If you're an avid skywatcher, no doubt you've seen many beautiful displays of natural skylight: halos, rainbows, sundogs, and crepuscular rays are a few examples. For the most part, however, these are unpredictable and can occur throughout the year. Only one can be said to be both predictable and rare: noctilucent clouds.
Noctilucent clouds (once referred to as luminous clouds) are cirruslike clouds that form in the mesopause of our atmosphere at the very edge of space (between 75 and 90 kilometers). At this altitude, which is some five times higher than most of Earth's weather-making systems, temperatures may fall to - 100°C (—150°F) at the North Pole in summer. These extremely cold conditions allow the slight amount of water vapor in the mesopause to condense onto meteoric dust or, more likely, electrically charged atoms and molecules. These ice-encrusted particles are thought to be the source of the clouds.
The feathery stratum of noctilucent clouds displays iridescent hues, like the inside of an abalone shell, and exhibit a range of structure from veils and bands to waves and whirls. Noctilucent clouds are often tenuous enough for bright stars to shine through, making them a subtle phenomenon, though perhaps not as delicate as the zodiacal light (March 8 - 14).
To see noctilucent clouds, you need to be in the right place at the right time. The right time (for those in the Northern Hemisphere) is June and early July. The right place is typically between latitudes 45° and 60°. (They are also visible from these same latitudes in the south during midsummer in the Southern Hemisphere.) Most noctilucent clouds appear low in the twilight sky, usually not more than 10° above the horizon. They first become visible at twilight, when the Sun is 6° below the horizon and the contrast between the darkening sky and the brightening clouds is perceptible. Once the Sun sinks more than 16° below the horizon, it can no longer illuminate the clouds and they vanish.
Around the time of the summer solstice, however, when the Sun is never far below the horizon at high latitudes, a robust noctilucent display may be seen up to the zenith and even into the other half of the sky. This is especially true at latitudes higher than 60°, where the Sun never gets more than 6° below the horizon this time of year.
Like aurorae, noctilucent clouds are more often reported in the Northern Hemisphere, because the population density is greater at the high latitudes there than it is in the Southern Hemisphere. The countries in which observers most often report noctilucent clouds are Canada, the British Isles, Scandinavia, and Russia. In the United States, they may be seen in Alaska, although they have from time to time been seen in New York state.
Interestingly, reports of noctilucent clouds have doubled since the 1950s. Although the increase may be due to greater public awareness of the phenomenon, another explanation invokes an environmental concern. The proliferation of industry and the automobile during the twentieth century has resulted in widespread pollution. Some environmentalists think that an increasing number of hydrocarbons and other pollutants in the upper atmosphere may be supplementing the natural condensing nuclei at high altitudes. If true, the luminous beauty of noctilucent clouds may have a decidedly darker lining.
Also this week:
• The Summer Triangle, consisting of Vega, Deneb, and Altair, has fully risen as darkness falls this week. (See July 26 - August 1.)
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