Noc­tilu­cent cloud sea­son is open

WHEN: From the last week of May, 90-120 min­utes af­ter sun­set or a sim­i­lar time be­fore sun­rise

Sky at Night Magazine - - THE SKY GUIDE -

The end of May is tra­di­tion­ally iden­ti­fied as the start of the North­ern Hemi­sphere’s noc­tilu­cent cloud sea­son. Noc­tilu­cent clouds (NLCs), are high-alti­tude ice clouds which form in a nar­row layer within the meso­sphere, 82km up. At that alti­tude any NLCs present are able to catch the Sun’s rays even when the Sun ap­pears be­low the hori­zon from the ground. As they are thin, their vis­i­bil­ity is del­i­cate and the Sun has to be be­low the hori­zon – typ­i­cally be­tween 6° and 16° – for the sky to be dark enough for them to ap­pear.

As they re­veal their wispy pres­ence by re­flect­ing sun­light, noc­tilu­cent clouds nor­mally make their evening de­but low above the north­west hori­zon be­tween 90-120 min­utes af­ter sun­set. They make a sim­i­lar ap­pear­ance low above the north­east hori­zon a sim­i­lar pe­riod be­fore sun­rise. If there, they ap­pear to glow against the dark­ness of the night sky with any nor­mal, tro­po­spheric clouds ap­pear­ing dark and sil­hou­et­ted against them. They of­ten ap­pear elec­tric blue in colour and ex­hibit del­i­cate net­work-like struc­tures.

Al­though there can be no guar­an­tee that there will be any dis­plays at all, it’s worth get­ting into the habit of check­ing for noc­tilu­cent clouds early on in the sea­son. This way you will be nicely trained for ob­serv­ing through­out the height of the sea­son dur­ing June and July.

Tro­po­spheric clouds are lower in the at­mos­phere than noc­tilu­cent clouds and so ap­pear dark against the eerie glow of the NLCs

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