Saturday, May 28, 2011

The Sky Has a Temperature!

While many on the web are discussing the boring legalities of scientific endeavor, I have been pondering the humorous contradictions facing certain political activists and how best to explain the greenhouse effect.

One of the better explanations is on Science of Doom in twelve plus parts. While the blogger does an excellent job, I have to admit I have dozed off a few times and found what I was awake for maybe a little complex for the average reader.

Many people still find it difficult to understand how a small trace of carbon dioxide, 390 parts per million, could change climate. It is a matter of scale which I have pounded on from time to time. So since I really don't want to work on the motorhome on this beautiful weekend, explaining climate change sounds like a good diversion. The sky has a temperature. There how was that? Maybe a little more detail huh?

Well, I can expand a bit, but that is the main point. Considering only the night sky, the sky has a temperature and the reason it has a temperature is that the gases that make up the atmosphere have a temperature. For about 20 bucks you can buy a cheap non-contact thermometer and check it out for yourself. The non-contact thermometer is an infrared temperature sensor. It determines temperature based on infrared radiation intensity. Just point it at the sky, pull the trigger and you get a temperature reading. On a cloudy night it is easy to see that the clouds are warmer than the clear sky patches. Depending on how good the thermometer is and how clear the sky is you get pretty big temperature differences.

Down here in the humid tropics, the clear sky may be -40 degrees and the clouds -10 degrees. It doesn't matter if the temperature is in C or F because the accuracy of the cheap non-contact thermometer is not all that great anyway. You will measure a difference though.

What throws most people off is that they can see clouds and they don't see anything in the clear sky. That clear sky is the important part though, even at -40 degrees it has a temperature much greater than the empty space beyond it.

The carbon dioxide is a fairly well mixed gas. Water vapor is not so well mixed. So while water vapor may make up as much as 4% of the local atmosphere, in a clear, dry sky is makes up much less, say 0.5 percent. 390 parts per million, 0.000390, is a small number, but then so is 0.5% or 0.005. At this water vapor concentration, CO2 makes up 7.8% of the greenhouse gas in the sky neglecting the other more rare greenhouse gases. As far as our matter of scale, 7.8% is significant. To simplify things, whenever CO2 has an impact of greater than 1% of the total greenhouse gases, it is significant.

Back at the start of the 20th century, CO2 was about 280 parts per million, so under these same conditions, CO2 would have been about 5.6% of the greenhouse gas concentration in a 0.5 percent water vapor sky. As long as there are greenhouse gases in the atmosphere the sky will have a temperature greater than it would if they were not there.

If there were no greenhouse gases, the temperature of the sky would be about 33 degrees C lower. That is not a hard and fast number. It would vary depending on the initial temperature of the surface under that portion of the sky and the thermal mass of the surface. Water has a high thermal mass, so it takes longer to heat and cool, so it would be warmer if the sky is over the ocean than over a sandy desert for example. Then since the ocean is water, there would be water vapor which is a greenhouse gas, so the 33 degree number is more of a concept than a real situation.

Understanding why the 33 degree number is a concept is kind of important. As long as there is water on the planet there will be water vapor. Water vapor is the dominate greenhouse gas because of the amount of water on the planet, so CO2 plays a limited role in limited regions of the atmosphere. The drier the air, the greater its role.

The physics behind the greenhouse effect is really not that complicated. The shape of the greenhouse gases molecules is different than the other diatomic gas molecules in the atmosphere. N2 nitrogen, O2 oxygen make up nearly 99 percent of the atmosphere. All the trace greenhouse gases have more than two atoms per molecule. Since the greenhouse gas molecules are not symmetrical, they behave differently when excited than the diatomic molecules. They are excited by infrared heat or radiation, while the diatomic molecule not so much. Once excited by infrared or collisions with other molecules, they gain heat energy which they can pass along to any other molecule or lose to space. The closer they are to the surface when excited the more heat the surface can retain. If the greenhouse gases molecules are high in the atmosphere, heat is retained, but has less impact on the surface.

Some will have a problem with my word "retained". I like it because retained does not have a fixed time associated with it. You can retain your earnings for a long time or spend it quickly, but it was still retained because you had the option to use it. Once you understand that greenhouse gases help retain heat, you can look at the significance of scale of the individual greenhouse gases to understand the relative impact they have.

It is pretty easy to understand that the higher the sky's temperature the more heat is retained, the warmer the surface can be. If you set a cold beer on a picnic table in the winter is will stay cold longer than it would in the summer. If you are not into beer, you could used a cup of hot coffee, which would stay hot longer in the summer than it would in the winter. The rate of heat flow depends on the temperature difference. If the sky is colder, the surface gets colder faster than when the sky is warmer. Heat flows from hot to cold and flows faster the bigger the difference between the hot and the cold.

The greenhouse effect is not rocket science, it should be easier for most to understand if people explaining it would stick to the basics and matters of scale.

Explaining how much an increase from 280 parts per million to 390 or 560 parts per million will increase the sky's temperature is a touch more complicated.

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