part 3 - Stunning Noctilucent cloud display!
A couple of
weeks back I posted images of a bright noctilucent cloud display over
Machynlleth on the evening of June 17th. Since then I've been
researching the topic and there have been a good number of displays,
mostly part to completely obscured by lower clouds. The early morning
of July 20th was a fantastic exception and with starry skies overhead I
dragged myself from bed to check for any reports on the internet. Good
news in that bright displays were being reported to the east, in Europe
- that normally bodes well as their dawn is of course some time before
ours. Soon, reports came in from England so it was time to have a look.
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Grabbing the kit, I set off on foot towards the edge of town - the plan
was to get clear of street-lights and their sodium-glow and to gain a
bit of height. The rougher-ground parts of Machynlleth golf-course
would suit both purposes. En-route I stopped to take in the amazing
A bright display stood
above the low bank of cumulus. Light levels were very low and the
cloud-motion made for a blurry image - plus the overhead cables were
Slightly further on. It
was about 0330 and the display was just getting brighter and brighter
as dawn approached!
The cloud-blur was easing now that faster exposure was possible. I then
climbed to get a better view:
A bit shakey this one: I included it for the classic "herringbone"
structure top L!
I then found a decent fencepost to rest the camera on and get some
"proper" non-shakey shots, of which this was my favourite, with a star
visible top L (on the hi-res version there are several more):
....while this one, taken with a wideangle lens, shows how extensive
the display was - pretty much from the horizon to overhead!
dawn approached, the display began to fade from the east:
As I wandered home, in serious need of
coffee by now, the last vestiges of the display continued to glimmer
aloft, before cloud moved in from the west.
I've written twice before about these mysterious "Clouds on the Edge of
Space", but haven't even tried to explain what they are and how they
are thought to form. So I thought I'd do a bit of research myself and
provide an account. There is a lot of ongoing technical research into
these clouds at the moment - if you Google "AIM satellite mission" you
will find reading material aplenty. This is just an introduction to the
subject, but here goes:
We'll start with a cross-section through Earth's atmosphere.
The bit we live in is the Troposphere. About 80% of all the molecules
that make up the atmosphere are contained within this bit, variably
8-17km deep (it's thinner at the poles, thicker at the Equator). In its
upper part there is a lesser density in terms of molecules - including
oxygen - and the air is said to be "rarefied" - that's why people get
out of breath on high mountains of over 4000m. What we collectively
refer to as "weather" all goes on in the Troposphere and thunderstorms
extend right up to its top - the Tropopause. Here, there exists an
abrupt boundary - up to the Tropopause it gets colder with altitude,
but above it gets warmer again. That's why convection stops at the
Tropopause and the anvil-shaped tops of thunderstorms spread out
beneath it. Warm air cannot convect up into warmer air - it's against
the laws of Thermodynamics!!
Above the Tropopause we enter the Stratosphere, a thick layer of
rarefied air, with about 20% of the molecules that make up the
atmosphere contained within. No weather as such goes on here, but
importantly it contains a layer enriched in ozone, which blocks a lot
(>>90%) of harmful UV light from
the Sun: without that ozone, life as we know it would not exist as it
At the top of the Stratosphere, the Stratopause marks the boundary with
the very cold, extremely rarefied Mesosphere. This is where most
meteors burn up. Containing just 0.1% of the molecules in the
atmosphere, it is a hostile place in which temperatures may be as low
as -100C near the top - the Mesopause. The latter zone, only a few
kilometres thick, marks the next boundary into the Thermosphere above,
which is a deep (several hundred km), virtually airless layer, where
satellites such as the International Space Station orbit the planet.
interesting feature is that Mesopause is cooler during the summer than
winter, the opposite of what one might expect. This is due to a
summer-to-winter circulation, in the form of gravity-waves, that
results in upwelling of rarefied air at the
summer pole (i.e. the North Pole in our summer) and downwelling at the
winter pole. The rising air expands and
cools, obeying as it does the laws of physics, and the result is the
colder Mesopause during the summer.
It is the mesopause that concerns us, for that is where, 80km up,
Noctilucent (or more correctly Polar Mesospheric) clouds occur.
WHAT ARE NOCTILUCENT CLOUDS MADE OF?
Clouds are made either of water droplets or ice particles, depending on
the temperature of the air in which they exist. In the freezing heights
of the Mesopause, ice is the key substance. It nucleates on any handy
"cloud-seeds", just as happens at lower levels. These seeds are almost
certainly microscopic meteoritic dust particles, which are common at
HOW DOES WATER-VAPOUR GET SO HIGH??
This is a tricky one. Normally, water vapour rises towards the
Tropopause, freezes and falls back towards the surface as rain or hail.
Only a very small amount gets through into the Stratosphere and even
less to greater heights.
One potential solution is that water is made in the Mesosphere by the
reaction of free oxygen and hydrogen atoms. It is thought that, while
water vapour condenses and falls at the Tropopause, a trace-gas,
methane, does not. It continues to rise until above the Ozone Layer,
where bombardment by UV radiation splits it into its constituents -
carbon and hydrogen. The hydrogen, being super-light, floats on up
towards the Mesopause, where it reacts with any oxygen it meets, to
form water. The diagram below sketches this out.
Another hypothesis holds that rocket exhausts supply some water vapour
to the Mesopause. This may account for an element of the water vapour
present, but it fails to explain why noctilucent clouds have been seen
since the 1880s.
Super-explosive volcanoes like Krakatoa in 1883 and large impacts like
Tunguska in 1908 have also been cited as having an influence. Some
scientists point out that noctilucent clouds were first described after
the 1883 eruption, and that after Tunguska there were some very bright
displays. However, noctilucent clouds have in recent years been seen
more often, been seen further and further south from their usual Polar
habitats and are getting brighter. Why?
AIM SATELLITE MISSION - PRESS-RELEASE FROM NASA, APRIL 11 2007
NASA AIMS TO CLEAR UP
MYSTERY OF ELUSIVE CLOUDS AT EDGE OF SPACE
WASHINGTON - NASA is
preparing to launch the Aeronomy of Ice in the Mesosphere (AIM)
spacecraft, the first mission dedicated to exploration of mysterious
ice clouds that dot the edge of space in Earth’s polar regions. These
clouds have grown brighter and more prevalent in recent years and some
scientists suggest that changes in these clouds may be the result of
AIM will conduct the
first detailed probe of this unusual phenomenon typically observed
approximately 50 miles above the Earth’s surface in the mesosphere. The
mesosphere is the region just above the stratosphere. Researchers know
very little about how these polar mesospheric clouds form, why they are
being seen at lower latitudes than ever before or why they have
recently grown brighter and more frequent. "These clouds are indicators
of conditions in the upper reaches of the Earth’s atmosphere, and are
an important link in the chain of processes that result in the
deposition of solar energy into Earth’s atmosphere," said Mary Mellott,
AIM program scientist, NASA Headquarters, Washington. "AIM will provide
an understanding of how and why these clouds form, an important
contribution toward the NASA goals of understanding the fundamental
physical processes of our space environment and how the habitability of
planets is affected by the interaction of planetary magnetic fields and
atmospheres with solar variability.
Bristling with instruments, AIM has been up there collecting data since
late April 2007. For lots more information, try going to the Mission
Or try this fact-packed site:
Are noctilucent clouds at a maximum this
year due to the fact that we are at a Solar Minimum?
Are they getting more extensive due to climate change?
How exactly do they form - which hypotheses are correct?
What can we expect to see in the years to come?
It will be
interesting to find out what the data being collected up there are
going to tell us!
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