Climate Change, Effect of Atmospheric Moisture

 

Climate Change, Scientific Considerations, H2O

To understand the role of water in atmospheric physics, we need some understanding of the basic chemistry of water. The molecule consists of two hydrogen atoms and one oxygen atom. Thus, the molecule is somewhat smaller than a carbon dioxide molecule, and for that reason it has been argued that it is less important as a greenhouse gas than carbon dioxide. However, that argument neglects a fundamental property of the water molecule: its polarity promotes bonding with other water molecules. This bonding results in the formation of aggregates of water molecules that can become so large and heavy that they become water, which is a relatively heavy liquid. Thus, water doesn't consist of separate H2O molecules, but of large aggregates that behave, insofar as their physical properties are concerned, like large heavy molecules – one could say, “super-molecules”.

We know that nearly all of the water vapor in the atmosphere results from evaporation from liquid surfaces. The evaporation process is interesting. The temperature of a substance is a measure of the random motion of the molecules that make up the substance. In a gas, the molecules fly around in random directions and bounce off of each other and off of wall boundaries. In a solid, like ice, the molecule locations are more constrained, but the molecules move slightly, and their components vibrate with an intensity that increases with temperature until the solid ice begins to separate into aggregates of molecules, as described in the previous paragraph. These aggregates are the components of water. When the water is heated, the increasing molecular motion causes the weak bond that holds together the aggregates of molecules to weaken, and the aggregates begin to break up into smaller aggregates. As the heat increases and the aggregates become smaller, they eventually become as light as the molecules in the surrounding air, and depart the liquid surface to become water vapor. (There are other factors that come into play, like the temperature and pressure of the air at the liquid surface, but this simplified model is sufficient to convey the basic concept.)

Once the water molecules become small and light enough to become gas, they take on a size that depends on the degree of saturation (percent humidity) of the moisture in the air. The saturation depends on the amount of water available as well as the local temperature and pressure. When conditions are such that the the aggregates are large they become visible as steam, or as clouds, or as fog, which can be so heavy and opaque that visibility is limited to few feet. Thus, the clouds shield the earth from solar radiation during the day, but at night they inhibit cooling by radiation into empty space.

When aggregates are smaller, they may not obscure vision entirely, but they may cause the atmosphere to appear hazy. This is the condition that may cause something of a greenhouse effect because the haze is more easily penetrated by the higher frequency incoming light waves than to the lower frequency heat radiation emitted at night.

When the aggregates become so small that they are no longer aggregates but single H2O molecules they are invisible, and transparent both to incident and emitted radiation.

The mechanics of the water cycle is complex and involves many factors, all of which has explained in textbook articles; so I will not reproduce that information here. My intent is only to call attention to the fact that the aggregates of water molecules in the atmosphere are much larger than CO2 molecules. Therefore, if any kind of greenhouse effect plays a significant role in the climate change discussion, one of the culprits is water, but not the highly transparent CO2 gas, which is not only innocent but actually plays a significant role in cooling by promoting plant growth.

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