Archive for the ‘Noctilucent clouds’ Category

Another display of NLCs last night.  Not the best or brightest display but here is a picture taken from Mitford, near Morpeth last night.

As an experiment I took a series of images – 30 in all – covering about 6 minutes in realtime and stitched them into a short animation.  Here is the result:

I wanted to get more pictures but (a) it was a work night and (b) a strange animal kept running at me from the darkness and it seemed to be getting braver with each attempt…

I wrote recently that the season of noctilucent clouds had started.  Last night, as twilight faded, I saw some appearing in the northern sky.  It was about 11.30pm – just as I was getting ready to go to bed that the first hints of them appeared.  I took a wide field shot with the camera from the road just outside the garden:

It’s a very subtle display at this point! Another five minutes or so confirmed to me that the sky was getting darker and those clouds appeared to be getting brighter.  Here’s a narrow field shot of some of the brighter parts of that cloud:

In this picture you can see the electric blue of the noctilucent clouds and some characteristic ripples in the structure.  Whether this display got better or subsided – I’ve no idea – really had to get to bed for an early morning start.

There’s some evidence that noctilucent cloud displays are inversely correlated to the sunspot cycle.  But strong displays can also happen during the solar maximum so the relationship is probably more complicated than that.  See here for an accessible discussion of this link.  The frequency of noctilucent cloud appearances is variable and it may be that we are in for a couple of lean years.  I hope we get at least a few more displays this year.

It’s noctilucent cloud season again.  In the northern hemisphere it lasts from roughly June 1st to August 1st each year.

Astronomers and clouds don’t generally get on.  However, during summer in the northern hemisphere some mysterious and beautiful clouds may appear in the sky long after sunset.  Known to astronomers as noctilucent clouds (NLC), these delicate and tenuous clouds are seen shining long after the ordinary clouds of the troposphere have darkened.  NLC are the highest clouds in the atmosphere and form within the mesosphere at heights of 85km (about 52 miles) above the ground (which means you’ll hear scientists referring to them as Polar Mesospheric Clouds).

Here’s a picture of a display seen from Northumberland in 2011.

The dark clouds are ordinary clouds in the tropisphere; they in darkness like the ground beneath them and are silhouetted against the brighter, more distant noctilucent clouds.

NLC formation is restricted to the summer months when conditions in the mesosphere near the poles are sufficiently cool enough (-120°C) to allow ice to form in the low pressure environment.  Conditions aiding the formation of NLCs tend to last from the start of June until the beginning of August.   The clouds can be seen from the ground between latitudes of 50 and 60 degrees north or south of the equator.  At higher latitudes the summer twilight is too bright for the clouds to be visible.  Northumberland (approximately 55°N) is ideally situated for NLC observers.

You can observe noctilucent clouds during the next 6-8 weeks by going out and looking towards the north after about 11pm.  Taking pictures of noctilucent clouds is also easy; for example, ISO200 for 5-10 seconds should pick them up.

NLC form when water vapour condenses onto a dusty ‘seed’ high in the atmosphere.  The precise nature of that seed has been the subject of much discussion.  The earliest reports of NLC came in 1885 just two years after the eruption of Krakatoa, an event which affected the Earth’s atmosphere and weather for a decade or more.  Could the seeds of NLCs be volcanic dust?  The association with Krakatoa is not accepted by all scientists but it is true to say that NLC have been widely observed ever since.  Aside from powerful volcanic eruptions like Krakatoa there are no plausible mechanisms for transporting dust from the lower atmosphere to the mesosphere.  Some scientists have speculated that the seeds of NLCs are the meteoric dust swept up by the Earth as it orbits the Sun.

NLCs are being seen twice as often as they were a few decades ago.  The traditional window of latitude through which the clouds can be seen is also increasing: NLC have recently been sighted from locations just 40° from the equator.  The temperature of the mesosphere is influenced by the presence of carbon dioxide and the humidity is increased by methane.  Both gases are being increasingly produced by humans and so NLCs could be regarded as a visible indication of climate change.  The validity of the connection is still controversial.  The exhaust gases of the Space Shuttle and other rockets have also been observed to contribute to the formation of NLCs.

NASA has a satellite studying NLCs from orbit: it’s called the Aeronomy of Ice in the Mesosphere (AIM) satellite.   The primary goals of AIM are to determine the processes which form NLCs, to measure the sizes of ice crystals in the clouds and to monitor the composition of the mesosphere over a period of at least two years.  AIM may eventually provide evidence to show whether or not these delicate and beautiful clouds are a manifestation of the destructive changes brought about by human production of greenhouse gases.