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8 minutes ago, magnemoe said:

Water vapor generate greenhouse effect in the atmosphere, co2 in the stratosphere. 

I assume you mean troposphere. Human emitted Co2 is in the troposphere (lowest storey of the atmosphere) where most of the weather and climate takes place. The stratosphere plays little role there.

8 minutes ago, magnemoe said:

And its an significant effect, its why deserts are so cold during night compared to other places.

There are a lot of different deserts. The ones i've been to (Sahara, Arabian) were hot during day and night.

8 minutes ago, magnemoe said:

Was about to say that clear winter days are colder than cloudy but that might be the other way around as in no clouds then its very cold. 

That is because of radiation weather. No clouds mean that the sun can radiate in during day without hindrance, but at night radiation escapes to space. Brrrr, space ... :-)

8 minutes ago, magnemoe said:

As water vapor is local it would contribute to urban heat islands, on the other hand an humid air help wash out pollution and its hard to reduce as its mostly because of weather. 

That's more an effect of the type of ground. An urban area heats up during sunshine because there is little foliage that could evaporate water, dark concrete or asphalt, trenches between reflecting house fronts, and local geography may play role if such an urbanisation lies in a valley with little exchange. Otoh, the often rising warm air above cities can actually be a reason for condensation and forming of clouds (cools) and draws air from the surroundings. Depends on the weather :-) The effect of water vapor can actually be cooling or warming.

8 minutes ago, magnemoe said:

Finally fossil fuel also generate lots of water vapor then burning, 

Lots is a relative word. The steam plumes you see over fossil power plants are from heat exchangers and cooling towers. For the flue gas, the carbon dioxide in it is much worse than the water vapor.

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Water is a "better" greenhouse gas (I hate that the name 'greenhouse' stuck, because it is a poor analogy), but we are saturated on impact of water vapor, so more of it does very little difference. CO2 happens to be a good IR absorber in bands were water isn't. It perfectly plugs holes were IR currently ecapes. Crucially, at high altitdes. That's why small changes in CO2 can make so much difference.

Desert temperature variations have a lot more to do with a) it almost always being clear weather, impact of which @Green Baron covered, and b) lack of bodies of water to regulate temperature. Humidity of the air has almost zero direct impact. Further, greenhouse effect is more about shifting mean by moving thermodynamic equilibrium higher up, into colder air, then it is about day-night variations. Clouds have more impact on later, because they scatter both visible light and IR.

Edited by K^2
weird typo
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Maybe this is superfluous to mention, but sometimes forgotten in detailed views:

The sun heats the ground, not the atmosphere. The ground emits IR, which is then either let out into space, reflected at clouds or absorbed (and maybe re-emitted) by atmospheric gases. The absorption part is the greenhouse effect that heats the atmosphere indirectly. So, greenhouse isn't all that bad, as a greenhouse is meant to trap the reflected heat from the ground that was warmed by the sun, like the gasses that absorb part of the radiation that would have otherwise escaped to space or played around elsewhere :-)

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Altitude plays a huge role in greenhouse effect. If greenhouse gasses were confined to the bottom 1km, the impact would be rather small. They'd mostly work like clouds already do, make IR stick around longer, which would make day-night variation less without impacting the mean temperature all that much.

What you have to keep in mind is that the atmosphere works like a refrigerant. It warms up when compressed and cools when it expands. Since convection currents currently circulate the air, this means that perfectly translucent temperature would establish a very predictable temperature gradient from ground to upper atmosphere, getting colder and colder as you go up. This is where things start getting dramatic.

First, picture a vacuum world. Lets put a shell of IR opaque, visible transparent material around it. For simplicity, lets say that albedo of the planet itself is constant across the band and << 1. All that happens is that the shell warms up to match ground temperature, at which point it starts to radiate as much heat as the planet did before. Surface temperature goes up by a few degrees to compensate for a bit more IR going up and down. If the shell is thin, no significant change on the planet. If it's thick, it can also act as clouds and reduce day-night temperature fluctuations.

