Updated 12th June 2017
During those short Summer Nights when the sky never really gets dark Noctilucent Clouds (NLC) 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 as 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 . Earlier 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 taken by 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. Unfortunately the movies are no longer available from his web site but one is available on UTube here.
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 use the spreadsheet described below. At present it is set for London UK but you can easily change this to your location. The number in the table shows how far the sun is below the horizon.. The numbers in the table are shown 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 spreadsheet is available on the link below:
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
Observations in the southern hemisphere summer are more difficult as there is little inhabited land far enough South to view the NLC.
It is very difficult to predict NLC but the Sky to the North of your location needs to be clear not only so you can see the clouds but so the Sun can shine onto the clouds. The normal weather satellite images help but there is not much information on how far north the sky has to be clear to allow the Sun to shine on the NLC due to the amount of refraction in the atmosphere.
In addition there has to be sufficient moisture in the Mesosphere and the temperature has to fall low enough ( The Mesosphere temperature falls in the Summer). A plot of the current Mesosphere temperatures can be found on this link ( Note this page does not refresh itself so you may need to reload the page to ensure you have the latest data). There is no exact threshold when NLC form but it provides some clue.
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 !
The AIM CIPS 'Daily Daisy' showing the location of NLC is shown on the left side of the spaceweather.com homepage. This shows the Southern Hemisphere during winter and is changed to the Northern Hemisphere during May.
May 2017 Announcement from AIM:
Spaceweather.com carried the following announcement from AIM on 16th May 2017
"Currently, we are not retrieving cloud data. The orbit of the spacecraft has precessed, and we are transitioning to a new way of pointing. We need a bit more time to settle the spacecraft pointing and hope to be back online by the end of May."
Hopefully this will get sorted soon so we can get a better idea of when NLC are far enough south to observe. Until then it is back to the Mark One eyeball !
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.
How far away are NLC ? added 12th June 2017
To observe the NLC the path between the observer and the NLC needs to be clear of other low level clouds. The table below shows the calculated distance to NLC for different observation angles, assuming a height of 80km for the NLC. Since the path is diagonal through the atmosphere the line of sight can pass above distant low level clouds. The distances calculated probably under estimate the distances at low elevations as the calculation does not include refraction. At high elevations the height of the cloud at 80 km / 50 miles causes a significant error in the actual distance as measured on the surface.
|Observer elevation assumed to be 0|
|NLC Elevation (degrees)||Distance (km)||Distance (miles)|
|0 °||1,013 km||629 miles|
|1 °||908 km||564 miles|
|2 °||814 km||506 miles|
|3 °||733 km||455 miles|
|4 °||662 km||411 miles|
|5 °||600 km||373 miles|
|6 °||546 km||339 miles|
|7 °||500 km||310 miles|
|8 °||459 km||285 miles|
|9 °||424 km||264 miles|
|10 °||394 km||245 miles|
|11 °||367 km||228 miles|
|12 °||343 km||213 miles|
|13 °||322 km||200 miles|
|14 °||303 km||188 miles|
|15 °||286 km||178 miles|
|16 °||271 km||168 miles|
|17 °||258 km||160 miles|
|18 °||245 km||152 miles|
|19 °||234 km||145 miles|
|20 °||224 km||139 miles|
|21 °||215 km||133 miles|
|22 °||206 km||128 miles|
|23 °||198 km||123 miles|
|24 °||191 km||119 miles|
|25 °||184 km||114 miles|
|26 °||178 km||111 miles|
|27 °||172 km||107 miles|
|28 °||167 km||104 miles|
|29 °||162 km||101 miles|
|30 °||157 km||98 miles|
|31 °||153 km||95 miles|
|32 °||149 km||92 miles|
|33 °||145 km||90 miles|
|34 °||141 km||88 miles|
|35 °||138 km||86 miles|
|36 °||135 km||84 miles|
|37 °||132 km||82 miles|
|38 °||129 km||80 miles|
|39 °||126 km||78 miles|
|40 °||123 km||77 miles|
|41 °||121 km||75 miles|
|42 °||119 km||74 miles|
|43 °||116 km||72 miles|
|44 °||114 km||71 miles|
|45 °||112 km||70 miles|
|46 °||111 km||69 miles|
|47 °||109 km||68 miles|
|48 °||107 km||67 miles|
|49 °||106 km||66 miles|
|50 °||104 km||65 miles|
Calculations were done using the triangle solution shown at https://en.wikipedia.org/wiki/Solution_of_triangles#Two_sides_and_non-included_angle_given_.28SSA.29. The two sides of the triangle are given by the Earth's radius and the Earth's radius plus the height of the NLC. The non-included angle is 90 degrees plus the elevation angle of the NLC.
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 version updated in 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.
Remote Control on Canon 10D + series cameras
To capture images of NLC it is desirable to leave your camera imaging for a couple of hours post Sunset and another couple of hours pre dawn. I have tried various software to control my camera but none is really satisfactory. The ideal is to have intervals of a minute or so and also to allow for mirror lockup which requires a double press of the shutter button ( once to lock the mirror up) and a second followed after a short interval to take the image.
One piece of software that will do this is IRIS but as with all the software for the earlier Canon 10D to 40 D cameras it needs an interface and special connection to the remote shutter connection to work. This presentation (pdf) shows the circuit I have used and also how it is constructed. This can be used for astro imaging as well.
Having constructed the interface and run some trials with IRIS I have identified a problem in that IRIS only allows you to set the interval from the end of one shot to the start of the next. As a result if you are using auto exposure as in the settings below the gaps between Frames increases. Perfectly acceptable for astronomical imaging but in my case I want to convert the still images into a movie. The Increase in intervals would result in the star motion and any NLC speeding up as the exposure time increases. It is possible to use IRIS but you need to ensure the sum of the mirror lockup time, the exposure time and the card write time adds up to the frame interval you desire.
As a result of this I have gone back to my original plan which is to use a Raspberry Pi to control the Camera via a PiFace interface. Originally the RaspberryPi would freeze when it lost the network connection when it was turned off at night. However updating the RaspberryPi using 'sudo apt-get update' followed by 'sudo apt-get upgrade' appears to have solved this problem. I have left my Pi running for several hours without any more lockup problems.
The camera is controlled via a very simple Python Programme. The only thing to be careful of is that the Raspberry Pi has a network connection when it starts up. If not the Pi's time will be set to the time it was last shut down. This does not cause problems at present but a future development is to start and stop the image sequence at specified times. If you don't have a network connection it is possible to set the time using a GPS module, this needs some editing of the Raspberry Pi configuration software. It is probably best to search on the web for the latest version of how to do this.
Details of the intervalometer I have constructed are on my intervalometer page.
Since the sky brightness will change a lot during the hours post / pre twilight the camera exposure will change. I use the camera auto exposure system to adjust for this which it seems to perform quite well. The camera settings I use are: