The worst time for clouds

(George Arents Collection | The New York Public Library)

This is the year’s best weekend to watch the sky. Unfortunately, as I write this several days in advance, the forecast is terrible. Still, you never know, so let’s arm ourselves for all possibilities.

The Geminid meteors look to be fabulous, and early Geminids are already lighting up the skies. The peak is Thursday, December 13; so, if it is mostly clear, go out as soon as darkness falls. Away from artificial lights and under a wide swath of open sky, you should see a meteor every minute or two. That will rise to a meteor a minute, starting at 10 p.m. and continuing all night.

The next night, Friday the 14th, there will still be many meteors, at a rate of one every two to three minutes. Face any direction. Also that night, look for the bright-orange planet Mars to hover just above the half Moon: a fine conjunction.


Then, Saturday and Sunday nights, I expect Comet Wirtanen to achieve naked-eye visibility – the first comet in years to do so. Look between the Pleiades star cluster and the bright-orange star Aldebaran to its left. If you don’t see it, sweep that area with ordinary binoculars. The comet will be a big blob, and will maybe have a tail by then. If you don’t know the Pleiades, sweep the binoculars to the upper right of Orion after 10 p.m. And look even higher up if the sky doesn’t clear until Monday or Tuesday. The comet may only dim a little by then.

You can see why we don’t want cloudy skies right now. But if we must have them, here are some cloud-tidbits you may find interesting.

First, you will not be amazed to learn that we live in a cloudy part of the world. Yet global cloud maps reveal that we’re not as cloudy as the Pacific Northwest, Buffalo or places farther north. The cloudiest zones on our world are centered over the Equator, and also along the 50th parallel of latitude – meaning north of Montreal.

Conversely, the world’s clearest areas are centered at 30 degrees north and south latitudes. It’s no accident those areas play host to the Atacama, Sahara, Gobi, Negev, Sonoran and Thar deserts.

In our region, the nature of clouds varies radically from our warm to our cold seasons. Right now we get a lot of solid sheets, meaning stratus. But May through September feature mainly puffy individual clouds: the cumulus. They’re mostly caused by hot ground heating the air, which ascends and cools at a rate of five degrees per thousand feet of rising, until its vapor is chilled to its dewpoint. Around here, that altitude – at which rising transparent vapor changes to white liquid droplets – ranges from 3,000 feet on humid days to 8,000 feet in drier weather.

A typical cloud weighs a million pounds, or perhaps ten percent more than that. Its greyness depends on how much water it contains, and whether the tiny droplets are colliding enough so that mainly big drops prevail. Bigger drops let more light through, which is why you can see farther through a downpour than through a drizzle.

Clouds keep the night warmer. The reason, first explained here a year ago, is quite simple: The warm sunlit ground loses its heat at night because its fast-jiggling molecules create infrared radiation that heads up toward space. On clear nights, the infrared keeps going, cooling the surface, which is why deserts get so cold at night.

Infrared continues upward in a straight line when it meets nitrogen, oxygen or argon atoms and molecules, which together constitute 99.9 percent of the atmosphere (excluding water vapor, which varies too much to merit a single percentage). But when infrared meets a molecule with three or more atoms – meaning water vapor (H20), carbon dioxide (CO2) or methane (CH4) – it doesn’t continue upward in a straight path. Instead, it is absorbed by the molecule, which then promptly radiates it in a random direction. The released infrared then goes either up, sideways or down. The down-traveling ones hit the ground and warm it a bit.

That’s why carbon dioxide, despite only comprising 1/25th of one percent of the air, contributes to global warming. But water vapor is an even-bigger contributor. Around here, water vapor typically makes up about one percent of the air. (It’s as much as four percent over tropical oceans). Needless to say, an overcast cloud cover has the most water vapor, and thus makes our nights several degrees warmer.

That may be welcome, but we still want it clear – at least for this weekend.

Want to know more? To read Bob’s previous columns, visit our Almanac Weekly website at Check out Bob’s new podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.

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