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Friday, March 16, 2012

Still Trying to Slay the Slayers

The Tyndall Gas Effect, Greenhouse Effect or the Atmospheric Effects,are all due to electromagnetic radiation interacting with the various elements that make up our atmosphere. As visual creatures, there pretty ones, rainbows, sundogs, aurora are all attention grabbers. The ones we cannot see, as with our own eyes, are more fun to explain. A book, Slaying the Sky Dragon, disputes the Greenhouse Effect, which is based on the Tyndall gas effect which is one of the Atmospheric effects invisible to the human eye. Humans tend to believe what they wish to believe until they "see" clear evidence to the contrary. The Slayers, have not seen the infrared light of the basic physics involved in our atmospheric effects. Mainly because the explanations or and analogies for the "Greenhouse effects" are not very well presented. Dr. Roy Spencer, has recently taken another stab at slaying the slayers.

I believe the main reason that the "greenhouse effect" is still causing controversy is the not that great understanding of the effect by the people trying to explain it. The impact of the Tyndall gas effect, AKA greenhouse effect is not just one, but a combination of effects. Describing one part while not acknowledging the rest, only increases the confusion.

One of the common mistakes is the insulation analogy. Everyone is familiar with home insulation and coolers for keeping things cool. Most everyone has a grasp that a cooler can keep beer cool can also keep food warm longer as well. It does both by reducing the rate of heat flow out of or into the cooler. Heat, a convenient form of energy, flow from greater to lesser or hot to cold. A cooler doesn't have a computer to let it know what it should do, the laws of physics, specifically, the laws of thermodynamics dictate what it does. They are laws, not guidelines.

Energy will find a way to find a lower level. The main paths for heat or thermal energy flow are conduction, convection and radiation. The rates of each flow depend on the medium of transmission because of thermal properties of that medium and the difference in energy or heat.

Conduction is the direct exchange of energy between molecules in contact. If you roast a marshmallow over a fire with a stick you will have a more pleasant experience than roasting the same marshmallow with a copper rod. Copper conducts heat much more efficiently than dried wood. If you use a green stick, you may find that your hand gets a bit warmer than if the stick is dry. Dry wood, copper and wet or green wood have different thermal properties.

The dry stick conducts the least heat of the three. The only difference between the green and dry sticks is the moisture. So water has a impact on heat transfer. The physical properties of water that cause the difference are it specific heat capacity and phase characteristics. Water though varies in volume. As a liquid, one kilogram of water occupies about one liter of space.

As ice, the water occupies more space. As a gas, water occupies even more space. How much space depends on the temperature and pressure of the space. Because of the physical properties of water, ice floats and steam rises. A huge amount of the Tyndall effect and all atmospheric effects is due to the properties of the substance water.

If you are in a desert, which has little water vapor in the air, You will be hotter in the daytime sun and colder at night than you would be on the ocean with the same amount of sunshine. That is because the air in the desert has a lower specific heat capacity than moist air on the ocean and it time time for heat to flow into or out of a volume.

With the green stick, it long to get hot at the handle than the copper rod and more time than the dry stick. The rate of heat flow varies with the physical properties of the medium, in this case the skewer for roasting a marshmallow.

The specific heat capacity of water is 1 calorie per gram degree C at 0 degrees C. That is one of the highest heat capacities of any substance. Since most table list specific heat in Joules, not calories, water as a liquid has a specific heat capacity of 4181J/kg or 4.18J/gram.

As a vapor, water has a specific heat capacity of 2.08J/kg and 2.11J/kg as ice. These values do change with temperature and pressure. Dry air has a specific heat capacity of 1J/kg at sea level and 0 C degrees. Water vapor has twice the heat capacity of dry air. Adding water vapor to dry air increases the heat capacity of the air mixture. Wikipedia has a nice list of the sensible heat capacities of substances. that is only a part of the story though.

Wikipedia also has a list of the thermal conductivity of substances. This list is a lot more complicated because different common substances has different compositions. Dry wood for example has a range of 0.04 to 0.17 Watts per meter-K, wet wood, at 12% moisture has a range of 0.09 to 0.4 W/m-k, but you can see that with about 12% moisture, wet wood conducts 4 times as much heat energy.

Water vapor, at one bar or at sea level conducts 0.04W/m-K or one tenth as much as the wet wood. Make a mental note of that.

The thermal conductivity of air 0.024, per the engineering toolbox. Adding moisture to the air increase the thermal conductivity. Then change in the thermal conductivity is a little confusing though. Any moisture changes the thermal conductivity, but there is no significant difference until the temperature of the moist air mixture is greater than 20 C degrees. As the temperature of the moist air approaches 100 C degrees, the difference is much larger. The reason for this it that that amount of water vapor that the air can hold is limited by its temperature and pressure.

