# Air temperature & water freezing

Could somebody please estimate the air temperature that water freezes at.

During a recent very cold night I left an open plastic container of ~50ml of water next to an alcohol thermometer in an uninsulated garden shed. By morning the thermometer was reading minus 3 degrees Celcius but the water in the container had not frozen. This temperature reading was confirmed with a mercury thermometer.

Please suggest an explanation for such a low temperature not causing the 50 ml of water to freeze.

Best regards,
inkpot

berkeman
Mentor
Could somebody please estimate the air temperature that water freezes at.

During a recent very cold night I left an open plastic container of ~50ml of water next to an alcohol thermometer in an uninsulated garden shed. By morning the thermometer was reading minus 3 degrees Celcius but the water in the container had not frozen. This temperature reading was confirmed with a mercury thermometer.

Please suggest an explanation for such a low temperature not causing the 50 ml of water to freeze.

Best regards,
inkpot

What was the temperature of the water?

K^2
Ground takes quite a while to cool. At night, air temperature can be significantly lower than ground temperature. And heat conduction from ground to the container is much better than from air to container.

In early spring, you can witness an exact opposite. Water can freeze during the day when air temperature is above freezing.

Unfortunately, I didn't think of measuring the water temperature, it must have been too early in the morning. However, the water had been next to the thermometer since the previous evening so it must have been in sub-zero temperatures for 8 to 10 hours.

Best regards,
inkpot

How the thermometer and the water are both mounted is important.

During the scuba-diving exam I went through a while ago, my instructor had a thermometer, and measured -0.4 deg C. Explained by the fact we were in flowing river (perhaps also because of poor calibration, hard to say).

In altitude, water droplets generally need a foreign particle to freeze around, otherwise, it won't be able to freeze until about -35degC.

Anyhow, zero celsius should be seen not as the freezing point of water, but as the melting point of ice. There is a difference. Phase changes are not symmetrical.

The thermometer is hanging about 200 mm away from the side wall of the shed and the 50 ml container of water is next to it on a shelf. Both of them are ~1.5m from the wooden floor of the shed.

Could you please elaborate a bit more on the difference between the melting point of ice and the freezing point of water.

Best regards,
inkpot

russ_watters
Mentor
The melting point of ice and freezing point of water are theoretically exactly the same temperature, but things can interfere with the freezing process. The previous post was describing that issue: supercooling.

There's another wrinkle no one mentioned: humidity. When the humidity is low, water will evaporate and carry away heat, which allows the remaining water to cool below ambient. So the freezing of water can be based on dew point as well.

Could you please elaborate a bit more on the difference between the melting point of ice and the freezing point of water.

When ice reaches 0degC, its molecules have too much energy to stay put. They break away from each other because they have to move.

But when water reaches 0degC, molecules can only form ice together if they meet long enough to not get bumped away from each other by something else (as russ mentions).

This is a very vulgar explanation, phase changes can be very complex, and H2O is a particularly special case.

K^2
Do you know if there is a fixed point at which water is guaranteed to freeze even if there are no nucleation points, or if it is probabilistic at all temperatures bellow freezing? I know that there is an experimental cutoff point, but that would exist in either of the two cases.

The -35degC figure is the best I have (from : http://en.wikipedia.org/wiki/Snowflake)
Hotter than that a nucleus is needed. But in general, I guess the cooling rate also has to be considered, and as it can allow -42degC.

Incidentally, freezing rain, which is common in Canadian Winter, is supercooled water droplets (< 0degC drops) that freeze on contact with the ground (and houses, and cars, and people, and most problematically roads and aircrafts, etc.). In this case, it's the object that acts the "nucleus".

K^2
Yeah, we get these in Ohio. Quite a lot of fun to come home and realize that you're wearing ice armor over your coat. I'm using the term "fun" very loosely, by the way.

Thank you all for your responses and for a very interesting discussion.

Last night was again very cold, so I put three containers adjacent to the thermometer, two plastic and one metal, see attached file. The thermometer reached a minimum of ~ minus 3 Celcius but water in the container on the right hand side in the photograph was the only water that froze. All three containers were filled at the same time and were in the same location overnight.