Fog and the vast dark nebulae

(Photo by Sean Chin)

(Photo by Sean Chin)

It’s now dark and moonless. October weather tends to be clear. It all adds up to our final optimum astronomy opportunity before the long months of overcast and stratus statistically begin, in early November. But first, one feature of the night remains mysterious to many: It’s this season’s predilection for morning fog, dew and frost.

It’s a bit of a paradox. The clearer and drier the night, the more beautiful is the starry firmament. And yet these conditions favor fog and frost, which you’d think should be associated with dampness rather than dryness. That’s because dry air allows maximum infrared radiation transmission.

The ground’s daytime heat gets released after nightfall. The nitrogen, oxygen and argon that compose the atmosphere all facilitate an unimpeded path for this infrared to head upward into space. As a result, the ground cools rapidly, and so does the air nearest the ground. This drops the near-ground temperature below air’s dewpoint. If above freezing, lawns and meadows are covered with dew. If below freezing – and areas exposed to the sky, like open meadows, cool more readily than those below trees or near houses – it’s frost.

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Meanwhile, the air a bit higher up – say, the first 500 feet above the ground – is chilled by the same process. It’s often enough to condense its moisture into a blanket of clouds, termed fog if the bottom boundary meets the Earth. At this time of year, it’s a particularly common process along the Hudson. The ample moisture just above the river is the perfect fog ingredient.

But all that is usually reserved for the wee hours. Before it happens and obscures the sky, if you can get to a place far from the lights of town, look up between 9 p.m. and midnight any clear night this week. It’s the very best time of year to observe a fascinating and often-overlooked facet of the Milky Way.

In truly dark skies, seek out the vast inky tentacles and blotches that form intricate black cameos in front of the Milky Way. These vast dusty hydrogen clouds, liberally sprinkled with organic particles, form immense structures that splay overhead this month.

They float nearer to us than the Milky Way. They are the material from which new baby stars will eventually form. These fascinating ebony patterns are utterly invisible from cities and suburbs. They require not good, but excellent sky conditions – far from any artificial lights.

Some imagine that these amazing “absorption nebulae” demand that the observer be in, say, rural Arizona, but it’s not true. Right here, a short drive from the towns in our region, on dry, moonless October nights, using eyes that have been allowed to dark-adapt for a few minutes, they are easily seen and truly gorgeous.

If you have binoculars, you can use them. But in truth, this is where low-tech rules. These vast blotches of abstract art are among the most impressive sights in the firmament. Relatively few in our modern world get to see them. But this week, in this region, you’re in the right place at the right time.

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