What is the temperature of the water in the river under the ice. Thermal regime of rivers

Why doesn’t the water in reservoirs freeze to the very bottom in winter?

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Temperature of the highest water density: +4 C, see: http://news.mail.ru/society/2815577/

This property of water is fundamentally important for the survival of living creatures in many reservoirs. When the temperature of the air (and, accordingly, the water) begins to decrease in autumn and in the pre-winter period, first, at temperatures above +4 C, colder water from the surface of the reservoir sinks down (as heavier water), and warm water, as lighter water, rises up and goes in the usual vertical direction. stirring the water. But as soon as T = +4 C is established vertically in the entire body of water, the process of vertical circulation stops, since from the surface the water already at +3 C becomes lighter than that which is below (at +4 C) and the turbulent heat transfer of cold vertically is sharply reduced. As a result, the water even begins to freeze from the surface, then an ice cover is established, but at the same time winter period the transfer of cold to the lower layers of water is sharply reduced, since the layer of ice itself on top, and even more so, the layer of snow that has fallen on the ice from above, have certain thermal insulation properties! Therefore, at the bottom of the reservoir there will almost always be at least a thin layer of water at T = + 4C - and this is the survival temperature of river, swamp, lake and other living creatures in the reservoir. If it were not for this interesting and important property of water (Max density at +4C), then the reservoirs on land would all freeze to the bottom every winter, and life in them would not be so abundant!

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A very important property of water is at work here. Solid water (ice) is lighter than its liquid state. Thanks to this, the ice is always on top and protects the lower layers of water from frost. Only very small bodies of water can freeze to the bottom in very severe frosts. In ordinary cases, under the layer of ice there is always water, in which all underwater life is preserved.

It all depends on the severity of the frost; sometimes even deep standing reservoirs can freeze to the bottom. if frosts below minus 40 last for several weeks. But basically, indeed, reservoirs do not freeze, which makes it possible for the fish and plants living in them to survive. And the point here is in such a curious property of water as negative coefficient expansion, which water has at temperatures from +4 degrees and below. That is, if water is heated above 4 degrees, then as its temperature increases, it will tend to occupy a larger volume, its density decreases and it rises. If the water cools below 4 degrees the situation changes to the opposite - than colder water, the lighter it becomes and the lower its density, and therefore the colder layers of water tend to rise, and those with a temperature of +4 - down. Thus, under the ice, the water temperature is set at +4 degrees. The boundary layers of water next to the ice will either flood the ice or freeze themselves, increasing the thickness of the ice until a dynamic equilibrium is established - as much ice melts from warm water, as much water will freeze from cold ice. Well, everything has already been said about the thermal conductivity of ice.

You missed a lot important point: the highest density of water is at a temperature of +4 degrees. Therefore, before the reservoir begins to freeze, all the water in it, mixing, is cooled to these very plus four, and only then upper layer cools to zero and begins to freeze. Since ice is lighter than water, it does not sink to the bottom, but remains on the surface. In addition, ice has very low thermal conductivity and this sharply reduces the heat exchange between cold air and the layer of water under the ice.

Temperature under the ice 0.1-0.3° above zero, in the spring during ice drift it does not exceed 1 °. During periods without ice, the water temperature depends mainly on the air temperature. The average daily water temperature before mid-summer is usually lower than air temperature, at the end of summer and autumn it is higher.

Below the reservoirs, the temperature of the river water in summer is significantly lower than usual, and higher in winter, which leads to the appearance of many kilometers of ice-free sections of the river. Abundant underground feeding of the river cools its water into summer period, in winter leads to a decrease in ice cover, and sometimes to the formation of polynyas.

Daily maximum water temperatures are 1-2 hours behind the air temperature.

On small and medium-sized rivers, the water temperature practically does not change in depth; large rivers it may decrease in summer in the lower layers by 1-2°.

Thermal sink(Wm in J or kcal) - the amount of heat carried through a given river section over a time interval (∆ t):

W m = L melt ·ρ·T·V, Where V- volume of water flow over the same time interval, T - average water temperature, ρ - its density, L melt - specific heat capacity of water.

Large rivers flowing in a meridional direction - transzonal rivers- have a water temperature that is not typical for rivers in the area.

Rivers by character ice regime They are divided into three groups: freezing, with unstable ice formation and non-freezing.

On freezing rivers, three periods with characteristic ice phenomena are distinguished: 1) freezing, or autumn ice phenomena, 2) freeze-up, 3) opening, or spring ice phenomena.

