There was a thread not long ago on how to cool things without power, and one of the suggestions was to set up a black pipe pointing at empty night sky, with whatever you want to cool at the bottom end. The idea is that the molecules attempt to achieve equilibrium with molecules in space? I don’t get this. Why or how would a length of pipe induce them to somehow ignore or not attempt to reach equilibrium with all the molecules between the end of the pipe and space? Why is the pipe needed at all? Does the object to be cooled have to be suspended or in some other way distanced from the ground? What am I missing?
Has anyone tried this idea out? I’m keen to, if anyone would be kind enough to provide more details - if there’s any ratio between the pipe diameter/length/mass of object, so for example would a small object placed at the bottom of a wide long pipe cool more then the same object in a wider shorter pipe, etc.
The principle is sound - radiative cooling does work. All objects emit radiation (mostly infrared, for things near room temperature), and are exchanging heat through radiation with the surroundings (i.e. emitting heat to, and receiving heat from, the surroundings). But the night sky emits very little radiation, so an object left outside at night would mostly emit radiation and receive very little. As a result, it cools down.
How far it cools down depends on many factors. The object still receives some heat from the air and ground (or whatever it’s sitting on) through conduction, and this amount depends on how warm the ground is and if there’s any insulation in between. The object also receives radiation from the ground and other warm things nearby (houses, people, whatever), if any. And the amount of radiatin emitted by the ground depends not just on the temperature, but also the material. Also the amount of radiation emitted by the sky can vary. A dry desert sky emits much less infrared light than a muggy or cloudy sky.
If you want to maximize radiative cooling, the best thing to do is put the object in a large metallic bowl, with insulation between the object and the bottom of the bowl. The bowl would block any radiation from the ground, and the bowl itself emits very little radiation because the surface is a shiny metal. (Polished silver would be ideal; aluminum is almost as good. Aluminized mylar would do fine if the aluminum side is facing up.) But it needs to be a bowl and not a long pipe because the object needs to “see” as much of the sky as possible. It would help to be in a dry desert, at high altitude if possible.
I don’t know who suggested black pipe, but that’s about the worst you can do.
Everything with a temperature above absolute zero is radiating heat at all times. Everything is also being hit by the heat being radiated by the stuff around it. The reason the Earth doesn’t cool off to near absolute zero is all that solar radiation, bathing it on one side like a turkey turning on a spit.
Additionally, when things of different temperatures are in contact, heat flows from the warm thing to the cold thing.
At night with a clear sky, everything is radiating heat every which way. Heat radiated out into the clear sky is lost, since a clear sky doesn’t radiate a whole lot back in return. But it takes a lot of heat loss for buildings, hills, the air and the ground to drop significantly in temperature. So objects left out at night under a clear sky do tend to get colder, but not to a great degree.
If you insulate your object from the ground, say with blankets or expanded polystyrene, you prevent heat from conducting to it from the ground. Now, if the only thing your object can “see” is the clear night sky, i.e. there isn’t a warm hill radiating down onto it, it can lose a lot of heat by radiation. Of course, some of this will be replaced by conduction from the air. One of my cites in the other thread calculated that you could still get to freezing with an air temperature of 60 deg. F. You may have to do it several nights in succession, keeping the cold object well insulated during the day to stop it warming up again too much.
Having said that, although the anecdote that this has been done is widespread, I’ve had difficulty finding a really solid cite. The one I gave in the other thread seems credible - it details the results of pointing an infra-red thermometer at the night sky, on a physics message board. There are a couple of comments at the end - one to the effect that someone had personally duplicated the ice-in-desert trick, and the other citing G. Gamow as having reported that desert Arabs make ice in this way. G. Gamow is the chap who coined the phrase “Big Bang” but I wasn’t able to track down his cite directly - I presume it’s in a book or paper, if it exists at all.