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9 minutes ago, Terwin said:

This link says nothing about mirrors, but says 'night sky cooling' can cause the surface of water to freeze in specially designed troughs with ambient temperatures as high as 41f:

https://www.realclearscience.com/blog/2018/07/09/how_people_created_ice_in_the_desert_2000_years_ago.html

I read that; and frankly it's not a very 'sciency' article for something that purports to be 'real clear science.'  It also does not comport with the other stuff I have read.  

 

Sadly there are a lot of articles on the internet that look like science reporting that end up being very slim on data / details to support the claim... 

 

EDIT:  Here's a little BBC blurb showing the inside of a Yakhchal - and it describes it as being used for 'ice storage' - not generation.  BBC - Travel - The world’s oldest ice cream?

Edited by JoeSchmuckatelli
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19 hours ago, JoeSchmuckatelli said:

I read that; and frankly it's not a very 'sciency' article for something that purports to be 'real clear science.'  It also does not comport with the other stuff I have read.  

Sadly there are a lot of articles on the internet that look like science reporting that end up being very slim on data / details to support the claim... 

EDIT:  Here's a little BBC blurb showing the inside of a Yakhchal - and it describes it as being used for 'ice storage' - not generation.  BBC - Travel - The world’s oldest ice cream?

Ice storage sounds a lot more logical. I can't wrap my head around radiating heat into space while the ambient air is above zero and then getting below zero to freeze water. Seems very counterintuitive, now, there are a lot of things that are counterintuitive. But this just seems to violate the laws of thermodynamics. 

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4 hours ago, lrd.Helmet said:

Ice storage sounds a lot more logical. I can't wrap my head around radiating heat into space while the ambient air is above zero and then getting below zero to freeze water. Seems very counterintuitive, now, there are a lot of things that are counterintuitive. But this just seems to violate the laws of thermodynamics. 

Not necessarily. In the dry conditions of the desert, water will still evaporate a little, even at low temperatures. Since temperature is an average of molecular motion, the warmer molecules are the ones that break free and evaporate, carrying away more heat and leaving the ever-colder water behind (if there's even less heat input).

No, you're not going to make ice cubes this way, but getting some thin rimes of ice on a cold desert night I could see, when conditions are right.

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1 hour ago, StrandedonEarth said:

not going to make ice cubes this way, but getting some thin rimes of ice on a cold desert night I could see, when conditions are right

I've spent a lot of time in several deserts across the world - and yes I've seen snow and ice rimes - but never in the way you describe.  Inevitably you need the atmospheric conditions to support snow or frost or any icing.  It almost always occurs in the late fall through winter and in to early spring* - in other words, when you would expect to see snow or ice given the conditions. 

Basically, every time it's happened, there was never any 'surprise' (i.e. no 'how did that happen' moment). 

Evaporative freezing might not violate the laws of physics - and might be doable in a lab - but it won't happen naturally / passively during the summer in any low to mid altitude desert with daytime temperatures above 100 and nights above 60.

 

 

 

* I have seen snow fall in late June on a high desert plateau above 8,000 feet, but it was one of those crazy whip up storms that happen at altitude. 

I've also seen snow falling when the local temp is 42 degrees, but it was obvious that the air immediately above us was quite frigid.  Again - no surprise situation - and it did not stick. 

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14 hours ago, lrd.Helmet said:

Ice storage sounds a lot more logical. I can't wrap my head around radiating heat into space while the ambient air is above zero and then getting below zero to freeze water. Seems very counterintuitive, now, there are a lot of things that are counterintuitive. But this just seems to violate the laws of thermodynamics. 

Nah, the principles are solid, but I've never seen it work as described. The correct setup involves some tricky materials that provide good insulation while being perfectly clear in thermal IR. These materials exist, but that makes the contraption a lot more complicated than simply a box with a hole and some mirrors. Nonetheless, on a clear night, you have fairly low amount of IR coming in from above - a lot less than you can radiate out even at sub-zero temperatures. So if you have good insulation, you get sub-zero temperatures with fairly warm ambient.

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14 hours ago, K^2 said:

Nah, the principles are solid, but I've never seen it work as described. The correct setup involves some tricky materials that provide good insulation while being perfectly clear in thermal IR. These materials exist, but that makes the contraption a lot more complicated than simply a box with a hole and some mirrors. Nonetheless, on a clear night, you have fairly low amount of IR coming in from above - a lot less than you can radiate out even at sub-zero temperatures. So if you have good insulation, you get sub-zero temperatures with fairly warm ambient.

So - it's doable in the lab.  But even if you took those materials and buried them in a hole hoping to passively create ice by radiating heat through the atmosphere into space... Won't the ambient ground temperature be pumping heat into your system? 

 

Shaded ground still takes in heat from the atmosphere during the day - and releases it through the night, but not dramatically enough to freeze water in a hole during the summer. 

 

Also   https://www.acoolcave.org/temp.html#:~:text=This temperature will be influenced,state the cave is located.

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6 hours ago, JoeSchmuckatelli said:

So - it's doable in the lab.  But even if you took those materials and buried them in a hole hoping to passively create ice by radiating heat through the atmosphere into space... Won't the ambient ground temperature be pumping heat into your system? 

Shaded ground still takes in heat from the atmosphere during the day - and releases it through the night, but not dramatically enough to freeze water in a hole during the summer.

