Thermal sheet

Thats interesting Peter, I gotta go now but I'll be back, see ya.

 

EH :D :D :D

 

Pete as PeterS points it is a latent heat thing the example I gave means the air passing at speed over the water drains the heat so quickly out of the water it freezes, the air that as passed the water is now warmer after robbing the heat(thermodynamics)In the case of a person stood in the wind the wind comming towards you at 5c would stay at that temp. until it passed you heat would be drawn from your body the air that as gone past would now be 5c plus, the stonger the wind the faster it draws away the heat(wind chill factor)it's only because we have a nice little furnace inside that we don't immediately freeze,so those tables are important if you are an outdoor pursuitist like me it gives me a good idea what clobber to put on.Sorry young daisy we must sound like a lot of boring farts

 

no you dont Walnut.. im just fick haha

 

Peter, I seem to remember reading something about those pourous bags on the roof of cars, and I dont disbelieve you that it works.
I assume its similar to the wet and dry buld thermometer used to determine humidity.

its just than in practice the plant will not be losing that amount of moisture.
Going back to walnut and Daves wind chill tables it states that, for instance, at 5C with a wind of 12mph the wind chill is minus 1C.
Now I am willing to bet that there is less chance of a plant freezing at 5C with a wind than there is on a still clear night at 5C. On such nights if a wind springs up the temp USUALLY rises slightly.
I will agree that once the air temp drops below 0C and any breeze happens, the frost does become much more severe that without the wind.

 

pete, why not just buy some frost fleece? Because my head is starting to hurt from all this science. david.

 

Because, as far as I'm concerned, fleece is too easily torn, its not much use against hard frost, and going by what I'm hearing from the others, when wet its probably useless, and it blows away when its windy.

 

Pete. Whilst I understand the principle I am not sure how big the effect is in practice.

But the illustration from the wind chill table is interesting. It is saying that a 12 mph wind is the equivalent of a reduction of 6C. You get ground frost because cold air sinks whilst the air above could be warmer. So a wind will mix the two air layers up and a thermometer, which is not effected by wind chill as its dry, may record a true air temperature of 3C higher, but the 12 mph wind will still make the plant feel 6C colder than the air temperature (ie 3C colder than without the wind)

Assuming that fleece works, I think it must be because it makes the air stationary. I doubt that the temperature inside the fleece would be any higher than outside.

If you want to experiment - stand outside on a frosty night with a wet and dry bulb thermomiter. When the wind starts to blow and the dry bulb temperature rises - I bet the wet bulb temperature will fall. Could be chilly trying to prove it. :D

 

As I seem to be weeing everyone off, I'm going.

 

Thankyou for your reply Peter.

 

Don't do it outside if it is freezing :D

 

no. :D

 

Pete - to go back to your original question about an insulating cover for a cold frame. I have copied this from a Salvia forum in the USA, in reply to a similar question. This was written by Rich Dufresne, who is a top class Salvia man, who used to run his own specialist Salvia nursery. He lives in Zone 7, ie a bit colder than the UK:-

Cold frames are great, but are labor intensive in terms of temperature control and venting. Twice or more a day, it is necessary to adjust the sashes, especially on clear days with wind (Arctic air masses). Cold frames are much more forgiving in cloudy weather. Cold frames are great, but are labor intensive in terms of temperature control and venting. Twice or more a day, it is necessary to adjust the sashes, especially on clear days with wind (Arctic air masses). Cold frames are much more forgiving in cloudy weather.

I use tarps to stop the flow of heat in either direction, especially at night. My cold frames have 8 x 4 feet sashes and I have 3/4 blue foam or aluminized yellow foam (wall insulation)cut to fit in the sash frames for the coldest nights. I have kept tropicals alive in such cold frames.

A radio frequency thermometer sensor inside the cold frame is a good idea, especially if the receiver has an alarm for too cold or too hot.

 

Thanks Peter thats interesting, its the kind of thinking I was looking for, dont know where you get hold of a good old fashioned tarpaulin these days, most are plastic.
Does he say how he keeps the wall insulation in place, although I think (and hope) thats not really nessesary for here or the types of plants that I'm growing.

Just to deviate a bit, hows you proteas doing?

 

I think the job of the tarpaulin is prevent the heat radiation at night - as we talked about. So any heavy fabric would do. What about a bit of old carpet.

I like the idea of that aluminized yellow foam. With the aluminium side down it would be doing exactly the job it was designed to do. With the aluminium reflecting the radiant heat. I would use a brick to hold it in place.

Out of ten Protea seeds (2 seeds of five different species) only 3 seeds germinated. One died very early on, I think I underwatered it - but of course seeds are not as drought tolerant as adult plants. There are two plants left - both of the same type. One has been rather sickly for some time, but still hangs on, and the other is in rude health, about 6 inches high. I have never tried to feed them.