Now we throw in atmosphere. At airliner altitudes, the temperature is already at -50oC, and there is still quite a bit of atmosphere to go. Lets throw that shell there. Black body radiation has T4 dependence. At -50oC the planet isn't dumping as much heat as it's receiving from the Sun. The shell has to warm up to the same temperature it would have been in the vacuum case before thermal equilibrium is reached. And that 50oC difference with ground? That doesn't go away. Atmosphere keeps pumping heat between layers until average temperature on the ground is 50oC higher. 0_0

This is why Venus is so %$#@! hot. No amount of "it works like a greenhouse, keeping IR in" can explain that temperature difference. You have to take atmosphere into account, and Venus has very thick, very active atmosphere. All of the heat exchange with Sun and space happens in upper layers, and the thick Venusian atmosphere keeps pumping heat to the ground.

This is why everybody's freaking out a little bit. Increasing IR absorption just a little bit at high altitudes will yield a significant increase in ground temperatures. That will raise cloud layers, pushing moisture up, further increasing ground temperatures. That will release various gasses from polar ice caps, making atmosphere thicker and adding greenhouse gasses, further increasing the temperature... And we have no idea where the tipping point is. I mean, the planet has definitely been hotter and had way more CO2, and it has recovered, so I don't think Venusian scenario is likely. But if it takes ten million years, it won't matter much to us.

Of course, panicking is pointless as well. Knowing how bad we're doing would be a good start. The system is so complex and has so many moving parts, that I still haven't seen a single model that puts either the "it's already too late, and it's a runaway process" or "it'll quickly recover if we cut back" outside of the error bars. We definitely need more research.

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Lets have neutron star or white dwarf.

Then we dump antimatter on it - for example 0.1% of that star mass per year or per day in continuous stream.

Antimatter crashes on surface emitting charged particles, that may or not may get trapped and gamma rays, that easily escape.

This means mass drops very slowly.

Will these stars will eventually puff up and be normal gas giants?

Or these stars will simply explode after dropping below certain mass?

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13 hours ago, raxo2222 said:

Lets have neutron star or white dwarf.

Then we dump antimatter on it - for example 0.1% of that star mass per year or per day in continuous stream.

Antimatter crashes on surface emitting charged particles, that may or not may get trapped and gamma rays, that easily escape.

This means mass drops very slowly.

Will these stars will eventually puff up and be normal gas giants?

Or these stars will simply explode after dropping below certain mass?

 

Wow thats a lot of antimatter, something big will happen...lemme just google some stuff real quick...

 

Ok, so, Im going to assume 1 solar mass, just for simplicity, but you can scale it up without changing my point too much:

0.002 (0.1% antimatter + 0.1% matter) solar mass, converted to energy, is about 3.6e44J

The sun's current power output is about 4e26W

So even if you dribble the 0.1% antimatter into the star over a year, the resulting energy release will increase the energy output by 2-3billion times

In other words, you will blow up the star.

Or at least severely disrupt it probably to the point of complete dispersal, the biggest problem with this prediction is that the annihilation may not be complete as unreacted antimatter may be blown away.

 

Matter-antimatter annihilation is powerful and we are talking about amounts measured in solar masses!

I dunno what would happen if it was a neutron star, but its such a colossal energy release that I will still go with "blows up".

 

There are ways in nature though, that a star can slowly lose its mass. There are examples in astronomy of binary pairs where one object is a neutron star or black hole - in close proximity to a more regular star, tidal effects from the intense gravity of the dense object can strip away mass from the normal star, eventually causing significant effects. Depending on various things, many things can happen, and a supernova is one of the possible outcomes.

Edited by p1t1o
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I may be totally wrong, but i have the impression that the question may be: What happens, if a neutron star steps over the chandrasekhar limit from below, i.e. its mass returns back into the range of a white dwarf. Will it fly apart ?

Yes probably, but it won't return to a white dwarf state and a white dwarf won't return to a red giant state. It is only the nuclei of the stars that formed the denser objects, the atmospheres where lost in the process. But afaik these processes don't happen irl, in contrary, dense matter attracts more matter.