That's why you need good insulation. A good dewar can slow the heat getting in from environment to the trickle, so you only have to worry about the opening where you let the light through. There are aerogel structures that are very translucent in thermal IR and are as good at insulation as space shuttle tiles. If you want, you can also add a thin metal layer deposit on top to reflect visible light while letting IR through. Something like that ought to work outside, sitting on the sand during the day. That's a neat trick, right? But we're talking space-age technologies at that point. What I don't know is where the limits are for simple materials discussed in the original post. It's clear that you can get some amount of cooling with insulation on the sides and an open hole at the top, and there are simple ways to improve on that a bit, but beyond that, I have neither knowledge nor experience.

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@K^2 - I'm just fascinated that it is possible at all.

The qanat system is quite good at bringing snow melt great distances - and when you look at how they're built, and consider the will it took to build with hand tools alone.  The ability to use those and windcatchers to passively cool structures shows just how innovative our ancestors were. 

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3 hours ago, JoeSchmuckatelli said:

Yes, but is it grammatically correct to say that something is in effect a tunnel, if it is in fact and literally, a tunnel? 

1. They dig a tunnel to pass below the place with higher potential energy (because it's above head).
2. They go without getting up, at the low potential energy state.
3. The suddenly appear at the opposite side of the hill. Usually with bags, sometimes with guns.

So, the quantum physics together with shovel can bring interesting unexpected results and emulate well-known physical phenomena in rather mundane conditions.

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19 minutes ago, Shpaget said:

In desert climates where relative humidity of air is low, dew point can easily drop below freezing even without freezing air temperatures. 

That's not really saying much.  Dew point is one measure of the amount of water in the air - thus in an arid location, saying that the dew point is below freezing merely means that in order to see condensation (or dew) the surface must be below freezing. 

 

It does not, conversely, mean that because it's arid and the DP is below freezing that water or water vapor will freeze passively in a shadowy hole with the ambient temperature above freezing. 

So if you are in the high desert and ground temps are below freezing you can get frost rimes - presuming the ground temp and DP cooperate (i.e. a 30 degree rock won't precipitate water from the atmosphere if the DP is 10 degrees) 

Edited by JoeSchmuckatelli
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9 hours ago, Shpaget said:

In desert climates where relative humidity of air is low, dew point can easily drop below freezing even without freezing air temperatures. 

This allows to condense water with a plastic bag but not a haloxylon in icicles.

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So... I found a stuff on Youtube that compressed a compilation of songs (around 60 minutes of songs) into 2 seconds-video. The idea is if you played the song at super ultra-slow speed, you could listen the entire song. So I'm wondering, if audio files can be compressed into shorter length (for example, to send a secret message in a particular segment of a song), is there a limit of message length that can be compressed into 1-second audio before it starts to be unreadable as a message (aka, just garbled noise no matter how slow it's played, like the pixels on "zoom and enhance" situation)?

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1. Download Audacity.

2. Record you speech.

3. Select All (Ctrl-A).

4. menu →"Effects" → "Change Speed..."

5. Enter a floating number multiplier (up to 50.000), "Listen", "OK"

6. Repeat 4 and 5 but now enter 0.02 to restore from 50x. "Listen".

For me, a 50 times compressed record after restoring sounds still listenable.

So, you can experiment by applying the multiplier twice. Say, 50, then 10, then 0.1 and 0.02.

Edited by kerbiloid
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36 minutes ago, kerbiloid said:

1. Download Audacity.

2. Record you speech.

3. Select All (Ctrl-A).

4. "Change Speed..."

5. Enter a floating number multiplier (up to 50.000), "Listen", "OK"

6. Repeat 4 and 5 but now enter 0.02 to restore from 50x. "Listen".

For me, a 50 times compressed record after restoring sounds still listenable.

So, you can experiment by applying the multiplier twice. Say, 50, then 10, then 0.1 and 0.02.

This confuses me. 
Say you sample at 44 KHz who gives the highest frequency recorded is 22 KHz, if you speed it up 10 times all above 2.2 KHz is lost, with 50 times cutoff is 440 hz.
Now in Audacity the signal is already digital and is compressed to mp3 or similar compression who drops information with little relevance, I guess mp3 compression algorithm in Audacity is smart enough to understand this is an very high frequency recording and handle it as so? 

And yes stuff like this has been used, old long distance phone systems tended run at say 1 MHz, you then speed your speech 250 times and used time multiplexing to have 250 voice channels on the line, on the other end this was split up and slowed down again, here cutoff was 4 Khz. 
It also has been used to send fast radio messages who is hard to track the location of, nice if you are an spy or an submarine, here you would typically use Morse who can be compressed a lot. 

Now day you only send an digital signal. 

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1 hour ago, magnemoe said:

stuff like this has been used

It's a steganography.

1 hour ago, magnemoe said:

Say you sample at 44 KHz who gives the highest frequency recorded is 22 KHz, if you speed it up 10 times all above 2.2 KHz is lost, with 50 times cutoff is 440 hz.
Now in Audacity the signal is already digital and is compressed to mp3 or similar compression who drops information with little relevance, I guess mp3 compression algorithm in Audacity is smart enough to understand this is an very high frequency recording and handle it as so? 

The signal of your speech gets less smooth and more distorted, but its shape stays recognizable.

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10 hours ago, kerbiloid said:

It's a steganography.

The signal of your speech gets less smooth and more distorted, but its shape stays recognizable.

Yes but in an megabyte sized data stream its way better methods to transmit data. Or other channels. 
And some other effects than just cutting high frequency is used here as described above. 

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