Clouds: Things you never knew

Occasionally at this time of year, when the Sun is lowish but not extremely low, look overhead and you may see an upside-down rainbow called a circumzenithal arc. (photo by Alan Bruce)

We are now in the middle of our region’s clearest time of year. Long-term statistical meteorological studies show that late-August-to-late-October has our fewest clouds. And it’s not a subtle phenomenon. In the mid-Hudson Valley and adjoining areas, the period November through April averages 65 percent cloud cover: two-thirds cloudy. And much of that includes full days of solid overcast.

But right now, it’s only half as cloudy. We average one-third cloud cover in early autumn. The changeover usually happens abruptly, around the first of November, so that it’s not wrong to generalize October as being a sunny month and November as a cloudy one.


The types of clouds vary, too. From May through August, we see lots of puffy individual ones. These cumulus clouds are often due to convection, meaning uprising daytime heating. So the common summer scenario is to awake with totally clear skies, but then see a few individual puffs forming between 9 and 10 a.m. as the daytime heating begins to take hold. Their number increases in late morning, and by afternoon they may take up half the sky. Some can develop into thunderstorms, and their scattered nature yields sunshowers, which in turn can produce rainbows (but only after 4 p.m., since a rainbow cannot appear if the Sun is more than halfway up the sky).

Our warm half of the year thus creates interesting-looking clouds with cauliflower designs that nature-lovers stop to admire…and possible rainbows. By contrast, our cold half of the year often features flat stratus clouds with few if any shapes to them, and no chance for rainbows. Thus it’s not just a matter of degree of cloudiness; their varieties are seasonal, too.

The average cloud weighs a million pounds. So when kids wonder how a cloud can stay aloft, it’s not a dumb question. The answer is that this weight is so spread out, each cubic yard (or cubic meter, if you prefer) can easily hover in any sort of updraft. Each cubic yard of cloud weighs just half a gram, or about 1/50th of an ounce. This is the weight of its liquid water, since there’s half a gram of water in each cubic yard of cloud. The air contained in the cloud has weight, too, but we’re not counting it since it’s the same as the surrounding air.

The other type of cloud we commonly get is cirrus, made entirely of ice crystals. These feathery or wispy clouds are always very high up, and are often semi-transparent. They are responsible for colorful halos around the Moon or Sun, and parhelia or sundogs, which are brilliant colorful blotches to the right or left of the low Sun.

Very thin cirrus can be invisible, and here is where their magic can truly shine. Occasionally at this time of year, when the Sun is lowish but not extremely low, look overhead and you may see an upside-down rainbow. This giant “smile” shape is an arc that precisely surrounds the zenith; hence it is called the circumzenithal arc, or CZA. Typically the arc is one-third of a circle, or 120 degrees, and never a complete circle around the overhead point. Its colors are astoundingly vivid – more saturated than even a rainbow’s. But it’s not a rainbow. It’s magically seen against a blue sky, not superimposed on distant rain. And it’s not opposite the Sun. And its source is ice crystals, not liquid droplets.

So there you have it. When water is in its gaseous form, it’s invisible. When it’s liquid droplets, it’s a cloud (or fog if it’s on the ground). When it’s ice crystals, it’s cirrus clouds, which create so much colorful magic.

Now in October we enjoy the final sunny period, before clouds rule our lives until late next spring.