Noctilucent Cloud Page

Updated 1st June 2011

During those short Summer Nights when the sky never really gets dark Noctilucent Clouds offer Astronomers something different to observe. While they are a bit closer to Earth than our normal objects ( with the exceptions of Meteors) surprisingly little is known about these clouds that form in the upper reaches of the atmosphere. Part of the reason that so little is known is that they occur at a height of typically 70 - 90 kms. This is a very difficult region to reach it is too high for aircraft and balloons and too low for spacecraft to operate. As a result the few measurements have been of a transient nature as spacecraft pass through this height on the way to orbit. These suffer from the problem of contamination by the rocket exhaust as one (in)-famous experiment on the Space Shuttle found.

These clouds are only visible for a few weeks each side of mid-summers day. To observe them you need to wait till the Sun is at least 6 degrees below the horizon so that the sky is dark enough and so that lower Cirrus clouds are not illuminated by the Sun.

One of the brighter displays on the 14th July 2006 is shown in the images on this page. They were photographed from the grounds of The Royal Russell School after a meeting of The Croydon Astronomical Society. At the beginning of the meeting I had showed some pictures showing what I initially thought were Noctilucent Clouds earlier that week - I realised my mistake when I was imaging them and saw an Aircraft creating another one - they were just high aircraft condensation (con) trails. Until you have actually seen a NLC it is an easy mistake to make but the NLC when I saw it was much lighter blue and has a luminous look against the Sky Background. This display was widely see from the Southern UK and lasted about an hour. I was very lucky to have a digital camera with me - I did not have a tripod and had to use a convenient fence post to try to steady the camera. Motto - carry a camera and miniature tripod !

A search on Google will find a number of number of sites on observing NLC's. A couple of the ones that I recommend are The Noctilucent Cloud Observers Home Page in addition to information on observing and recording image NLCs there is a page that shows the latest sightings. This page also shows images of the NLCs. However you should be aware that images appear on this page of cloud formations that do not appear to be NLCs so you need to be careful if using these for confirmation of your sighting.

The International Association of Geomagnetism and Aeronomy NLC web site has a PDF copy of a guide to observing NLCs. This contains a lot of interesting and useful information. Unfortunately the link on the page that should get you to the images does not work but the images in the PDF file are of reasonable quality.

Some more of my images of NLCs taken on the 14th July 2006 can be found here. Examination of the images will show some evidence of camera shake - hence my recommendation to carry a tripod. At the time the first image was taken at 22:15 BST the Sun was about 8 degrees below the horizon from Croydon. The last was taken at about 22:30 BST when the Sun was about 9.5 degrees below the horizon. By the time I had driven home at around 22:50 when the Sun was just over 11 degrees below the horizon there was very little visible.

There are some good movies of NLC on the website of Jacek Stegman of MISU . These are taken with a standard digital camera fixed to a North facing window. A computer is used to take the time lapse images and store the result. Beware the file marked recent is the latest movie uploaded and may change contents between visits. You need to look at the file size and date to confirm if it has changed. They play better if you download them.

One useful tip from the PDF guide above is that the light from NLCs tends to be quite strongly polarised. They recommend using a polarizing filter to aid visual confirmation - it is not clear if the use of a polarising filter will help imaging - I am waiting for more NLCs to find out.

It is easy to confuse high cirrus - either natural or from aircraft con trails with NLC. I have created a page with some images of False NLC here to aid you spotting the real thing..

To observe NLCs the sun has to be between 6 and 16 degrees below the horizon. At less than 6 degrees the sky is too bright and cirrus clouds as well as the NLC will be illuminated. When the Sun gets below 16 degrees the Sunlight does not illuminate the NLCs. It should be noted that as the Sun gets lower the point that the NLCs are illuminated moves towards the horizon in the direction of the Sun.

To aid observations I have computed a table for the latitude of 51 degrees 30 min & 0 deg 0 min W (roughly Croydon, England, UK) showing how far the Sun is below the horizon. The numbers in the table are shaded in three colours - black for when the Sun is too high or low, Dark Blue for when the Sun is between -6 & -12 degrees - this offers the best chance of observing the clouds. Between 12 & 16 degrees the numbers are in pale blue, the NLC will be much lower in the Sky and you need a clear NW to NE horizon to see them. The table is available in:

If you live or are observing away from London the US Naval Observatory have a calculator here that will show you the Sun's altitude but unfortunately only calculates the altitude to -12 degrees.
I have revised my calculation spreadsheet so you can now enter your location and time zone and the table will automatically update. The latest version (V2) now updates the year automatically. Please note that to get the calculations to work you need to enable the macros in this spreadsheet - otherwise the cells will be filled with a #NAME? error. The only note of caution is that the calculation allows for atmospheric refraction as do the calculations above. I need to investigate what values are used for refraction when the Sun is below the Horizon ( it's about 0.8 degrees at the Horizon. This leads to the interesting question 'Do the quoted values for the amount the Sun has to be below the Horizon allow for refraction ?' If you know the answer please contact me.
An additional spreadsheet (NLC-Now) is available here that shows the Sun's altitude and azimuth in real time ( derived from your PC Clock). You need to enable the macros for the spreadsheet to work. The accuracy is dependant on your PC Clock. To use the spreadsheet you need to enter your observing location as well as the time zone and daylight saving time. The colour of the displayed Sun's altitude changes when the Sun is at the best height for NLC to be observed.

