Is 0.0'C a frost?

Jumalauta. -7C is some severe shit.

The point of this thread was just to find out which was more used so I'd know which to use myself. Smhi defines a frost as temps below 0'C, and that's technically the definition of freezing temps too. The only reason I got defensive is because a lot of you guys seemed to think that it's to not regard 0.0'C as a freeze. Because it really isn't.

You just reminded me of supercooled water which, if you haven’t seen it, is incredibly cool. If you put a bottle of water into a freezer just below the freezing point (like -1C) it won’t freeze until you disturb the water (by tapping it) at which point the whole mass of water will instantly turn into dense icy slush.

If water is stored at exactly 0.0000000000000C, it just kills itself because it doesn't know what to do.

Also, if that -7C eventuates, it could be a new record low for September.

Water stays at exactly 0°C while it's freezing and the crystal structures of ice are formed so it's a definite yes.

Don't name the schrödinger's cat for this, it is just all another subject.


So that this thread is starting to deviate toward a really weird direction, I think that we need to define with a little bit more of precision what the freezing point of water actually is.

First of all, the phase transitions of pure water follow this graph:

This graph is the phases diagram of water, and represents as a function of temperature and pressure which phase (solid, liquid or gas) is the most stable from a thermodynamic point of view. The lines between the areas of stability of different phases correspond to the equilibrium conditions between the two phases on the two sides of the line, while point B is called "triple point" and is the point in which the three phases are in equilibrium conditions. If you like the genre, the extended diagram gets quite interesting in the solid part [1]

What does equilibrium conditions mean? It means that the two possible phases (for example, liquid and solid) are equally stable at those conditions of temperature and pressure. It is a dynamic equilibrium condition, which means that the amount of molecules (or other) which pass from phase 1 to phase 2 is the same as the one of molecules that pass from phase 2 to phase 1, so the relative amounts of the two phases don't change over time. This is very different from a "schrödinger's cat"-like situation, in which you would not be able to tell which the phase is (or, at least, not before opening the cat's box).


If you look at the standard freezing point of water in that graph you will see that it is a quite random point along the equilibrium line between solid and liquid phase (at the crossing between a pressure of 1 atm and a temperature of 0°C). If you changed the pressure, also the temperature of the phase transition would be a little different. And the situation becomes even more complicated in a stystem which is not only made by pure water.

Also, this graph is only about thermodynamic equilibrium, and it doesn't take into account other particulars like the nucleation of the first solid crystals, amorphous phases and so on.


Now, this thread is about the temperature required to have an air frost. In this situation the diagram above can't be applied, because we are evidently don't talking about a pure water system. You can't even say that all the water present in the air suddently falls down as ice when the thermometer reaches 0°C, actually there isn't any sudden change in the air mixture at that temperature which could make you choose a specific temperature for a real physical reason.
What the weather agiencies did was to choose a totally arbitrary (but reasonable) value as a threshold for defining the conditions for having an air frost, and they decided to use the temperature of the freezing point of water for a pressure of 1 atm. They could also have chosen any other random value, their porpoise is only to define a parameter with which you can evaluate the number of air frosts and that you can use for comparing different data series.

The official temps are measured in a Stevenson Screen sited 1200mm ( 4') above the grass level. Grass level can be substantially colder than at Screen height.
For example, a preliminary frost warning is issued in Alice Springs with a forecast min of 5c and a moderate frost risk at 2.2c. Many AWS sites in AU have instruments that record grass temps as well as Screen temps.