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