Note: Updated 26th May 2010

The time when the Sun is between 6 and 12 degrees below the horizon is known as Nautical Twilight and can be calculated using some astronomy planetarium programmes. When the Sun is between 12 and 18 degrees below the horizon this is Astronomical Twilight - at the end of Astronomical Twilight the Sun is 2 degrees lower than the figure quoted for the end of visibility of Noctilucent Clouds. As a result we can use the times for Nautical Twilight for the times for the best opportunity but have to be aware that the NLC are not likely to be visible near the end of Astronomical Twilight.

Two calculators for the times of Twilight can be found here from spectralcalc.com (use the Solar Calculator Option) and from the US Naval Observatory here select Nautical or Astronomical twilight from the 'Type of Table'.

An article on the visibility of Noctilucent Clouds can be downloaded from ADS here and a more general article here and the BAA Harold Jeffries Lecture on NLC

Some Tips:

Carry a camera at all times. Modern digital cameras are quite capable of recording images that need exposures of a few seconds.
Know how to use the camera in low light - to set the focus to infinity and turn the flash off - often the 'Landscape' mode is a good start. Also you might need to increase the ISO rating if this is not automatic.
Practice on aircraft con trails after sunset - there are plenty to image !
The NLC will be in the direction of the Sun i.e. North West to North East
They may be visible in gaps in lower (dark) cirrus cloud)
Carry a small tripod - it is impossible to hand hold the camera.
Carry a polarising filter to aid identification
If possible have a polarising filter that attaches to your camera - difficult with most point and shoot cameras
The sky needs to be clear for in the direction of the NLC - even over the horizon - a bank of thick cloud will stop the Sunlight as it skims the Earths surface. This condition may be achieved by a 'High Pressure' area to the North of your position but beware that Summer Highs can easily become polluted and not very transparent. Keep looking at the weather charts and Satellite Images. More information of sources of Weather Information can be found on the Croydon Astronomical Society Weather Page.
Look out every clear night and morning at the times shown in Dark Blue in the spreadsheet
Beware that bright moonlight can illuminate high cirrus clouds even when the Sun is well below the horizon - these clouds will not be as blue as the true NLC.
Low powered binoculars with a wide field of view and a large exit pupil may be useful
Beware that moonlight can illuminate high normal clouds and aircraft con trails so they look a bit like Noctilucent Clouds. Aircraft con trails are much lower than NLCs.
Keep a record of your observations for Morning and Evening for every day during the season. It is important to know if there were clouds, if you were not able to observe, if it was clear and there was no NLC or it was clear and you observed NLC

AIM Mission:

On the 25th April 2007 NASA launched the AIM ( Aeronomy of Ice in the Mesosphere) mission specifically to study Noctilucent Clouds, they have published some images from space but 2007 was a poor season at least from the UK. There are two sites that contain information from the mission - the NASA summary site here and a more detailed page from the mission managers at Hampton University (USA) located here.

Details of the CIPS imager on the AIM spacecraft are here look also at the highlights and the data page to see what the NLCs you spotted look like from space !

Weather Charts:

One of the things that need investigating is what were the meteorological conditions when NLC was sighted. One useful thing to look at is the Surface Pressure Analysis or Synoptic Chart. An archive of these is available here. Enter the date of the chart you require in the box in the bottom left, click the 'los!' button and you will get the 00:00 UTC chart. You can then select the other charts for that day by using the arrows on the bottom right. Just be careful with the date of the 00:00 UTC chart - this is the first for the new day not the last of the previous day.

More Information :

Search the web for 'Noctilucent Clouds' and their alternative scientific name 'Polar Mesospheric Clouds'. Else have a look at this Wikipedia entry and the associated links.

Recording your results :

I use a simple spreadsheet to record the results for each morning and evening during the Season from May till August. The template has been modified so it records the Sun's altitude at the start & end of each observing session.This also has a link to a plot of Mesosphere Temperatures and Water Vapour Levels. A copy of the latest 2011 Excel Template can be downloaded here. You need to enter your name, observing location and latitude and longitude. Use the F9 (recalculate) key if you want to update the time and altitude calculation.

The spreadsheet is intended to show when NLC was NOT observed as well as the rare occasions it was. The entry 'NO' for no observation means just that - for one reason or another you were not able to make an observation. If it was cloudy use C1 to C8 with your judgment of how many 8ths of the sky was covered. I usually just record the NW to NE sector as that is the only place that NLC will be seen. If you were observing and no NLC were seen put an X in the NLC column, this applies if it is clear or partially cloudy - if you think you would have seen them if they existed put an X in the column.. If you observed NLC put 'NLC' in that column. Times are in UTC - the clock at the top is just to remind you what the UTC time is if you are on BST. Observers in other time zones will have to edit the formula for the appropriate offset. You need to create a new spreadsheet for each month and set the top left date cell to the first of the month & correct year - the rest should then fill in automatically.