Layman struggles with Science

[Richard111]

With regard to the water vapour feedback effect, I find relying on  data from the web results in much confusion so I will discuss a personal experience and experiment that anyone can replicate.

Way back when the UK Met office forecast a “barbeque summer” I had read about a homemade cardboard box solar oven. I also happened to spot a couple of sheets of tempered glass in a skip at the local recycle centre. Since no one seemed to be looking my way the glass transferred to the boot of my car which was nearby as I off loaded my own defunct glassware. ( bottles and stuff)

Having acquired the glass this encouraged me to do the rest. Two cardboard boxes were found. The smaller one about 14 inches square and fitted inside the larger box with about a 2 inch space all round and underneath. The 2 inch gap between the boxes was packed with polystyrene sheet cut to fit. Any remaining air gaps were stuffed with cardboard on the side away from the inner box. The top of each box was trimmed off such that both boxes and the packing were level and there was a 14 inch square hole. This would be the oven and was accordingly lined with cooking foil and the bottom painted black with school board paint. As the foil was quite delicate I placed, upside down, a small black baking tray on the bottom to support the cast iron cooking pot. The lid was hardboard cut just larger than the outer box such that  lengths of half inch batten screwed to the edges made for a tight fit over the box. A square hole and a single sheet of glass, gaffa-taped down firmly, completed the oven. I also lined a separate small sheet of hardboard with cooking foil to act as a reflector.

Did it work? Yes! But with failures and the failures are what I will talk about. Firstly, a sunny day with a clear blue sky, the darker the blue the better, guaranteed a well cooked dish. Temperature in the oven would reach and hold around 200 degrees F. Any small clouds did not effect the oven temperature as long  as they did not hide the sun for more than about ten minutes or so at a time.

Now failures in detail. Obviously a cloudy day was a no-no. But there were many clear sunny days when the temperature would not rise above 150F. I noticed that these days, although the sky was clear, it was not blue-blue, it tended towards blue-white, the more white the less heat in the oven. That high level haze has a marked effect on reducing the incoming solar energy.  The most surprising failures were caused by jet contrails. This seemed to be allied to the hazy sky problem in that although the oven reached cooking temperature, if a jet contrail was in the way of the sun the temperature dropped off and the cooking had to be completed on the kitchen stove. Jet contrails during these hazy conditions could persist for hours and often expand into full blown cirrus stratus. So any jet contrails hanging about indicated another no cooking day. Out of about ten attempts only three were fully successful.

What I ended up doing was placing the oven outside and adjusting the reflector and noting the temperature change on the oven thermometer inside. With no pot inside the temperature would climb fairly quickly to about 150F and then rise more slowly. The temperature would have to rise and hold at 180F for at least an hour before my wife would consider trying to use the oven. I noticed that the glass top would get very hot to the touch which indicated heat loss from the oven. But this turned out to be a good indicator of when cooking would fail because the pot inside the oven acted as a heat store and you would not notice that incoming heat was much reduced.

So, while I have not made a very efficient solar oven, I produced a workable INSOLATION METER.
The results are quite startling. Even when you think you have a clear sunny day, that high level haze has markedly reduced incoming heat energy. That is not just into my little box but my local environment for possibly many miles around. And what is that haze? It is not water vapour. You cannot see water vapour. But you can see condensed water vapour, tiny drops of liquid water and possibly even ice. Any thickening of this layer, such as jet contrails, shows a further drop in available energy at ground level.

The exact mechanism for this energy loss I surmise is reflection. If much heat energy was being absorbed the water droplets would vaporise and you would have clear skies again.
So as far as water vapour feedback goes I find no change even though relative humidity may be different each day, but if there is any condensation causing haze or high level stratus then  energy levels at ground level fall off sharply. To me this implies negative feedback. It would be interesting to compare the performance of a solar panel at the same time.

Richard111

Quote:Water and global warming
Water is the main absorber of the sunlight in the atmosphere. The 13 million million tons of water in the atmosphere (~0.33% by weight) is responsible for about 70% of all atmospheric absorption of radiation, mainly in the infrared region where water shows strong absorption. It contributes significantly to the greenhouse effect ensuring a warm habitable planet, but operates a negative feedback effect, due to cloud formation reflecting the sunlight away, to attenuate global warming. The water content of the atmosphere varies about 100-fold between the hot and humid tropics and the cold and dry polar ice deserts.