Limitations of temperature in a large body of water

sgstudent

In a river, water flows so how does it resist freezing? Or does it freeze at the same rate just that at the point where the cooling agent is at, the water keeps getting replaced. So the average kinetic energy of all the water molecules drop so as a whole the temperature drops?

But a river is huge so if I place a thermometer in one area and then in another, there's a good possibility that the 2 readings are different. So I'm quite confused about how temperature works. Temperature is the average kinetic energy of all the molecules but what are the limits to this?

Thanks for the help

CWatters

I'm not quite sure I understand your question but...

On a lake heat is lost to the atmosphere and that allows the temperature of the top surface to fall to freezing point while the depths remain warm(er). The surface of a river tends not to freeze because of mixing with warmer water from the depths.

sgstudent

sorry for being vague. What i meant was what is temperature measuring actually? When we put our thermometer into a small tub of water, we find the average KE of the molecules of all the water molecules? Then what about a large body of water like a river, when we place our thermometers into it what are we measuring now?

SteamKing

When you use a thermometer, you are measuring the temperature of the substance directly in contact with the device. In general, it takes special care to ensure that temperature is uniform in a given mass of a substance, whether it is solid, liquid, or gas. In bodies of water like rivers and lakes, it is very difficult to make the temperature of the water uniform throughout.

sgstudent

But technically what is the substance directly in contact with the thermometer? Are they just those few molecules around it? But if that's the case how can it be because isn't temperature the average KE of all the particles? That causes a new problem too, as what would quantify 'all' the particles present.

Hope you can help out

mfb

I would not call something like 10²⁰ (give or take some orders of magnitude depending on the type of measurement) "a few", even if it is a tiny fraction of 1 liter.
You measure the average kinetic energy of those 10²⁰ molecules, which is the temperature of them = the temperature directly at the thermometer.

sgstudent

Oh that explains why its so difficult to freeze running water, the molecules around the cooling agent leaves before enough energy is removed from them?

etudiant

You point out that the very idea of a temperature is not readily applied to anything big or fluid, such as a lake or a river.
Temperature applies to one locale at one moment, how big and long depends on the sensor and the measurement interval.
So the river may have frozen eddies and warmer stream runs at the same time. As it freezes over, your measurement space shrinks as well.
The only way to get an overall temperature is to moniter the radiation from the object in question. Otherwise, confusion reigns, as with global warming today.
Al Gore had championed a satellite at one of the LaGrangian points that would observe the entire earth at once from about 1.5 million miles away and measure its temperature. Unfortunately, that was not funded, so there is still no broadly agreed measure, just lots of local thermometers with lots of adjustments.
You too will need lots of adjustments to measure your river temperature, but don't feel bad, everyone else has the same problems.