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Planetary Boundary Layer question

thestug

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Lasers seem to end after a few hundred meters because of the planetary boundary layer. According to this this is because of the effects of particles that exist because of air interaction with the particles on the ground. However, what the article fails to explain is when the PBL is at its thickest. Does anyone know of when the PBL is thick to make the longest apparent beams?

Laser Pointer Safety - Why laser beams outdoors seem to end
 

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I believe the altitude and density fluctuates with the every changing atmospheric and weather conditions but I am not 100% sure. That's a good read I linked it in another thread. Did you click on the link that was in the page you posted? It has the troposphere ending at 10 Kilometers and the PBL is the upper part of that. So id estimate around 6-8 kilometers. Here is the link. RASC Calgary Centre - The Atmosphere, Astronomy and Green Lasers

Yea, here it is it was in the link you posted maybe you didnt see it.

The graph above was produced from elements of the "Standard Atmosphere Model" [1] [2]. Actual temperatures and pressures and the specific heights of the layers - especially those lower in the atmosphere - depend greatly on Solar and local conditions as well as local weather.

The Troposphere - Where the Atmosphere Begins and Where We Live
varies in thickness
begins at the Earth's surface and extends to 6 km (4 miles) high at the poles and up to 20 km (12 miles) high at the equator
The summit of Mount Everest at 8.848 km or 5.498 miles) is a little less than 1/2 the height of the Troposphere.
at +/-50° Latitude it is about 8.9 km (5.5 miles) thick
Almost all weather occurs in this layer of the atmosphere.
The gases become thinner with height and the temperature also decreases from "surface temperature" to about -51°C (-60°F).
The transition boundary between the troposphere and the layer above is called the tropopause.
Both the tropopause and the troposphere are known as the lower atmosphere.
50% of the atmosphere by mass is below an altitude of 5.6 km.
90% of the atmosphere by mass is below an altitude of 16 km.
The common cruising altitude of commercial airliners is about 10 km.
The commonly known "Jet Stream" varies in altitude between 7-16 km.
 
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Atomicrox

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This has been discussed before. There was no consensus if the cause of the beam ending was the PBL or just perspective. IMHO it has more to do with perspective, else beams pointed at the horizon would look longer.
 

Teej

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There are a few factors to consider...

The beam is going to ONLY be visible if it hits something that then reflects light to your eyes....say water vapor or dust particle, etc.

So, the more stuff in the air to hit, the brighter the beam will be able to appear to an observer.

The other is the resolution of your eye as range increases. When you look at say a railroad track receding into the distance, it APPEARS that the tracks get closer and closer together as the distance from you increases.

At some point, you will lose the ability to see two tracks, and see only one line. This is because the farther away something is, the smaller the percentage of it is in your field of view...and the less resolution you have.

How far away from you would you be ABLE to see something that was small, perhaps a few inches across?

Is there a distance that you might be able to make out a turkey buzzard but not a hummingbird?

The dowel shaped laser beam is a small target to resolve, and, at longer ranges, will be such a small percentage of your field of view, that you will no longer be able to see it.

Just like RR tacks "converge than disappear into the distance" so will your beam.

Add to that that the stuff in the air is gradually reducing as you enter the upper atmosphere, so there's less beam visibility...and now its like those RR tracks are not only really far away, but being painted a non-contrasting color to boot.

So, at some point, the beam will simply be to hard to see from where its emitted from.


At the RECEIVING end, the opposite is happening...the tracks are getting CLOSER, and easier to resolve. Now, in space, I don't know how much stuff is floating around to reflect the light to your eyes/camera, etc...but, by the time the beam is in space, its fatter, and, closer at least.
 
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Atomicrox

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There are a few factors to consider...

The beam is going to ONLY be visible if it hits something that then reflects light to your eyes....say water vapor or dust particle, etc.
Not true. Pure nitrogen also scatters light (but "dirty air" helps a lot!).
 

Cyparagon

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If the effect were adequately explained by the atmosphere ending, laser beams would appear much longer when pointed at the horizon (since there is a lot of atmosphere to our left and right, but comparatively little above us), but they don't.

Even if the laser went on forever, it would still appear to end because of the angle you're looking at it. Imagine if the earth were flat, but infinitely large. It would still have a horizon. That is, it would still appear to "end" at the vanishing point.
 
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Teej

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Not true. Pure nitrogen also scatters light (but "dirty air" helps a lot!).
I don't think you'd see the beam in any real sense in "Pure Nitrogen"...as the scatter would not be visible under most circumstances.

Laser scatter particle measurement devices can be zero'd in N2 for example, etc.

:D
 

Atomicrox

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I don't think you'd see the beam in any real sense in "Pure Nitrogen"...as the scatter would not be visible under most circumstances.

Laser scatter particle measurement devices can be zero'd in N2 for example, etc.

:D
Scattering in air gas molecules (mostly O2 and N2) is one of the chief reasons the sky is blue. From Rayleigh scattering - Wikipedia, the free encyclopedia

Rayleigh scattering (pronounced /ˈreɪli/ RAY-lee), named after the British physicist Lord Rayleigh,[1] is the (dominantly) elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. (...) The particles may be individual atoms or molecules. It can occur when light travels through transparent solids and liquids, but is most prominently seen in gases.
(...)
Rayleigh scattering of sunlight in the atmosphere causes diffuse sky radiation, which is the reason for the blue color of the sky and the yellow tone of the sun itself.

For historical reasons, Rayleigh scattering of molecular nitrogen and oxygen in the atmosphere includes elastic scattering as well as the inelastic contribution from rotational Raman scattering in air, since the changes in wavenumber of the scattered photon are typically smaller than 50 cm−1.


I know nothing of those devices, but I'm guessing the need for zeroing under N2 is precisely to account for the natural scattering from N2 ;)
 

Teej

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Scattering in air gas molecules (mostly O2 and N2) is one of the chief reasons the sky is blue. From Rayleigh scattering - Wikipedia, the free encyclopedia





I know nothing of those devices, but I'm guessing the need for zeroing under N2 is precisely to account for the natural scattering from N2 ;)
LOL


The sky is not blue if you look at a section of it...only up into outer space, etc.

IE: There's not enough N2 scattering to see...in this context at least.

The zeroing uses filtered N2, or even particle free dry AIR, as it doesn't cause enough laser scattering to register above zero in a detection chamber.
 
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