It's Time Again for Noctilucent Clouds

As we move into late summer, the opportunity to observe noctilucent clouds is at its best. Many people who have lived in the northern latitudes for years have probably noticed them before without having a proper appreciation for what they really are.

Noctilucent (night-shining) clouds ride in the sky above 99.9 percent of the atmosphere and over 40 miles above the highest clouds associated with weather. At an average altitude of 50 miles (80 km), they actually skirt the lowest fringes of the aurora, and are above the height at which meteors are observed. For reasons which are not well understood, they occur only at higher latitudes and almost exclusively during the summer months.

Noctilucent clouds are usually seen in the western sky 10 to 20 degrees above the horizon after the sun has set. During this time, the local area is in darkness and nearby clouds appear dark against a lighter sky. Noctilucent clouds, however, are high enough above the earth that they are still illuminated by the sun and appear bright against a darker sky. They are a beautiful, wispy blue-white and usually display a wave-type structure that changes quickly with time. They are so tenuous that starlight shines through them without diminuation.

What are they made of and why are they there? These questions have never been answered to anyone's total satisfaction, but some rocket borne observations have provided clues. The first of these studies was made in Sweden in 1962. A Nike-Cajun rocket with a payload designed to trap particles of a cloud and return them to earth was fired into a noctilucent display and successfully recovered.

Under an electron microscope, the surfaces on which the particles were captured revealed millions of minute motes of dust as small as 0.05 microns in diameter (a micron is one-thousandth of a millimeter; a millimeter is about half the thickness of pencil lead). Obviously, chemical analysis of samples so small is extremely difficult, but electron bombardment indicated that the particles contained nickel. Nickel is an element quite rare on earth, but common in meteorites. Hence, an extraterrestrial origin is suggested for the components of noctilucent clouds.

In addition, the collecting surfaces showed halos surrounding some particles. This implies that they originally had a coating of something that had evaporated between the time they were collected and the time they were observed in the laboratory. The coating must almost certainly have been water ice.

The picture which therefore emerges is that noctilucent clouds are meteor dust particles covered with ice. Knowing what they are, however, in no way explains why they behave as they do. It would be expected that meteoritic particles would be evenly distributed in the earth's upper atmosphere. Why, then, are noctilucent displays localized; why do they occur only occasionally; why only during the summer months; and, why only at the higher latitudes? These questions about the rare and beautiful spectacle remain to be answered.