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I am in O'Fallon, Missouri and today it is 5 degrees Fahrenheit outside. I was taught water freezes at 32°F. I could understand if it was exactly 32°F that the water might not be turning to ice, but how can it be 27°F below water's freezing point and still be liquid water? The water is still moving due to the wind, which I think shows it has no plan of freezing. How can this be?

My question may be too broad because there may be more than one reason why it isn't freezing, but I don't know of any. I tried to narrow it by giving my exact location and temperatures. It is a pond in my backyard. From what I've seen through Google, it may have something to do with crystallization rate? Thanks in advance.

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related: – Zaibis Jan 12 at 16:33
Try splashing it with a flat paddle or similar. You MAY be able to precipitate quite sudden freezing. Measure pond temperature with an indoor/outdoor thermometer. Indoor = by pond. Outdoor = probe in pond. Report back :-). – Russell McMahon Jan 13 at 11:52
up vote 45 down vote accepted

Water melts at 0 °C (32 °F) but freezing is a more complicated affair. It is safe to say water gains the ability to freeze at 0 °C, but it can get much cooler before it actually does so resulting in supercooled water. Water in this state can rapidly solidify when suitable ice nuclei are introduced. For example, in convective clouds, liquid water can be observed at temperatures as low as -40 °C. However, the water in your pond isn't supercooled.

You say the air is 5 °F, but what is the temperature of the water (probably above 0 °C)? Ultimately it is the water that needs to be below freezing for ice formation to occur. How deep is the pond in your backyard? Water, compared to air, has a much better ability to retain heat and the bigger the volume of water, the more of a heat reservoir you have to deal with. Freezing itself is an exothermic process and ice formation will heat the surrounding water. The pond is probably not pure water and full of ions (e.g. salts) it has picked up from the ground, which lowers the melting point of water. All of these effects make it more difficult for water in your pond to actually freeze and could explain what you are seeing.

Of these effects, the heat capacity of the water and the temperature of the ground underlying the pond are probably the primary contributors. Underneath the pond is the ground and it too is a heat reservoir. Both are probably warmer than the melting point of water even though the air temperature is much colder. If a fluid is cooled from above, the cool fluid sinks toward the bottom. If a fluid is warmed from below, the warm fluid rises. Only one of these needs to be happening to start convection but it is likely that the ground below the pond is warmer than the pond and both the warm ground and cool air will drive convection in the pond. This, in turn, means your pond is well mixed and you will need to extract enough energy from the system to cool down all of the water and cool the ground below the pond to give ice the chance to form on the surface. This will take time (on the order of days/weeks) of continual sub-freezing air temperatures to accomplish. You'll have a much easier time freezing your pond than a large lake, but it still won't be an overnight process.

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It is a couple feet. I accepted this bc I like how many possibilities you bring into your answer. – Seth Kitchen Jan 10 at 19:20
-1 (if I could), this answer appears to be grasping at straws. A pond is certainly not going to be supercooled; the salt in the water is not going to bring the freezing temperature below 5°F; and the wind will add so little energy it's not even worth mentioning. – BlueRaja - Danny Pflughoeft Jan 11 at 0:12
The only thing you can do right now IS to grasp at straws, but the fact remains that the pond is not frozen. You don't presume casey go and retrieve a sample do you? – Nelson Jan 11 at 5:03
It is indeed the heat capacity and ground temp that are mostly responsible for this. During the daytime, energy added from sunlight also contributes, of course. If that pond sits at 5 F for a day or two, its surface will almost certainly freeze (though the ice likely wouldn't be very thick unless it stayed that cold much longer.) It's worth noting the temperatures were unseasonably high the day before it got this cold, so the ground and the water were still quite warm. – reirab Jan 11 at 16:20
Wind can also prevent freezing as a solid for some time by disrupting any "skin" that forms through stirring. Think of an icecream maker - eventually the liquid freezes but it can get well below the freezing point of the ingredients because motion keeps ice crystals from growing much. There may be little bits floating around frozen, but these will be more susceptible to destruction by other methods. – Josiah Jan 11 at 16:31

Water is a rather strange substance. With most substances, the solid phase is denser than is the liquid phase. This is not the case with water. Ice is less dense than liquid water. A side effect of this effect is that liquid water very close the the freezing point is less dense than is slightly warmer water. That very cool water sinks.

