The effect of cloud cover on the progress of climate change has not yet been settled. This is a subject that I have been paying more and more attention to lately, using my customary habit of getting as much information as possible directly from a study of the weather maps. I like it that the maps depict the intensity of cloud cover and rainfall together on one map. This has made it easy to observe that heavy rainfall makes a big difference in the total cooling effect of cloud cover. Heavy rainfall seems most likely to require thicker clouds than light rainfall, allowing less light to pass through and reach the surface. The energy carried by the incoming light may just get trapped and stuck inside the body of the cloud, warming the air at some distance above the ground, on balance, rather than the air close to the surface and the surface itself. Temperatures carried by raindrops could also have an effect because the relative coolness of the air where they originate. With respect to albedo effects, concerning only the amount of sunlight reflected from cloud tops, I am wondering if there is any difference between reflection from the heaviest clouds that produce a lot of rainfall and that of clouds of medium thickness, like those producing no rain at all?
One thing I’ve taken note of is that any observation of heavy rainfall on a map is most often accompanied by the presence of a heavy concentration of precipitable water (PW), probably even more heavy than that of any nearby air which may have thick clouds but no rainfall. That makes good sense in view of the basic rules of saturation. What makes this all so interesting is that sometimes we see PW with very high concentration and yet the air is perfectly clear. Such situations are almost sure to produce very warm anomalies at the surface because there is nothing in place to offset the greenhouse impact. Then we may see places where clouds form with patches of rain, while the amount of PW stays about the same. Each such change is likely to have its some kind of effect on temperatures below. I am personally convinced that nothing happens here that would change the energy generation of the greenhouse effect by more than a small amount, as H2O molecules change from one state to another through condensation. That may be true for outgoing radiation, but a change of state can have a totally different effect on incoming radiation. The overall cooling effect of any blocking of radiation, while difficult to assess with accuracy, is undoubtedly significant, on scales that can be surprisingly high.
We still need to expect a flow of variations that depend on seasonal factors, like length of day and angle of the suns rays, that occur on a wide scale. Overnight shifts can have the same effect on daily results. There is nothing of this sort applicable to the steadiness of greenhouse effects. Looking into the future, could climate change have an effect on incoming radiation that has not yet been discovered? I can see how how a trend toward either more or less heavy rainfall would have considerable leverage as a means of offsetting the ultimate greenhouse effect of the higher concentrations of PW that can be expected, but which of these is the more likely?
I’ll close by showing three images that can be studied by looking for connections between today’s warm and cold anomalies, cloud cover and rainfall, and PW values across the globe. The one thing I find consistent is the regularity of high PW values, almost certainly well above historical averages almost everywhere in the NH, but low values can be seen in the southern parts of South America and Africa. The power of clouds and rainfall to offset the strength of these values is obviously considerable.
Carl