The Sciences

Sky Lights

The sky turns cloudy this month. But even on overcast days, there is still plenty of spectacle to see up above

By Bob BermanNov 8, 2003 6:00 AM


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The bond that many people establish with the night sky during summer evenings comes rapidly undone as autumn progresses. It’s not just the chilly air that gets in the way. Data from the International Satellite Cloud Climatology Project, the Global Resource Information Database of the United Nations, and the National Weather Service reveal another, more fundamental impediment: November skies suddenly turn cloudy.

Across much of the United States and Canada, the frequency of blue-sky days plunges 20 to 25 percentage points between October and November. In many American cities, November is the cloudiest, or at least the second- or third-cloudiest, month of the year. It might therefore seem that the game is essentially over for sky watchers after Halloween. Fortunately, the overcast days of autumn deliver their own kind of sky pleasures. Several dramatic phenomena become visible only when the sun or moon shines through a partially obscuring deck of clouds.

The strangest of these events occurs when sunlight passes through the edges of white midlevel clouds. Quickly evaporating droplets within these clouds, which have a wide range of diameters, cause light waves to travel varying distances along the path to your eye. As a result, the peak of one arriving light wave may coincide with the trough of another wave, canceling each other and eliminating the color associated with that wavelength. With one of sunlight’s components obliterated, the remaining light takes on a psychedelic appearance. This phenomenon, called cloud iridescence, displays exquisite pastels that lie outside the familiar rainbow: aquas, maroons, purples, and pinks. Using nothing more than a pair of sunglasses to highlight the fringes of clouds, you can observe colors that cannot be seen anywhere in the universe, through any telescope.

Another subtle color effect emerges when the sun or the moon passes behind a thin cloud and becomes surrounded by a disk of light known as a corona. It often appears as a brownish or orange smear, as several bluish rings, or even as a subdued circular rainbow. It is caused by diffraction, a result of the way that waves of light slightly bend around the edge of a water droplet. Coronas around the moon are visible every few nights from most places. Solar coronas are often lost in the glare of the sun; they are easier to see when the sun is viewed as a reflection in water, which diminishes its dazzle.

If the high, partially obscuring clouds contain tiny hexagonal ice crystals, a halo will form around the sun or the moon. A halo is simple to distinguish from a corona because it is vastly larger and forms a well-defined circle with an unilluminated interior. Hold out your hand at arm’s length, with the fingers spread fully apart. The distance from pinkie tip to thumb tip will neatly span the 22-degree radius of the halo, its unvarying size dictated by the geometry of the ice crystals. Light refracting through those crystals creates the halo and ensures that red, the color that bends least, will paint the inside of the ring.

High cirrus clouds often produce another dramatic type of refraction. Within two hours of sunset, a small, brilliant white or vividly colored spot often hovers 22 degrees directly to the right or left of the sun. This extremely bright effect is called a parhelion by scientists and a sun dog by everyone else. It is also caused by those hexagonal ice crystals, which tend to settle in a horizontal orientation—hence the left-right arrangement.

Of course, there are some moments when a clear sky is what you really want. One of them arrives at 8:06 p.m. EST on November 8, when the moon will slip into the center of Earth’s shadow and vanish, or at least turn deep red. Even then, a touch of halo or corona could be just the thing to turn a lovely astronomical event into something truly spectacular.

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