Liquid fresh water achieves its maximum density at a temperature of 4 °C (40 °F). This means that a pond or lake cannot freeze until the entire body of water is cooled to 4 °C. Only then can the upper surface of the water cool to below 4 °C, and then eventually freeze.

In addition to the water itself needing to be cooled to 4 °C before freezing can commence, the water-saturated ground beneath the pond has to be cooled as well. Until then, that warmer ground will transfer heat to the pond and keep it from freezing.

How long it takes before a body of water to first start freezing depends on a number of parameters. These include the size and depth of the body of water, the nature of the ground beneath the body of water, the weather leading up to the cold weather, and windiness. The question doesn't mention the depth of the pond, but it's only been below freezing in O'fallon, Missouri for about 24 hours, and temperatures had been quite balmy before that. That 24 hours span of sub-freezing weather is more than enough time to freeze a small puddle, but certainly not a lake, and probably not even a pond.

The question also mentions that it's windy. For a body of water to freeze, it needs a cooler layer of water atop the 4 °C thermocline. Winds act to keep the water well-mixed.

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Thanks. I would accept this answer if I could accept two! – Seth Kitchen Jan 10 at 19:21
+1 this is the correct answer. The process described in the first two paragraphs is called convection. – BlueRaja - Danny Pflughoeft Jan 11 at 0:15
Surely the water close to freezing point is less dense and therefore rises to the top (in absence of other effects)? – copper.hat Jan 11 at 5:25
"liquid water very close the the freezing point is less dense than is slightly warmer water. That very cool water sinks." As observed above, cool water (<4C) rises not sinks. This then insulates the warmer water below from the cooling effect of the wind. – Vince O'Sullivan Jan 11 at 10:33
@VinceO'Sullivan - Not necessarily. There's a phenomenon in northern lakes called fall turnover. Once a lake cools to a near-uniform 4C temperature throughout, winds can cause the entire entire lake to mix. This is a pond, so turnover won't be so high -- unless the winds are strong and sustained. The cold front that recently blew through O'fallon MO brought strong, sustained winds from the northwest, from noon Saturday (when the temperature first fell below freezing) until 4 PM Sunday. – David Hammen Jan 11 at 11:40

Due to convection (the cold water sinks while the warm water rises), the entire pond needs to be brought to near-freezing temperatures before the surface can freeze. With only the top of the pond in contact with the cold air, this takes a long time.

​‌‍​‌‍Additionally, the ground (which is not cooled by convection) will take even longer to cool down, meaning the ground will warm the bottom of the pond. That warm water will rise to the top, lengthening the freezing process.

It takes time for things to freeze. The above conditions cause the pond to freeze slowly, but if the air were to remain below-freezing forever, eventually the entire pond would freeze over (starting with the top layer and working its way down - when the top turns to ice it no longer sinks because ice has lower density than water).

This is why, if you travel north to Minnesota where ice fishing is common, they'll tell you the weather needs to be (mostly) below freezing for weeks before the lake is safe to walk/drive on.

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The water in a pond is in contact with the ground and the ground is not even close to freezing even if the air temperature is 27° below freezing.

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Lots of science being thrown around here, while the simple answer is that the ground is still too warm. If the ground is frozen past the bottom of the pond, then we can get into all of that super-cooled-ice-nuclei jargon. – Mazura Jan 10 at 20:59
@BlueRaja-DannyPflughoeft, you're absolutely correct. I made my answer a simple as possible. – BillOer Jan 11 at 0:14

This could be because the air cools and heats faster than water. Air has a lower Specific Heat than water. It may take some time for the water to freeze.

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In addition to the accepted answer from Casey, it is likely that the sun is warming the pond over the course of a day more than it warms the thermometer used for the temperature readings you are quoting, as thermometers are kept inside a Stevenson Screen.

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If the ground is warming the water, condensation should be rising from the water.Take a temperature reading from the pond and add it to your question. – user5434678 Jan 12 at 8:08

No-one has mentioned fish yet. Do you have fish or other animals in your pond? Not only do they provide a small amount of warming, their movement will help prevent ice forming.

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Yes it does contain fish! :) – Seth Kitchen Jan 13 at 14:56

Pure water freezes at 0 °C (32 °F) and most of the time pure water is rarely found in ponds and other open water bodies. When impurities get mixed in water its freezing point drops.

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