450nm area is the result.

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Album name:lights*plus / Lamp Spectra
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Date added:Aug 04, 2016
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rjluna2
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Aug 04, 2016 at 06:10 AM Author: rjluna2
I wondered what is the spectrum absorption of water vapor?

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Aug 04, 2016 at 10:25 AM Author: Medved
By the way what instrument are you using to acquire these spectra? It seems to be pretty sensitive...

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Aug 04, 2016 at 02:47 PM Author: lights*plus
I used the same "setup" employed throughout this Lamp Spectra gallery: A modified camera with my home-made spectrogrphic box. The exposure for the above spectrum was 900 seconds but stopped down to f4. Needed to resolve the lines better to discern the 611nm triphosphor peak. But it wasn't really necessary..my skyglow is bright enough to record in all its glory.

As for the absorption spectrum of water vapor, see here or here. Mainly an IR absorber.
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Aug 04, 2016 at 03:16 PM Author: Medved
And where did you get the grating?

And I admit, for the water it was just a wild guess theory. The fact is, the atmosphere is changing over night and the only component that significantly changes it's concentration is the water vapor...
Other hypothesis (this time taking into account the absorbing spectra of the water vapor):
Initially at the evening it is high (because the air is warm, so easily holds a lot of water in a gas form). That means all wavelengths longer than blue are suppressed. How much depends on how far they are from the blue end. Then till the morning it cools down, the water condenses (so it occupies less volume; the droplets then scatter the light in a broadband fashion, so no effect on the spectra alone, so the net effect comes from the less water vapor in the air), so the air contains less and less water vapor. That reduces the absorbtion of all the longer wavelengths, so it lets them rise, so the whole spectrum is tilting. So it tilts so the blue appear to go down, just because the rest goes up. But according to the water spectra this effect should be rather broadband (really the whole spectrum "tilting") and not affecting only some small part of the spectra (a "hole" or "peak" appearing/disappearing)

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lights*plus
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Aug 05, 2016 at 10:02 PM Author: lights*plus
Medved, you're on the right track, but not exactly correct in saying: "so the air contains less and less water vapor" by the morning. I studied meteorology & I can say that everyone has a misconception of the distinction between water-vapor and mist/fog/clouds which are water droplets (or ice crystals).

Like any molecule in the gaseous state, H2O gas, is tiny & will scatter just like oxygen & nitrogen molecules - the intensity of which is wavelength dependant given by I ~ 1/λ4power (Rayleigh scattering). Our atmosphere can hold only from 0 to 4% water by volume yet the role of water is quite important in "reflecting" light in our atmosphere. Turns out that nocturnal cooling does not "change" the content of water-vapor but what changes is the "relative humidity" which is tied to the "Saturation Vapor Pressure" which is the ability or capacity of air to hold water in the gaseous state for a given temperature.

Water is continually evaporating, condensing & precipitating in a vast cycle. Water evaporates into a gas because a fraction of the surface molecules on bodies of water acquire sufficient kinetic energy to escape their intermolecular bond. This vapor, a dry gas, exerts a pressure which is temperature dependent. For every temperature there is a maximum vapor pressure (tor or millibars) that can be exerted whereby the space above the liquid surface becomes saturated. This maximum value is the Saturation Vapor Pressure. Air at 30°C (86°F) has the capacity to hold 5 times the amount of water-vapor as air at 5°C (41°F). At any time, the actual vapor pressure can be some value less than the saturation vapor pressure. Condensation will occur if the rate of evaporation is sufficient so that the actual vapor pressure reaches or matches the saturation vapor pressure. The excess moisture will form fog, mist or clouds. When this occurs, Rayleigh scattering is gone; the actual scattering is independent of wavelength (becoming simple reflection not scatter).

Yet even without condensation (fog & mist) the higher the temperature, the greater the vapor pressure. Governed by the laws of probability, a tiny fraction of this vapor condenses back into a liquid becoming suspended as tiny water droplets. Warm air means greater water droplets, cold air means smaller numbers of water droplets (this is without fog, mist or clouds)..so that is what you alluded to as the night cools.

