Mystery Clouds at the
Edge of Space
Noctilucent clouds (NLCs)
are unlike common clouds that we see in the troposphere,
such as cumulus, stratus and cirrus clouds. Cirrus clouds,
generally the highest tropospheric clouds, are found
no higher than about 10 kilometers (around 33,000 feet)
above the surface. Occasionally giant thunderstorms
will poke through the tropopause and top out in the
lower stratosphere at around 18 km (about 60,000 feet).
This is not nearly as high as NLCs,
which are found in the mesosphere above 82 km (51 miles).
This is much higher than research balloons can reach
and only about half the minimum altitude for an artificial
satellite. Thus, it is very difficult to study NLCs,
which are suspended at the edge of space.
Noctilucent clouds are actually polar
mesospheric clouds (PMCs). They are called noctilucent
clouds when they are seen by an observer on the ground.
Because they are very thin, NLCs cannot
be seen during daylight. Instead, they can be seen only
during twilight when the surface of the Earth below
is still dark and the NLCs are illuminated by the rays
of the rising or setting sun.
NLCs can be seen from the ground only
at locations between about 50 to 70 degrees from the
Equator and then only within several weeks of the summer
solstice. During summer, sites farther than 70 degrees
from the Equator don't have sufficient twilight to allow
NLCs to be seen. Sites closer than 50 degrees from the
Equator rarely have NLCs.
NLCs pose several scientific mysteries.
Why are NLCs so high? Where does the water originate
that forms their microscopic ice crystals? What provides
the condensation nuclei for the water vapor to condense
on? Are NLCs increasing or decreasing in frequency?
Photographers and sky watchers are
helping professional scientists find answers to these
and other questions about NLCs. Their key contribution
is photographing and documenting NLCs that they observe.
Astronauts also provide important NLCs
observations and photographs. Soon, their unique perspective
on NLCs will be shared by NASA's AIM satellite. AIM
means Aeronomy of Ice in the Mesosphere. The AIM satellite
is scheduled for launch in 2006. The satellite has instruments
that will measure the abundance and movement of PMCs.
Also measured will be aerosols, water vapor and various
gases that might contribute to the formation of the
clouds. An important objective of AIM is to determine
if changes in PMCs are possible indicators of changes
in global climate. Hampton University's Center for Atmospheric
Sciences is the lead science organization for the AIM
mission, and you can find out more by visiting their
AIM web site.
Finland's Pekka Parviainen is among
the best NLC photographers. You can see his NLC photography
and many other images of atmospheric phenomena here.
Parviainen has been photographing the sky since 1973
and has amassed more than 80,000 images.
Observer's of NLCs can submit reports
and images to the Noctilucent
Cloud Observers' Homepage.
Recently I have been working with a
team from Hampton University to prepare an NLC observation
protocol for the GLOBE
program. The objective of the protocol is for students
to learn about and report noctilucent clouds. The protocols
will be usable by sky watchers and photographers of
all ages. If you live between about 50 and 70 degrees
away from the Equator, look for the GLOBE NLC protocol
later this year and get set to start observing clouds
floating at the very edge of space during the period
several weeks either side of the summer solstice.
Forrest M. Mims III 
|
