From Watts Up With That?

Guest Post by Willis Eschenbach

I got to thinking about the clouds again. They’re easily the least understood part of the climate. So let me start with what we do know.

Overall, clouds cool the planet. Here’s the effect of clouds on the surface.

There are some interesting things about Figure 1. First, clouds cool the planet everywhere except the poles and the deserts.

Next, over the ice-free parts of the ocean, the clouds only cool the ocean surface. No part of the ocean is warmed by the clouds.

Finally, the more clouds, the more it cools the surface. That’s why the moist tropical areas, with their extensive cloud cover, are receiving the most cooling from the clouds.

With that as prologue, here’s a thought experiment. Suppose you could control the amount of clouds, and you wanted to create a thermoregulatory system that would stabilize the temperature of the earth. How would you go about it?

Well, it seems to me that you’d want a system where if the planet started out cold, the warmer it got the fewer clouds it would have. Fewer clouds mean less cooling, and that would encourage the system to continue warming …

… but you’d only want less clouds up to a certain point. Once you reached the desired temperature, then you’d want the reverse to happen. Above that temperature, if it warmed further you’d want more clouds, to tend to drive the temperature back down to the desired point.

So you’d want the response of the clouds to look something like this:

In Figure 2, the amount of clouds decreases as you go from freezing to about 20°C. But further warming brings increasing clouds, tending to drive the temperature back down.

And to move from our thought experiment to the real world, here’s the actual relationship between cloud area and surface temperature. I’ve used only the unfrozen sea surface temperature, to avoid the complications of ice, mountains, and deserts. However, the full globe shows the same pattern, just with more scatter.

Note that what is shown in Fig. 3 is that the preferred temperature occurs in the tropics at around 25° – 27°C. Colder than that and clouds decrease with increasing temperature. Warmer than that, and the clouds increase very rapidly, putting a cap on the tropical temperature.

In closing, there are other variables in the equation. For example, at the warm end, the clouds are composed of more and more thunderstorms, which cool the surface in a variety of ways, not just the effects on radiation.

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And that’s all the fun I’m allowed to have in a 24-hour period … my floor-mounted gas heater died, and I need to crawl under the house, pull out the thermocouple, test it to see if that’s the problem, and if so, go to town, buy a new one, and crawl back under the house to replace it.

Do I know how to have a good time, or what?

Best to all,

w.