Noctilucent Clouds taken from 36,000 feet above the South of Nunivak Island, Alaska on June 20, 2006. Image credit: John Boardman; copyright, reprinted by permission.

Credit: Peter Dalin

Valkeakoski, Finland, July 28, 2001 Credit:Tom Eklund

Noctilucent clouds photographed on 28 July 2001 at 61.12 N and 23.59 E. Southern Finland on the shore of the Lake Vanajavesi and between the towns of Valkeakoski and Toijala. Credit: Tom Eklund

July 24-25, 1996 Credit: John Lambert

1989 July 21/22 22:48 Type IVb: whirls A simple curve consisting of one or more bands with angular radius of curvature of 3 to 5 degrees. Bands, IIb, showing a diverging perspective. Credit: Pekka Parviainen.

Credit: Pekka Parviainen.

A noctilucent cloud photograph taken at Utah State University, 41.7 degrees north latitude, by AIM Co-Investigator Dr. Michael Taylor. 22-23 June, 1999.

Credit: NASA/Donald Petit.

Credit: Tom Eklund

1988 Jul 8/9 22:45 Type I: veil At the upper border, diffuse bands and billows are present. Upper left: diffuse bands, IIa. Upper centre: diffuse billows, IIIb. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1988 Jul 27/28 21:48 Man-made noctilucent cloud, no classification possible. The trail of a rocket created an artificial cloud showing irridescence. On this occasion, the height was estimated to be 80-90 km. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1978 Jul 16/17 21:25 Complex Type O A form which does not fit any of the types I to IV. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1987 Jun 23/24 22:45 Type Vb: whirls Veil, I, is present too. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1985 Jul 25/26 22:30 Type IIb: bands An example of brightness 3 noctilucent cloud. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1993 Jul 8/9 22:05 Fish-eye lens photograph. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1993 Jul 8/9 22:05 Fish-eye lens photograph, taken with a polarizing filter. Compare the with no.15, to see how the use of a linear polarizer (with the axis parallel to the northern horizon) can enhance the contrast of the cloud field. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1982 Jul 24/25 22:00 (approximate time only) Type II: bands Sharp bands (IIa) and diffuse bands (IIb). There is an area of complex structure, type S, in the centre of the cloud field. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1982 Jul 18/19 22:00 (approximate time only) Type IIa: bands Streaks with blurred edges. Some billows, IIIa, in low center and left. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1983 Jul 31/Aug 1 21:47 Type IIb: bands Streaks with sharply defined edges; diffuse billows, IIIb, are crossing the principal streaks. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1989 Jul 21/22 23:15 Type IIIa: billows Short, straight and narrow streaksm with bands, IIb, at the side of the bands. In the upper left corner there is a complex struxture, type O. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1988 Aug 8/9 00:27 Type IIIb: billows Wave-like structure with undulations. Diffuse bands, IIa, and knots, type S, in the centre. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1988 Aug 8/9 00:18 Type IVa: Whirls Small angular radius of 0.1 to 0.5 degrees; a bright whirl in a highly structured display. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.

1988 Aug 8/9 00:43 Type IVc: whirls LArge scale ring structure with tropospheric clouds (altostratus) in the foreground. Upper left: diffuse billows, IIIb. Upper right: complex structure, type O. Credit: Pekka Parviainen. This photograph is from the book "Observing Noctilucent Clouds" by M. Gadsden and P. Parviainen, edited by IAGA in 1995.