A physicist may find a better explanation, i only have very basic knowledge :-)

----------------

 

Edit: that brings me to a question: The sun has Schwarzschild radius of 3km or so (?). Can we imagine the part below this horizon as a black hole or is it only degenerated and will "reappear" if we stripped the sun of its mass ? Same question for a neutron star, where the ratio between radius and Schwarzschild horizon is much bigger.

Edited by Green Baron
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25 minutes ago, Green Baron said:

Edit: that brings me to a question: The sun has Schwarzschild radius of 3km or so (?). Can we imagine the part below this horizon as a black hole or is it only degenerated and will "reappear" if we stripped the sun of its mass ? Same question for a neutron star, where the ratio between radius and Schwarzschild horizon is much bigger.

The Schwarzchild radius is just the radius you need to squeeze the entire mass of the object into, in order for it to collapse into a black hole.

For object like our Sun, the Schwarzchild radius is largely irrelevant, it doesnt do anything nor is the mass underneath it anything special.

If you remove mass from an object, the Schwarzchild (Is there a shorter notation for that??) radius shrinks.

Edited by p1t1o
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30 minutes ago, p1t1o said:

The Schwarzchild radius is just the radius you need to squeeze the entire mass of the object into, in order for it to collapse into a black hole.

For object like our Sun, the Schwarzchild radius is largely irrelevant, it doesnt do anything nor is the mass underneath it anything special.

Thanks. That clarifies my misconception.

 

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16 hours ago, raxo2222 said:

Or these stars will simply explode after dropping below certain mass?

I would guess that the white dwarf would not explode if you took mass away.  It's already fused all the elements it can with the mass it already has.  It would probably become some kind of weird planet.

The neutron star on the other hand might.  Electrons in the neutron star have been forced into the nuclei of atoms turning protons into neutrons (ok bad explanation but I'm on the fly here).  I would imagine that as you took away the mass that holds the neutron star together away neutrons would start to reorganize back into atoms and some kind of fusion would take place blowing the star apart.        

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On 9/20/2018 at 1:58 PM, KG3 said:

I would guess that the white dwarf would not explode if you took mass away.  It's already fused all the elements it can with the mass it already has.  It would probably become some kind of weird planet.

The neutron star on the other hand might.  Electrons in the neutron star have been forced into the nuclei of atoms turning protons into neutrons (ok bad explanation but I'm on the fly here).  I would imagine that as you took away the mass that holds the neutron star together away neutrons would start to reorganize back into atoms and some kind of fusion would take place blowing the star apart.        

We have found some very dense planets around other stars, typically very close to the star and an density higher than lead. Think its the degenerated core of an gas giant there all the gas has been blown away so degenerated matter it at least to some degree metastable. 

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On 9/17/2018 at 10:02 PM, K^2 said:

The only torpedo in operation that I'm aware of that benefits from pointy nose is VA-111 Shkval,

What that article doesn't show is the actual tip of the nose cone.   It's not pointy at all.   It's actually blunt and flat, and looks similar to a hot water tap in some designs, and a large disc in others.   This large flat surface, immediately followed by a hollow area behind it, creates the cavitation bubble the torpedo travels in.   Since the rest of the torpedo doesn't actually touch the water, you only need the force to push the little flat surface at 250 knots through the water. 

As to the original question... 

First off, most torpedo drive propellers have a top limit to how much force they can produce.   Any faster and the blades start cavitating and lose power.  So while a pointy nose might get them a little more speed, it's not really worth it. 

Secondly, Space.  The amount of room you have on a submarine is very limited.   Even more so inside a torpedo tube.    Having a long pointy nose that doesn't hold anything useful just takes up space that might be used for other more useful things. 

Thirdly, acoustics.   Most modern torpedos have some sort of active and possibly passive sonar system in them.   This allows for the fish to lock onto and home in on their intended target.   A pointy nose cone won't allow sonar systems to work as effectively.   This also allows for some interesting tactical plays by the firing sub.   The sub can fire off the torpedo, running at a slow speed for stealth,  at a large offset angle to the target.  When the fish gets to a predetermined location, it can turn towards the target, fire off it's sonar, and max out it's speed.   So now, even if the target evades the torpedo, it will think it was fired from a different direction than it actually was. 

