These beautiful clouds are seen from Mid-Latitude
locations (50-65 degrees) in the twilight arch of long
summer nights, from about late May to mid August, with a
peak around the first week of July. They are about
80-85km above the Earth's surface and as such the highest
clouds ever seen, some ten times higher than cirrus, the
highest tropospheric cloud. They are still in sunlight
long after sunset. They are often a bluish colour, or
intense white, sometimes golden when low near the
horizon, and they shine in the sky whereas the lower
tropospheric clouds show up dark against them.
The clouds are extremely thin and tenuous, made of tiny
ice crystals which brilliantly reflect sunlight. When a
sheet of NLC appears edge-on to the observer it appears
bright but if NLC is overhead it is usually faint and
difficult to distinguish from cirrus or cirrocumulus
weakly illuminated by the twilight glow or the moon.
The fact that these clouds appear at all is something of
a mystery. Evidently water vapour has convected from the
lower atmosphere. The air pressure at the height of the
Mesopause, about 80 km, is only one ten-thousandth of the
pressure at sea level, and the temperature is about
-130C.
Data is required on when and where the noctilucent clouds
appear. They are seen perhaps 3 or 4 times a year from
any one locality but this is mainly because in our
Western European climate most places are oftener cloudy
than clear. So the more observers there are looking out
for NLC, the less likely it will be that occurrences will
be missed. It is important, too, to keep a record or
negative nights if possible, i.e. clear nights during
which, over a complete period of darkness, or
near-darkness, no NLC was visible.
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Observations -For convenience, printed observing sheets are available for download as PDF file.
NLC,
unlike the aurora, change fairly slowly during a night
and observations should if possible, be made at 15 minute
intervals, i.e. on the hour, quarter past etc. They can
then be compared with those or other observers. Drawings
are very useful, and more especially, photographs. For a
200 ASA film 8 seconds at £ 2.8 should give good
results. Keep a note of times and exposures on the back
of the form.
Your report should include -
Place of observation with latitude and
longitude to the nearest half-degree.
The date. Always use the
"double-date" for a single night. Thus 'June
13/14 means the night of the 13th and the early morning
of the l4th.
The time. If you use local time, state
how it relates to UT, or use UT. Always state the time
system you are using e.g. BST.
If you use the forms, write in column 2 :
NLC if there is definitely
noctilucent cloud present,
0 if noctilucent cloud is definitely
absent, if clear,
X if no decision is possible because of
mist, cloud etc.
AZIMUTHS - if NLC is present try to
measure the left-hand and right-hand azimuths of the
total limits of the display, but azimuths of the
individual bits of it are not necessary. If you use a
prismatic compass correct the magnetic bearings to true
bearings. Use the 0 - 360 convention, i.e. 000 is north,
090 east, 180 south, 270 west.
Elevation - measure, if you can, the
elevation of the upper border and, if there is one, that
of the lower border. There are various kinds of simple
alidade you can make for this.
Brightness - If the NLC is faint or
visible only with binoculars write "1", if
intensely bright and vivid call it "3",
otherwise write "2".
Structure - I,II,III,IV or any
combination of the four ( see illustrations below).
Observing Conditions - it helps to
describe these very briefly, e.g. cloud cover, haze,
bright moon.
Aurora - if you see an aurora please
describe it in detail on the back of the form because the
simutaneous occurrence of the two upper atmospheree
phenomena is unusual and is of great theoretical
interest.
Blank report forms are available:
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VEIL - a simple structureless sheet, sometimes as a background to other forms. |
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BANDS - lines or streaks sometimes parallel or crossing at small angles, not particularly organised. |
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WAVES - sometimes called BILLOWS. Fine herring-bone structure like the sand ripples on a beach at low tide. Very characteristic of NLC. |
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WHIRLS largescale looped or twisted structures. |
The Parallactic Photography programme
There is a current programme of noctilucent cloud
photography to measure its exact height. This requires a
camera fixed to a permanent bracket and exposures to be
made exactly (to 5 seconds) on 0, 15, 30 and 45 minutes
past each hour. The precise orientation of the camera has
to be determined by photographing a star-field, for
example in April or September, when the sky is
sufficiently dark to give a large number of stars. The
films are projected in a standard manner and pairs of
pictures from different observers matched to give
triangulation of the clouds.
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