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As you can tell from the banner image of my website, I love noctilucent clouds (NLC). I may not be as obsessed about them to the same degree as recorder extraordinaire Mike Noble, or filmmaker Adrien Maudit, but NLCs stand out as one of my key skywatching passions.
I have created some observational tools on nlcplanner.com as a stopgap measure until I can develop something more sophisticated.
Having a career in operational meteorology, and being frustrated by the unpredictability of NLCs from evening to morning, or morning to evening, I have wanted to put some time researching and more particularly into developing numerical model output to improve the situation. As of 2018, most global operational weather models reached a height of 65 km or so, well below the mesopause at 81 km [I have not done an exhaustive survey, so this may not be entirely accurate]. With the availability of inexpensive computational resources (e.g. Alberta's Cybera Rapid Cloud Access) I figured now is a good as time as any to extend the modelling upward.
One must crawl before one can walk before taking a run, so until we have reasonably reliable NLC forecasts, here are some tools available at nlcplanner.com:
- Timetable: when is sunset? Local midnight? The jump? Solar depression of 4 degrees? Solar depression of 16 degrees?
The rapid change of "when should I start scanning the evening sky?" has always caught me off-guard, thanks to the busy-ness of daily life and intervals of poor weather. So instead of relying on standard outputs of civil and nautical twilights, or websites that force me to interpolate, here is a table of values for my location (click the table to go to the site):
The screening height is the depth of the troposphere that casts a shadow up onto the NLC layer, rendering them invisible. A typical value is 10 km, but when there are more particulates the value is lower. The key effect is to reduce the altitude at which NLC are still lit by sunlight. Max height is when only the solid Earth is used to cast a shadow, while the practical max altitude includes the obscuring effect of the lower atmosphere.
- Tropospheric and NLC cloud motion schematics (click to go to the site). Often, moonlit jet contrails masquerade as wisps of NLC. Sometimes, cirrus wraps around a blocking high pressure zone, arriving from the east or northeast, confusing the proper identification of NLC. It would be helpful to know what direction altoculumlus and cirrus are expected to be moving in.
It's also a problem of 3 dimensions collapsed to 2 on the sky and different angle of viewing. For example, a flow almost directly from the north appears to move up the sky when you look at azimuth 0, but appears to be moving horizontally when you look at azimuth 90 or 270.
Caveat emptor! These schematics show an artificial NLC motion from the east at 15 m/s. A few years ago, when I had full access to the Canadian Meteorological Centre's Global model, I used output at 66 km, but today I am retired and as a member of the public can only get the 1 mb level which is insufficient. Until I get my own model running, this is not a horrible estimate.
- Annual visibility graphs (click to go to the site)
These annual graphs are intended to be used for general interest and inquiry. They depict the typical northern hemisphere dates when NLC may be active, along with the location's sunset, 4 degree solar depression, and 16 degree solar depression times. These values correspond better to NLC visibility than do the nautical and astronomical twilights.
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