Thursday, 17 July 2014

Post 157 Winter and the dark side of the moon.

I was away at the time but I'm told that winter started here around the end of July when the daily max dropped under 20 and everyone rushed to close the windows and put on something warm. Since then it's got really cold, seriously it has. Just last weekend it was 4C in the morning, that's about the coldest I can remember. And there was frost in the low lying areas. How does that work frost at 4C.

Well yes I looked it up and apparently it's all about radiant heat dissipation and the universe's pursuit of equilibrium. Sounds profound but like so much it's just magical science and nothing supernatural. On a clear night the relatively warm ground radiates it energy in an effort to reach equilibrium with the close to absolute zero temperature (-273C) of deep space. The air in between isn't much of a barrier and so even with small above zero air temperature the ground can go sub zero causing moisture condensation and freezing.

This would suggest that the ground temperature falls below the air temperature all year and perhaps that is true too. Even at warmer times of the year I've noticed a similar thing happens with dew forming on the grass at dusk and on the metal roof of my house. At dusk as the day cools and the sky darkens the relative humidity of the air can reach saturation. At the same time the radiant heat transfer of the metal roof chills it to below the ambient air temperature and so condensation forms. But interestingly it forms in patterns and those patterns correspond to the areas where the insulation is in direct contact with the sheeting. Where the sheets are secured to the roof structural battens and so exposed to more warm thermal mass the condensation doesn't form.

Energy production comes out of temperature differentials but I guess something like 4 or 5 degrees C might be just too little to be useful. Still it could probably drive an IPO.
The thing I don't understand is why this dusk ground condensation does not form on the ground where there is a roof or covering structure but otherwise open to the air. It's like there is a perpendicular line from the edge of any overhang and the dew does not form on the overhang side of the line. But then you don't have to understand it to use it. I'm trying to maintain the condition of a couple of my evergreen trees at the moment for a show and in the absence of a heated plant house all I can do is put a cover over them at night and save them from a couple of degrees of cool.
For a sub-tropical/temperate area we can get a lot of rain in the year but it usually falls in summer storms and our winters are dry. Warm dry winters make for parched dry landscapes and that's our lot at the moment. The gardeners amongst us are working hard to keep plants alive.

The one thing the chill has been good for is the colour of some of our deciduous bonsai. I have a couple of what I think are malus trees, one a cutting from the other. It is remiss of me but I have no record of the exact species/variety. I've not seen flowers or fruit on them, perhaps they need more chill for that than we can offer. They certainly develop some mean thorns but right now they have coloured up to a beautiful claret colour and the longer the cool goes on the leaves go to a pink red.


Meanwhile the Celtis turn a beautiful clear lemon yellow. The next picture is from one of my yamadori stumps.

I've let them grow this year and so they look pretty untidy but the colour is great and if I can ever tame them they will make a great show.

And just to finish up with the dark side of the moon. It shows just how powerful radiant transfer to the wider universe can be. While the side facing the sun can run to 120C without any atmospheric insulation, the dark side can drop to minus 150C. Dusk must be a pretty interesting time on the moon. No complaints from me about frost at 4 degrees!

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