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into the troposphere, which took two years to be transported to the mesosphere,
eventually becoming seeds for the ice particles that formed these clouds. The
ice particles were large enough to scatter sunlight and be seen by the naked
eye. Excellent scientific and historical references are provided by Gadsden and
Schröder (1989) and Schröder (2001). A less-well-known NLC event followed
the great Tungiska event of 1908 when a comet or meteor struck Siberia. During
the next two nights, the postsunset sky over England was bright enough to
read a newspaper (Whipple, 1930). Recent studies of the aftermath of solstice
launches of the space shuttle have been accompanied by brilliant NLC displays
within a few days (Stevens et al., 2003, 2005). The associated injection of 300
meter tons of water vapor in the height range 100-115 km is clearly a sig-
nificant perturbation but its transport to polar regions was remarkably rapid
and must be related to the huge winds reported by Larsen (2002) described in
Chapter 5. Equally curious is the rapid dispersal of the clouds, implying an
anomalous horizontal diffusion, perhaps due to two-dimensional turbulence
(Kelley et al., 2009). Kelley et al. (2009) connected these results with the Great
Siberian Meteor Event of 1908 (Whipple, 1930) and concluded that the impact-
ing object was a comet.
These clouds are only observed during the summer months and usually present
a wavy pattern, an effect that is attributed to their interaction with passing grav-
ity waves. The ideal zone for viewing the scattered sunlight due to the largest ice
particles is between 53
◦
and 57
◦
latitude because of the long twilight and the
polar location of the clouds themselves. The photographic example presented
in Fig. 7.1 displays some of the characteristics of noctilucent clouds. As will
be seen, the charged component of the polar summer mesosphere allows radar
Figure 7.1
Noctilucent clouds as observed at 2255 UT on July 19, 1997, from
Glengarnork, Ayshire, Scotland. (Photo courtesy of Tom McEwan.)
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