NOCTILUCENT CLOUDS (NLCs)

WHEN: LATE MAY-EARLY AUGUST

Night darkness is in short supply during the middle of the year, as sunset is late in the day and sunrise early. In addition, the Sun’s position below the northern horizon in the middle of the night means much of the UK never sees proper darkness at all. Although this is a challenging time for stargazing, there is another phenomenon that can only be seen at this time of year.

Noctilucent clouds (NLCs) are ice-sheet clouds forming at an altitude of around 80km (50 miles) in a thin region within the mesosphere. The mesosphere is a defined layer of Earth’s atmosphere, extending from an altitude of 50-120km (30-75 miles). During the northern hemisphere’s summer, the temperature of the mesosphere drops and any water vapour present becomes supercooled. If small particles pass through the supercooled vapour, tiny ice crystals form, creating NLCs. If you’re wondering what the astronomical connection is here, the natural supply of seeding particles is the dust left after a meteor vaporises in the atmosphere.

If present, NLCs can normally be seen low above the northwest horizon between 90 and 120 minutes after sunset, or low above the northeast horizon before dawn. Extensive displays may appear low in the northwest, track through north and end low in the northeast.

Despite the Sun having set for us on the ground, from the altitude of the NLC sheet the Sun is still up. Consequently, the sheet reflects sunlight and, from our perspective at least, appears to shine at night. This is where they get their name: noctilucent means ‘night shining’.

NLCs can be very beautiful, glowing against the deep twilight sky with a vibrant electric blue colour and often showing fine, net-like structures.