Now water vapor can change the thermal conductivity as I ask you to note, but it is limited by the properties of the mixture of all atmospheric gases, temperature and pressure from making much change under normal atmospheric conditions. That limit is called the saturation vapor pressure. Air can be saturated with moisture which would be 100% relative humidity. Once it reaches saturation, it cannot hold any more moisture unless the temperature increases or the pressure decreases. Since the temperature in the atmosphere decreases with decreasing pressure, warm moist air can rise further in the atmosphere until it reaches a point of 100% relativity humidity and the water vapor begins to condense. Since water has a higher specific heat than water vapor, there must be a change in heat content and volume for there to be a phase change from vapor to liquid or vapor to solid. Since this change is not easily seen, it is call the latent or hidden heat of fusion or vaporization. Heat has to be gained by the water molecules to progress from ice to water liquid to water vapor, and heat lost to progress in the opposite direction. As a vapor, water is limited by temperature and pressure to the amount of energy it can gain or lose until it reaches saturation.

So water vapor adds to the specific heat of the air, causes a small increase in the thermal conductivity of air which is limited by temperature and pressure and the amount of water vapor the air can hold is limited by temperature, pressure and available water.

Back to the marshmallow roast. If it is a cold night, you will notice that the fire warms one side of you, the one facing the fire, and the back side stays cool or even seems to get colder. Since the heat of the fire is expanding the air, cause it to rise, most of the heat you feel is due to radiant heat transfer. You put your hands over the fire to warm them, you are getting added warmth from the conductive heat transfer from the rising warm air plus the radiant heat transfer. The actual convection does not warm you, it actually case cooling as colder denser air moves toward the fire to replace the warm rising air. So the convection transfers the heat from point to point, but does not transfer heat from surface to surface. The surface to surface transfer is due to either conduction or radiant heat flow. For the water in the air to condense, it also has to lose heat by conduction or radiant heat transfer.

So without including carbon dioxide, there are quite a few physical processes that have some influence on the rate and amount of heat that can be transferred from one surface to another.

Just like with water vapor, adding CO2 changes the physical properties of the atmosphere. Just like water vapor, only a small amount is required for some of the change and more will increase other changes and temperature and pressure are factors that also must be considered. Unlike water vapor, CO2 is non-condensable for most of the temperature and pressure ranges in the atmosphere.

The specific heat capacity of CO2 is 2,470J/kg at 0 C degrees. A little over half the capacity of water and slightly HIGHER than water vapor. The specific heat capacity of CO2 is much more variable with temperature and pressure than water vapor, ranging from 36,400 at 30 C degrees to 1,840 at -50 C degrees. The thermal conductivity of CO2 also ranges from 0.07 at 30C to 0.115 at -20C degrees. Both of those ranges are for one atmosphere of pressure. While CO2 is considered non-condensable in the atmosphere, it has a freezing point of -78.5C sea level and approximately -89C at 0.38 atmospheres. That means that the impact of CO2 on the properties of air are highly dependent on temperature and pressure for conditions of the atmosphere. Just as a small quantity of water vapor can can impact air temperature, a small concentration of CO2 can have an impact on the air temperature.

The big question is how much does a change in CO2 have on the atmosphere? The answer depends on the range of change possible. That really hinges on another physical property not yet mentioned, thermal diffusivity. The thermal difusivity of air is roughly its thermal conductivity divided by the product of the density and specific heat of the air. CO2 changes both the thermal conductivity and the specific heat capacity of the air and both in a non-linear manner. The specific heat capacity increase with temperature exponentially, which decreases the thermal diffusivity. The thermal conductivity increases with a decrease in temperature with a maximum value at -20C degrees, then begins to decrease with temperature. In a greenhouse, where temperature, pressure convection are controlled, the impact would be the greatest. In an open atmosphere, convection is not limited, advection, horizontal winds are not limited and pressure is subject to change both locally and with convection. This makes it fairly easy to show greenhouse warming in an experiment, but difficult as all hell in an open system like our atmosphere.

Just to give a real world example of the impact of conductivity and radiant energy flux, consider a radiant barrier or space blanket. Using the US R-value, an air space of 3/4" has an R value of one. Adding a clean, shiny, brand new radiant reflective barrier increase the R-value to three. That would imply that conductivity is at least one third as important as radiant heat transfer between surfaces. It is the transfer between surfaces that creates the convective and latent heats that can transfer energy to another surface at another point in space and time.

BTW, since atmospheric physical properties are related to pressure which is dependent on gravitational force, gravity doe impact climate. But the change in gravity does not appear to produce significant changes in the lower troposphere where we live. It likely does have a small impact in the upper atmosphere, but how much is far from known.

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