OT Anybody seen the articales about the ultra white paint?

die Fuhrer vill like dat

Yeah, and they turned comments off; that's important, because the effect of white paint MIGHT be to reject thermal input in daylight, or might just reflect light onto a different target, so the ground, and vegetation etc. all get extra hot. Also, at night... it doesn't radiate heat OUTGOING into the night sky very effectively. White isn't a one-way heat valve. If it turns off incoming, it turns off outgoing.

Full diurnal cycle net effect of white paint might be interesting, but it isn't the paint character that matters, it's the ENVIRONMENT AROUND IT that has to be considered. Study of the incident-light-onto-paint capture informs us inadequately.

If it's dark around 10 microns, it will radiate heat into the night sky.

Most organics are nearly black at thermal wavelengths. The ideal "cold" paint is white in the visible and black at long wavelengths.

Yeah, but of course the white pigment, barium sulfate, isn't organic. Any paint (or other object) will radiate into the night sky, but our CO2 problem is that the night sky isn't transparent to all those long wavelengths. Heat hasn't left until it's on the OTHER SIDE of the sky, in vacuum. Departing a surface at ground level is only a beginning.

That is flat out *wrong*. White paint was traditionally used to keep observatory domes passively cool during the daytime. It was the whitest white paint that money could buy at the time. The big problem is that in the thermal IR band white paint is as near as damn it black. The dome would super cool badly at night and cold air would drip in through the dome slit causing very bad turbulence as it poured over the edge.

Modern domes are much better insulated internally and have aircon. The dome is painted with a combined white and metallic aluminium paint that is even more expensive than the old super white but which maintains a near neutral thermal profile when facing the night sky.

This trick as all but eliminated the worst of dome seeing.

Your understanding is woefully lacking. That isn't the same thing at all. There are meta materials now available one even on the verge of mass production that can passively cool in direct sunlight.

They are in essence simultaneously a mirror finish or white for optical and near IR wavelengths whilst being black in thermal IR band. It will be quite a breakthrough if the stuff can survive being used outdoors.

So far most trials have been demos to other scientists.

whit3rd snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:

Death valley is many many square miles of white salt deposit, 3 miles deep.

That is also the hottest place in the nation.

I want to build a tarp 5 yards off the ground above it for a couple acres and paint it with this medium. Then do some space observations of it and the surrounding air.

Under my tarp city, I would have a huge array of 100 foot diameter tanks as deep as can be managed, all seperate from each other and filled with polar ice water. The desert regions around the world need to build these arrays and start storing fresh water, as we have very little of it directly available to us, and the polar ice will harm us as it melts away.

So I say start storing it. A few great lakes sized arrays around the world with white tarps over them, and mine, in Death Valley would have a city over it. No steel or concrete or trucks or cars. Just a college campus. Put water curtains at the wind facing edge and make cool air that way too. Hang large weights at oints around the peimeter that the people crank up on occasion that operate small generators that power the street and sidewalk lighting.

The lightest weight city ever made. The lightest color. In the exact right place... flat for miles.

But bring a parka if you plan to stay overnight.

snipped-for-privacy@highlandsniptechnology.com wrote in news: snipped-for-privacy@4ax.com:

That is wind chill. Stop the wind, as in enclosed spaces, and trap the day's heat. No need to worry about night temps. Plenty of day heat to trap using plenty of methods for night utilization.

Solar array nearby, one making electric, the other heating up a liquid sodium reserve. Could also place a nearby fusion reactor plant, since that will be ready in ten years (right?) and this will likely take that long to build. Just in time technology!

Radiation cooling will kill you. Look it up.

google Persian Ice House too.

It's completely correct.

Not an issue being addressed; the daytime incident light heating is small on a surface which is visibly white. Now, tell me why the white building, and not the entire locality, is relevant to global warming? When you see a white object, light HITS YOUR EYE because the white object didn't absorb it; but, your eye does. So do all other nearby objects, because the light reflected, but didn't yet leave planet Earth. To retro-reflect, as with corner cubes, would have some of the desired effect.

We cannot know the character of 'white paint' that applies to the new barium sulfate material. Maybe there's a good observatory paint for seeing, with greenhouse effect negative, but that's not the same paint.

Barium sulfate is not such a metamaterial. The white-paint surfaces that we can keep clean and white, are not generally aimed to retroreflect light back into the sky, but will simply illuminate nearby objects and fill in shadow areas... a south-facing wall in sunlight will reflect, and be cooler, and the north-facing wall will be in shade, and will not radiate heat effectively, day OR night. The effect of conduction-only heat transfer on an observatory is relevant because it minimizes temperature gradient, and convection, inside the enclosure, and improves seeing. That means white or metallic paint is preferable, regardless of its effectiveness in climate change moderation. It improves the air temperature gradients, THAT is why it is beneficial, not because it changes net heat flow.

Wind is part of why Death Valley gets so hot; warm air that comes from higher altitude (and all the surrounding area is higher altitude) gets warmer by adiabatic compression when it dips down into the lower (higher-pressure) places. Death Valley is the lowest region in this country.

The inverse effect happens when prevailing winds (from the west) hit a mountain range; the air cools, so the Hoh river gets rainfall (and a rain forest forms). This is also the reason we hear 'snow level at 4500 feet' announcements: the temperature drop is predictable, and the humidity and temperature of ocean-surface air is known.