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https://en.wikipedia.org/wiki/Flute#Acoustics

Quote

Acoustics

A flute produces sound when a stream of air directed across a hole in the instrument creates a vibration of air at the hole.[33][34] The airstream creates a Bernoulli or siphon. This excites the air contained in the usually cylindrical resonant cavity within the flute. The flutist changes the pitch of the sound produced by opening and closing holes in the body of the instrument, thus changing the effective length of the resonator and its corresponding resonant frequency. By varying the air pressure, a flutist can also change the pitch by causing the air in the flute to resonate at a harmonic rather than the fundamental frequency without opening or closing any holes.[35]

Head joint geometry appears particularly critical to acoustic performance and tone,[36] but there is no clear consensus on a particular shape amongst manufacturers. Acoustic impedance of the embouchure hole appears the most critical parameter.[37] Critical variables affecting this acoustic impedance include: chimney length (hole between lip-plate and head tube), chimney diameter, and radii or curvature of the ends of the chimney and any designed restriction in the "throat" of the instrument, such as that in the Japanese Nohkan Flute.

 

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5 hours ago, ARS said:

What caused bombs dropped from aircraft to "whistle"? I know not all bombs dropped have a whistling effect, but what caused it in the first place?

Its a siren attached to the bomb powered by airflow, in order to increase the effect on morale. And in the case of the famous Ju-87 Stuka divebomber, the siren was on the aircraft, not the bomb.

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20 hours ago, ARS said:

What caused bombs dropped from aircraft to "whistle"? I know not all bombs dropped have a whistling effect, but what caused it in the first place?

Along with @p1t1o's answer, some bombs would have a little propeller on the nose.  This would spin in the airflow, driving a clock mechanism, since it was delivered from a known altitude, the time to the ground was also known, and it could be set to detonate some altitude above the ground.    That may have also been a source of the noise. 

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The propeller on bombs are usually for arming the detonator. There is a pin holding the propeller fixed. When bomb drops away from the plane the pin is pulled to allow the propeller to spin and arm the detonator. Depending on the fuze design this might be completing an electrical circuit or positioning a firing pin for a mechanical fuze. There might be propellers front and rear attached to different types of detonators, say a contact fuze is the tail and an airburst fuze in the nose, so the bombardier chooses which pin is pulled for the desired effect when the bombs are dropped. If the bomb load needs to be jettisoned over friendly territory the bombs are dropped with the arming pins still attached. If there is a timer it's typically to set a time delay before detonation. A time delay contact fuze would start counting down when hitting a hangar roof, but not detonate until inside the hangar. Timing from bomb release was unlikely because precise height above ground is difficult to know about a couple thousand feet. It's easier to use a barometric or radar altimeter to detonate at a specific height above the target

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I need my math checked :P

So, I'm trying to find the geostationary height for a planet 0.37x the mass, 0.66x the radii or Earth (~0.85 g), and I got the value of around 21,308 km. Which seems right. BUT, I calculated my finished value as follows: 295948.832 km x 0.072, why did I multiply it by 0.072? FIrst, I tried out the equation with Earth values (Several times), and multiplying it by 0.072 was the only way I could figure out how to get Earth's geostationary height. So when I did the equation for my planet, I did the same thing after converting the initial value from m to km (If you haven't figured it out by now, this was the first time I've ever used the equation :/)

I used a solid 24 hr day in seconds (86400 s), multiplied by 0.798 for the planet (19.152 hr day), Earth's mass in kg (expanded out to get rid of scientific notation), then times 0.37 for the planet, and Earth's radius in meters, multiplied by 0.66 for the planet. I used parentheses on all values with an exponent. And I used the calculation on the KSP wiki, which used cube root instead of square root on another website I found, which gave me even worse answers. (a = cube-root(G x M1 x t^2/4pi^2) - R) was the one of the KSP wiki.

So uh, yeah, where did I go wrong? And what's the correct answer, if I didn't get it?

Edited by Spaceception
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