It's a truely complex topic which is beyond the scope this lighting board. I only wish that I heven't offended or turned-off anyone by going on too much. Thanks for bearing!
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Aug 07, 2016 at 12:13 PM Author: Medved
You are exactly right about what happens, but not exactly about what I had in mind.
What I was speaking about was the condensation that uses to happen mainly during clear or nearly clear nights (so the air cools down a lot) mainly after a hot day (so the atmosphere picked up a lot of water vapor to start with). This condensation then alters the absolute water vapor content a lot (the absolute content, I really do not speak about relative humidity, as that does not matter for the light at all). And since quite short time after sun set, the RH uses to rise to 100% and then stay there the whole night (or even a bit above 100%, if we use the RH to characterize even the supercooled vapor state). Then it just starts to condense and that is the moment, when the water content and so the light distribution starts to change.
Moreover unless there is dust or so in the air (to form the condensation nuclei, so the water does not supercool that much), the water vapor pressure becomes varying along the height, as in lower levels the ground objects provide the required condensation spots, so the air gets dryer there.
This layering then should have it's effect on the light distribution as well.

The light spectrum is indeed influenced by all air components, but the water is, what changes a lot. Plus it uses to alter if it is gas, liquid droplets or ice crystals (or nothing at all), each form influences the light in a different way.

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Aug 07, 2016 at 02:52 PM Author: lights*plus
I wholeheartedly agree with what you're saying, especially that light waves are influenced by the entire height of the atmosphere. But I need to emphasize that air does not get dryer as condesation occurs; the capacity of air to hold water has decreased due to coolling.

Water exhibits an "either-or" situation when it condesnses into droplets or ice, which are far larger than any wavelengths of light (400-700nm). When condensation ocurs, boom, droplets & ice crystals (no matter what their size) scatter all light independent of wavelength. They'll become READILY visible to us as mist, fog on or near earth's surface, or various clouds at increasing heights. The droplets will take on the color of the dawn, the morning sun, or the color of street lights since the scattering is independent of wavelength.

There are condensation nuclei that are at about the size of wavelegths of light.. Chemical smogs, fine pollutants, the finest suspended dust and sea-salt particles as well as finer spores and pollen in spring & summer will scatter light in a different manner. Such particles tend to be concentrated by local meteorological conditions known as inversions and are clearly visible in the daytime as a greenish-brown or yellow layer commonly visible above an urban center (always below a blue upper sky). A more complete theory of scattering was developed by Gustav Mie (circa1908) to account for scattering by any particle, including for particles much larger than any wavelength of light. Mie's mathematical theory revolves around numerical solution of Maxwell's equations that is beyond my grasp. None the less, for particles with sizes of about the wavelength of light the simplified relation becomes: Scattered Intensity ~ 1/λ (or just one over wavelength) instead of the classic Rayleigh scattering (one over 4th power of wavelength). My point continues to be, it's a complex topic.
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Aug 07, 2016 at 03:26 PM Author: Medved
The decreased capacity of holding water is, what then leads to the increase of the RH and the condensation in whatever form (dew on object on the ground level, mist/fog/cloud if there were condensation nuclei in the air)...

I agree the droplets/crystals are far larger than the wavelength, but beside the scattering, they cause different diffraction effects. And these are mainly important when they are not yet so dense, so you do not see them as a clear mist/haze.
The dirt forming the nuclei is usually by itself not enough to cause any visible effects, but the main influence comes when it presents a spot, where the supercooled vapor starts to condense. Less of these, fewer these spots are, so deeper supercooled the vapor becomes before it turns into liquid/solid (it does not condense by itself, it has to hit something to stick on, preferably a water droplet or ice crystal, but any object causing some surface effects will do the job). So when the air is really clean (just very few such nuclei, so they get loaded by the water and wiped off the air pretty soon), the air may cool down way below the temperature that should lead to condensation (so the water holding capacity drops below what water vapor is still in there; that is what I called when "the relative humidity exceeds 100%"), while the water still remains as a vapor (even when the amount of water vapor exceeds the capacity of the air at that temperature, until it reaches some nuclei, it still remains as vapor).

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May 03, 2017 at 11:19 PM Author: Lumex120
Do you live on Jupiter or something?

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