Quite uncommon -8°C here in Hamburg at the moment I write, and I thought: Surely our local rivulet is frozen over. In retrospect this was silly, the "rivulet" is 3rd order and has a very visible flow. Thus: Is there some rule of thumb to decide whether some waters will freeze at least at the top to give a continuous ice cap? The variables would be width, depth, flow speed, outside temperature and the duration of it. I'd guess the flow is most relevant.

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    $\begingroup$ The air in Hamburg has a temperature of −8 °C; however, the water of the Elbe has +4 °C. $\endgroup$
    – user323792
    Dec 26, 2021 at 9:28
  • $\begingroup$ @Loong: Any water has +4°C as a whole, loosely speaking. :-) (BTW, "local" meant "at my home place", i.e. the Tarpenbek feeding the Alster, not the Elbe. It has to be brass monkeys until you see ice on the Elbe...) $\endgroup$ Dec 26, 2021 at 19:36


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