A surface that exposes a lot of area to the sun necessarily exposes a lot of area to space. Especially if it's flat horizontal, like a roof or a street or a parking lot.

Painting cities white wouldn't have much effect on global temperature, but it would cool the locals some and save on a/c.

Death Valley bottoms out around -280 feet. The pressure/temperature effect is very small.

The gradient on the western Sierra slope is, by my measurement, about

So, from the nearby mountains, at 8500 feet, you expect a temperature change of about 30 F. Interesting.

But, the sun is a half-degree spot in the sky, only 1E-4 steradians, while the blackness of space is 6E0 steradians; the white material doesn't contribute to LOSING HEAT either, and while that outgoing radiation isn't dramatic, it does have about five orders of magnitude more exposure involved. You can't calculate a thermal balance without considering it.

Consider, too, that my roof is at 45 degrees north, straight-up doesn't mean aimed at the sun. It isn't clearly visible from space (Google Earth does depict tree cover, though), and doesn't stay clean (I have to rake it off annually). Parking lots also don't stay clean, nor are they painted white.

Save on a/c for south-wall apartments at noon, not for north-wall nor at night.

About right. The top of Sugar Bowl is 8383, but some resorts are higher.

The winds along the crest routinely exceed 100 MPH, which can make it feel a bit colder.

At this instant, the gradient is 1 deg F per 77 feet.

LMDDGTFY. Search term "selective coating", fourth result:

For opaque objects, at any given wavelength, the emittance equals the absorbance. You can show this from the second law of thermodynamics, see e.g. this sci.optics discussion from awhile back:

This fact is probably what you were thinking of, and it does show that you can't make a "one way valve" between two objects at the same temperature (more precisely, two objects in thermal equilibrium).

The kicker is that the system under discussion is very far from equilibrium. The source (the Sun) has a thermodynamic temperature around 20 times higher than the Earth, and subtends only a tiny solid angle. Seen from the Earth, the zenith Sun subtends about 0.5 degrees, a solid angle of

Omega_Sun = pi*(0.25 * pi/180)**2 = 0.000060 steradians,

compared with 2 pi steradians for a hemisphere, and the rest of the sky is very cold. A surface radiating into free space actually sees a projected solid angle of pi steradians, on account of obliquity, but that's a nit at this point in the argument--the zenith Sun occupies only _60 parts per million_ of the sky.

The peak emission wavelength is proportional to the thermodynamic temperature, so there's plenty of space for the transition band of a filter: you can have high absorption in the visible (where incoming solar radiation peaks) together with low emittance at terrestrial temperatures (where outgoing thermal radiation peaks).

Greenhouses, Earth's atmosphere, and selective coatings for solar thermal collectors are examples. The result is that the equilibrium temperature of the coated object in sunlight is higher than it would be if its emittance were wavelength-independent. (i.e. if it were a so-called "gray body".)

Selective absorber coatings look black because they absorb in the visible. However, you can also make coatings that selectively reflect the visible but are highly absorbing in the thermal IR. Ordinary white paint made from TiO2 or MgS powder with an organic binder is a good example. A sunlit object so coated will have a lower temperature than a gray body would in the same conditions.

On a surface that is locally flat, very nearly 100% of the emitted radiation will escape unless it's intercepted by the atmosphere. Retroreflection would send most of the light back in the general direction of the Sun, but that's in no way required. Any upward direction is as good as any other from the energy balance POV.

All common nonmetallic materials are highly absorbing in the 10-15 um thermal IR region. YCLIU.

Cheers

Phil Hobbs

That 'flat' doesn't mean planar, it means horizontal. Horizontal, at temperate latitudes, is never sun-directed. The wall can get the same insolation as a roof, and need the same white paint... the wall, though, isn't horizontal, and directs light toward the ground by reflection. That's a very-far-from-100% scenario.

Not really; i f there's clouds in the sky, they'll also reflect back at the ground, and in Seattle, and that's a very nearly 100% probability during a sun break.

Not easy to look it up; I do find a claim, however

The old observatory domes (pre aircon) were painted the most brilliant matt white (at cost no object). The new paint is a deliberate compromise including a fair proportion of aluminium flake which behaves like a mirror and makes it look greyer and become hotter on the surface. It isn't as effective at reflecting heat in daytime but at night it doesn't radiate heat too fast either so it prevents dome currents forming.

He's right. Almost every common material that isn't transparent in the

There are a handful of designer materials with very odd properties but they are far from common and still some way from commercial applications. They may find a place as passive aircon one day. A small number of setups have been deployed but I am suspicious of how carefully you would have to clean them and keep them clean to benefit.

I'm not sure why this barium sulphate paint has suddenly become a hot topic it was nothing much to write home about and historically the first use of such pigments was a 1870's patent by Du Pont. aka Lithopone or Pigment White 5 also used as barium meals in medical applications.

OK this one the BaSO4 is RI 1.64 might be a bit purer and stay white but it strikes me that TiO2 RI ~2.6 ought to be able to have it for lunch. The trick seems to be in the particle size distribution but I expect TiO2 can do that as well once the rules are understood.