Aug 08 2007
Do Greenhouse Gases Act As Blankets?
There is a common belief that blankets warm you in the same way that greenhouse gases warm the surface. On the surface, this seems like a good hypothesis. Without a blanket on a cold night, you would be much colder than if you placed on over your body. Without greenhouse gases, the surface of the Earth would be much colder. But that’s about as much as they have in common. You see, greenhouse gases and blankets affect the heat distribution in two separate ways: radiation and convection, respectively.
To see this, consider four ice cubes, two of them exposed to warm air (~80F) and two of them right next to them, but covered with a blanket. Which will melt first? I’m glad you asked.
Below you’ll see a picture of the experimental setup. The two ice cubes to the left will be totally exposed to the air in the room, while those on the right will be covered by the two “blankets” (actually dish towels).
|
Here is how the experiment looked while running, except I lifted the towels every 5 minutes to take a picture.
|
And here is how the experiment ended. Note that the ice cube under the towel is still there.
|
And here is the link to an animation of the ice cubes melting. [1.4 MB]
If blankets acted like greenhouse gases, the temperature under the towels should have been greater than the ambient air temperature. This should mean that the ice cube under the towels should have melted faster. However, we see the experiment has shown the opposite. The ice cubes under the towel took longer to melt. This is because blankets, or towels, don’t affect the radiation greatly, but they do act as great insulators. At night while you’re under the covers, the blanket acts to insulate you from the cold air outside. In this case, the towels acted as an insulator to keep the warm air outside.
Blankets act to suppress convection and have a negligible effect on radiation; greenhouse gases do not suppress convection and have an important role in the radiation balance.
(1 votes, average: 1 out of 5)
Loading ...Related Posts:
3 Responses to “Do Greenhouse Gases Act As Blankets?”
To reduce spam, comments are automatically closed 30 days after the last comment. If you would like to comment on any closed thread, please use the contact form at the top of this page.
I’ve resisted the temptation to say “Cool video!”, but I do have to ask if I can use this as a worthy alternative to the cliched analogies of “watching grass grow” or “watching paint dry”? Also, is that a tub of icecream in the top right corner of the frame? If so, why was it not similarly covered with a teatowel? Enquiring minds want to know these things.
Doh. I mean “top left” of the frame. Blame hemispheric confusion.
It was actually a tub of cake frosting. Both it and the cake are now gone.