NH Spring/SH Autumn 2020 Discussion Thread

Well let's see how much snow you get once the pattern changes in the next few days. 

Please post pics. We all want to see your snow. 

I'm not a physics professor or anything but I believe 0.0'C is a kind of eqiilibrium where things will neither freeze nor melt. Water that's 0.0'C will stay as water until it dips below 0.0'C and ice that's 0.0'C will stay as ice until it dips above 0.0'C. I.e. frozen doesn't necessarily mean colder than the freezing point of water, it could be exactly the freezing point of water, and water needs temperatures below the freezing point to start to freeze.

Ice can be colder than 0'C if it's been cooled down by subfreezing temperatures, but snow that's falling during above freezing temperatures and is in the process of melting will be exactly 0.0'C. Melting snow can't freeze water.

There is obviously a simplification, and there is some nuance to this that I'm sure urania93 is going to be upset about. Or I could just be plain wrong too, idk

Sounds like there is some confusion about energy and temperature equilibria, yes

In thermodynamics, the general idea is that for a isolated system the total amount of energy is conserved (but can change from one kind of energy to the other), while the temperature tends to equilibrate until all the system is at the same temperature.

If you have some water at exactly 0°C at 1 atm of pressure and without any solute in it, and if there is not any energy exchange, it will maintain its state (either liquid or solid). But here we are considering a system containing two elements (the plant and the snow) which, at the beginning, are at a different temperature and can exchange energy, so the situation is a little different. The fact that we are close to a phase transition makes the phenomenon less intuitive too.

If we were far from the phase transition, some thermal energy would have passed from the warmest part of the system to the coldest one until they were at the same temperature. From the microscopic point of view thermal energy is related to the average kinetic energy of the molecules in the system, and the macroscopic observable corresponding to this average kinetic energy is the temperature of the system. So in this case the exchange of energy straightforward cause a change in the temperature.

But when we are at a phase transition the situation is a little more complex, you have to consider latent heat. It is true that at a phase transition the temperature remains constant, but energy could continue to be exchanged. The reason why energy continues to flow but the temperature does not change is simply that in this case the energy is not simply transferred as kinetic energy of the molecules vibrating, but it also change their potential energy (allowing for different arrangements of the molecules, which macroscopically correspond to the phase transition). This means that if you have liquid water at 0°C and keep subtracting energy from it, it will maintain the temperature of 0°C but the system will also progressively turn into ice, and only once the all of it turner to ice and if you keep subtracting energy it you will observe the temperature decrease. Vice versa, if you have ice (or snow) at 0°C and you keep introducing energy, the temperature will remain constant but the ice would also start to melt, and once it completely melt the temperature can rise as further energy is provided [1]. This is valid for a pure system, in case of mixtures the situation is a little more complex.

Anyway, in the case of snow at 0°C on a slightly warmer plant, we will have thermal energy moving from the warmest part (the plant) to the coldest (the snow). The plant is above its freezing point, so to release energy corresponds to a decreasing in its temperature. The snow, on the other hand, in this case is considered to be exactly at its phase transition, so to take some thermal energy will make it start to melt but will not increase its temperature. At the end, if the snow is enough and does not completely melt in the process, it is possible that both the plant and the snow equilibrate at a same energy of about 0°C. Still, the plant would probably not freeze completely because the water in its cells contains a lots of solutes, and salts in water lower its freezing point. This is evidently a super-simplified model, because I'm not even considering the air nor anything else.



Apart from the all of this, many plants suffer low temps even without freezing nor snow (I'm thinking about the tomatoes in my parents garden...). I also agree with Doña Jimena who pointed out that snow on trees with leaves on its more likely to break the branches (more available surface = more snow which can sit on the tree = more weight). Talking about fruit trees, late snow can also be quite bad on the pollination process side, because flowers are particularly delicate and because there are not bees around when it snows, and problems in this phase usually correspond to very few fruits later in the summer.