6.3: Cloud Organization

Clouds frequently become organized into patterns during stormy weather. This organization is discussed in the chapters on Fronts, Midlatitude Cyclones, Thunderstorms, and Tropical Cyclones. Also, sometimes large-scale processes can organize clouds even during periods of fair weather, as discussed here.

Cloud streets or cloud lines are rows of fairweather cumulus-humilis clouds (Figure 6.9) that are roughly parallel with the mean wind direction. They form over warm land in the boundary layer on sunny days, and over water day or night when cold air advects over warmer water. Light to moderate winds in the convective boundary layer cause horizontal roll vortices, which are very weak counterrotating circulations with axes nearly parallel with the mean wind direction. These weak circulations sweep rising cloud-topped thermals into rows with horizontal spacing of roughly twice the boundarylayer depth (order of 1 km).

Mesoscale cellular convection (MCC) can also form in the boundary layer, but with a much larger horizontal scale (order of 5 to 50 times the boundary layer depth; namely, 10 to 100 km in diameter). These are so large that the organization is not apparent to observers on the ground, but is readily visible in satellite images. Open cells consist of a honeycomb or rings of cloud-topped updrafts around large clear regions (Figure 6.9). Closed cells are the opposite — a honeycomb of clear-air rings around cloud clusters. Cloud streets changing into MCC often form when cold continental air flows over a warmer ocean. [WARNING: Abbreviation MCC is also used for Mesoscale Convective Complex, which is a cluster of deep thunderstorms.]

Figure 6.9 Illustration of organized boundary-layer clouds, such as could be seen in a satellite image over an ocean. Grey = clouds.

A SCIENTIFIC PERSPECTIVE • Cargo Cult Science

“In the South Seas there is a cargo cult of people. During the war they saw airplanes land with lots of good materials, and they want the same thing to happen now. So they’ve arranged to make things like runways, to put fires along the sides of the runways, to make a wooden hut for a man to sit in, with two wooden pieces on his head like headphones and bars of bamboo sticking out like antennas (he’s the controller) and they wait for the airplanes to land. They’re doing everything right. The form is perfect. It looks exactly the way it looked before. But it doesn’t work. No airplanes land. So I call these things cargo cult science, because they follow the apparent precepts and forms of scientific investigation, but they’re missing something essential, because the planes don’t land.”

“Now it behooves me, of course, to tell you what they’re missing. ... It’s a kind of scientific integrity, a principle of scientific thought that corresponds to a kind of utter honesty... For example, if you’re doing an experiment, you should report everything that you think might make it invalid – not only what you think is right about it: other causes that could possibly explain your results; and things you thought of that you’ve eliminated by some other experiment, and how they worked – to make sure the other fellow can tell they have been eliminated.”

“Details that could throw doubt on your interpretation must be given, if you know them. You must do the best you can – if you know anything at all wrong, or possibly wrong – to explain it. If you make a theory, ... then you must also put down all the facts that disagree with it, as well as those that agree with it. There is also a more subtle problem. When you have put a lot of ideas together to make an elaborate theory, you want to make sure when explaining what it fits, that those things it fits are not just the things that gave you the idea for the theory; but that the finished theory makes something else come out right, in addition.”

“In summary, the idea is to try to give all the information to help others to judge the value of your contribution; not just the information that leads to judgement in one particular direction or another.”

– R. P. Feynman, 1985: “Surely You’re Joking, Mr. Feynman!”. Adventures of a Curious Character. Bantam.