Freezing of rivers. When the water temperature drops to zero, autumn ice phenomena begin in the river. Grease - floating spots of ice film, consisting of ice crystals in the form of thin needles. Around the same time, banks form - strips of motionless ice off the coast. When water is supercooled (to fractions of a degree below zero), intra-water ice can form in its thickness and at the bottom - an opaque, spongy, ice mass of chaotically fused ice crystals. The accumulation of inland ice on the surface or in the thickness of the flow forms slush. Its movement is called sludge movement. At the same time, ice floes are formed on the surface, consisting of crystal ice. Their movement is the autumn ice drift. Blockage of the riverbed by slush is called a jam, and by ice floes - a jam.

Freeze-up is the formation of a continuous, motionless ice cover. Small ice-free areas are polynyas. They are connected to the exits groundwater or with rapid current, sometimes with the discharge of warm water into the river by industrial and municipal enterprises. As the thickness of the ice cover increases, the cross-section of the channel decreases. Under the influence of the resulting pressure, water can flow onto the surface of the ice. When it freezes, ice forms.

Opening up of rivers. With the onset of positive air temperatures in the spring, snow and then ice begin to melt. Stripes of clean water form on the river near the banks - edges. The adhesion of the ice cover to the shore ceases, and cracks appear. Sometimes after this, small (several meters) displacement of the ice fields is observed - ice movements. Then the ice cover is divided into separate ice floes, the movement of which forms spring ice drift. More often than in autumn, congestion occurs, especially on large rivers flowing from south to north. On small rivers, the ice cover often melts in place without ice drift.

Nature surprises us unexplained phenomena. One of them is water crystallization. Many people are interested in this unusual question, why at sub-zero temperatures ice forms on the surface of a reservoir, but under the ice the water retains its liquid form. How to explain this?

Why water under thick ice does not freeze: answers

At what temperature does it begin to harden? This process begins already when the temperature drops to 0 degrees Celsius, provided that it remains normal level atmospheric pressure.

The ice layer in this case performs a thermal insulation function. It protects the water underneath it from being affected by low temperatures. The layer of liquid that is located directly under the ice crust has a temperature of only 0 degrees. But the lower layer is different elevated temperature, which fluctuates within +4 degrees.

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If the air temperature continues to decrease, the ice becomes thicker. At the same time, the layer that is located directly under the ice is cooled. At the same time, all the water does not freeze, since it has an elevated temperature.

In addition, an important condition for the formation of an ice crust is that the low temperature must be maintained for a long time, otherwise the ice will not have time to form.

How is ice formed?

As the temperature decreases, the density of the liquid decreases. This is precisely what explains why more warm water is at the bottom, and the cold one is at the top. Exposure to cold causes expansion and decrease in density, resulting in the formation of an ice crust on the surface.

Thanks to these properties of water, the temperature in the lower layers is maintained at +4 degrees. This temperature regime ideal for inhabitants of the depths of reservoirs (both fish and shellfish, plants). If the temperature drops, they will die.

It is interesting that in the warm season the opposite is true - the temperature of the reservoir on the surface is much higher than at depth. How quickly water freezes depends on how many salts are present in its composition. The higher the salt concentration, the worse it freezes.

The ice crust helps retain heat, so the water underneath is slightly warmer. Ice prevents the passage of air into the lower layer, which helps maintain a certain temperature regime.

If the ice crust is thick and the reservoir is of sufficient depth, the water in it will not freeze completely. If there is not much of it, there is a possibility that the entire reservoir will freeze when exposed to low temperatures.

The reason for this is one of the water anomalies. As far as everyone knows, the density fresh water equal to 1 g/cm 3 (or 1000 kg/m 3). However, this value changes depending on the temperature. The highest density of water is observed at +4°C; with an increase or decrease in temperature from this mark, the density value decreases.

What happens on reservoirs? With the arrival of autumn, when cold weather sets in, the surface of the water begins to cool and, therefore, become heavier. Dense surface water sinks to the bottom, while deeper water floats to the surface. In this way, mixing occurs until all the water reaches a temperature of +4°C. The surface water continues to cool, but its density now decreases, so the top layer of water remains on the surface, and mixing no longer occurs. As a result, the surface of the reservoir is covered with ice, and the deep waters cool very slowly, only due to thermal conductivity, which is very low in water. Throughout the winter, bottom waters can maintain their temperature at 4°C. With the arrival of spring and summer, the reverse process occurs, but deep waters again maintain their temperature.

Thanks to this interesting feature relatively large bodies of water almost never freeze to the bottom, which gives fish and other aquatic life the opportunity to survive in winter.

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