Tuesday, August 21, 2007

The Big Heat Pipe In The Sky

The atmosphere has been described by the Profits of CO2 Doom as a blanket that traps the incoming solar energy and warms the planet. Which is true. At a short time scale. At a little longer time scale the atmosphere is more like a heat pipe. This is on the scale of weather. Day to day changes. I'm going to first explain how heat pipes work and then show how the atmosphere is similar. We will also look at how the ocean is the primary determinant of climate on longer time scales (5 years time constant).

What is a heat pipe? It is a sealed metal tube (quite often copper) partially filled with a fluid that also has a wick running its full length. The wick is arranged so that it is in contact with the walls of the tube. Here is a simple diagram of a heat pipe.

How does the heat pipe work? It has a cold end and a hot end. At the hot end the external heat source boils the fluid in the heat pipe the vapor created then condenses on the cold end and the wick then carries the fluid back to the hot end and the cycle repeats.

Because of the evaporation/condensation method of heat transfer the temperature drop between the hot end and the cold end is much smaller than if the tube had been made of a solid piece of metal. That kind of heat transfer is very efficient. You can get into more of the details at wiki on Heat Pipes. If you want to learn a little of the math and some of the practical difficulties here is a good article on how to build a heat pipe.

Let me start with an article I discussed earlier at Feedbacks Misdiagnosed. It is by Roy Spencer and discusses the nature of water vapor feedback in terms of weather and climate.

Let me start out by saying that water vapor is the most prevalent and most effective greenhouse gas in our atmosphere.

Here is a bit of what Roy has to say.

For instance, everyone believes that water vapor feedback is positive, and conceptually justifies this by saying that a warmer surface causes more water to evaporate. But evaporation is only half the story in explaining the equilibrium concentration of atmospheric water vapor; precipitation is the other half.
Which is to say that the whole heat pipe, not just the hot end, must be considered when studying the atmosphere. I covered some of that in Clouds and More Clouds and Clouds In Chambers.

Roy Spencer discusses how water vapor is the atmosphere's natural air conditioner. Which is not bad. An air conditioner is in many respects a mechanized heat pipe. It can actually transfer heat from a cold space to a hot space. The atmosphere can't do this. So the heat pipe analogy is more apt. Other than that he has some good simple diagrams and pretty pictures to explain what is going on.

So let me sum up:

We don't live in a greenhouse. We live in a heat pipe. Actually that is not strictly true. We live in a greenhouse and a heat pipe. The greenhouse slows heat transfer by radiation. The heat pipe increases heat transfer by conduction and convection and evaporation and condensation. Because of heat storage in the ocean there is about a 5 year time constant from the time the extra energy starts coming in until balance is mostly restored.

In terms of delayed response, the climate problem is similar to the capacitor soakage problem in electronics.

There is a primary time constant and a number of secondary time constants.

The secondary time constants are generally not very influential except at very high precisions. Even then their influence is limited to very low frequency signals.

Roy Spencer and a number of others have worked out the primary time constant by other means and have also come up with a numbers around five years.

In control theory to assure system stability you generally want a system where a first order lag is dominant. This appears to be the case in the climate system according to a number of different analysis methods.

In addition because of water vapor evaporation/condensation the atmosphere is more like a heat pipe than a blanket at the time scales (five years) in question. At shorter time scales it is more like a blanket due to the lags. In fact the primary time constant is determined by the evaporation/condensation time constant according to Stephen E. Schwartz.

I'll go into where the five year number comes from in another post (can't say when).

For those of you who can't get enough on how the consensus is breaking down you might want to read this link rich piece by a Congressional staffer.

Cross Posted at Classical Values

2 comments:

triticale said...

Be careful with that capacitor analogy or the Gorebal Warming crowd will point out that sufficient overload can burn thru the dielectric.

Snake Oil Baron said...

The climate oracles recently informed us that, rather than worrying that global warming would cause the atmosphere to expand and bring down satellites, as they had told us a year before, we should be concerned about CO2 causing upper atmospheric cooling resulting in space junk staying up longer than it would have. Wouldn't a cooler and denser upper atmosphere pressing down on a warming lower atmosphere drive the moisture heat pump - not to mention simple convection - faster? It would be like putting a cold pack on a lavalamp causing the blobs to fall faster.