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Politics : The Environmentalist Thread -- Ignore unavailable to you. Want to Upgrade?

To: Sam who wrote (19062)	12/27/2007 3:43:54 PM
From: HPilot	Respond to of 36921

A much better discription. You can also heat a building by painting it black. The only similarity is the radiant heat.

To: Sam who wrote (19062)	12/27/2007 3:56:54 PM
From: neolib	Read Replies (2) \| Respond to of 36921

The point is comparing a greenhouse to the earth and seeing how the equations are similar or different. The earth prevents both convection and conduction. So does a well designed greenhouse. Both the earth and the greenhouse are then warmed by radiation. This results in their internal temp increasing. ASSUMING THAT THEY REMAIN IN CONVECTIVE AND CONDUCTIVE ISOLATION, they will both achieve an equilibrium temp which is higher than if they blocked out the incoming radiation. The actual equations of what this equilibrium temp is depends on the internal properties of the system (greenhouse or earth) and includes all the mess of reflection, absorption, atmospheric dynamics, oceans, etc. but in the end, assuming convective and conductive isolation, the temp is set by the net radiative energy balance. Trying to claim that a greenhouse is "heated" by convective isolation is like claiming you can push on a rope. You can pull on a rope, just like you can cool a greenhouse by allowing outside air to blow in and replace inside air. But imagine if the sun is not shining, and the greenhouse is at the same temp as the outside air. Can you heat it by preventing convection? Of course not. You can understand this better by the following thought experiment. A greenhouse is used to grow plants in a cold environment where there is reasonable sunshine. Say at 6000 ft in the eastern Oregon desert in the winter. A well insulated, glass enclosed greenhouse is used. The greenhouse is nearly airtight (convective isolation) and the glass panels are triple-pane, gas filled, so they are close to conductive isolation. The design is thus, within the confines of economic reality, convectively and conductively isolated. The glass windows are all equipped with shutters having high reflectivity. With the shutters open and the sun shining, the inside heats up. When the desired temp is reached, the shutters, or some of them are closed as needed. The equilibrium temp that the greenhouse reaches can now be controlled by radiation only, no convection, plus you can choose the temp. Please note that you can even drop the temp inside the greenhouse BELOW the outside temp (assuming the building is well insulated) using this method. If you don't understand how the latter can be, think about it for awhile.