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Physics

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    $\begingroup$ In a water cycle, the water itself undergoes the thermodynamic cycle: it evaporates into water vapor and condenses back into liquid water, which makes a cycle. In ocean circulation, salt should be considered a part of the system since it plays a part in changing the density of the water. You may also consider the salt in a water cycle: evaporation is due to heat of the sun and salt is responsible for changing the evaporation rate. However, I would say the effect is so minimal that you may as well ignore it. $\endgroup$
    – hendlim
    Commented Jul 3 at 4:51
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    $\begingroup$ If you want to model something, it is inevitable for you to decide on what you would consider as the system. You may model it however you see fit. For example "one kilogram of water" is a legitimate physical model. You then have one kilogram of water plus the sun as a basic system and from there you may add everything else (wind, salt, etc.) to make your model more complicated, but more realistic. As I stated in my answer, I think your usual textbook does not use the phenomena I mentioned because they are more complicated than man-made machines, hence more effort to model correctly. $\endgroup$
    – hendlim
    Commented Jul 3 at 5:29
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    $\begingroup$ Then you consider either one drop of water or all the water that is spread out all over the world. The choice is yours. Being spread out means that another interaction is at play, e.g. wind. You can technically observe a simple water cycle by containing some amount of water in an isolated container where the bottom part is heated and the upper part is cooled. The water will evaporate and the vapor will condense near the top, drop back down via the wall of the container, and repeat the cycle. You may not observe any cloud, though. $\endgroup$
    – hendlim
    Commented Jul 3 at 5:42
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    $\begingroup$ I think Cepheid variable stars constitute thermodynamic cycles. They have a well defined photosphere radius (and thus volume) and surface temperature that very cyclically. $\endgroup$
    – shockburner
    Commented Jul 3 at 15:31
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    $\begingroup$ @ether For a standard refrigerator the refrigerant doesn't stay in a single coherent thermodynamic state through the cycle. Instead, its a continual process and different bits of refrigerant take on different thermodynamic states at different points in time. In advanced systems, you have multiple cooling loops and split distribution and multiple condensers, such that any particular drop of refrigerant doesn't even have a consistent path through the system. -- Similar things apply for the water cycle, just with additional layers of complexity, as befits a world-spanning process. $\endgroup$
    – R.M.
    Commented Jul 3 at 17:10