Noctilucent clouds

Explanation

Noctilucent clouds (NLC) are silvery-white thin clouds that can be seen in some summer nights towards the northern horizon. Unlike other cloud types, which reach a maximum height of 13 km, noctilucent clouds appear at an altitude of 83 km. They can only be seen when the sun is between 6° and 16° below the horizon. Then, the noctilucent clouds are still illuminated by the sun, while the rest of the sky is already dark. For the formation of noctilucent clouds, the temperature of the mesopause must be very low. This low temperature occurs between mid-May and mid-August. Especially in June and July, noctilucent clouds can be observed on some nights. Here, they reach a height of about 20° above the northwestern to northeastern horizon. In exceptional cases, they can even be seen near the zenith in our latitudes.

NLC Warning List

NLC Forum

Origin

It is quite obvious that Noctilucent Clouds consist of water ice. To form ice at an altitude of 83 km with the very low water vapor concentrations, very low temperatures below 140 K are required. Furthermore, either dust particles are needed as condensation nuclei, or so-called water cluster ions are formed due to the dipole nature of water molecules.

It is known from measurements that the temperatures necessary for ice formation are only reached between June and August due to inter-hemispheric circulation. Larger winds also occur in the summer, transporting ice particles over greater distances. The lifespan of individual ice particles is likely in the order of a few hours, until they, for example, sublimate again through sinking and southward displacement. This also points to the possible rapid changes in the NLC.

Long-term influences are much harder to track. A connection with solar activity seems likely, as changes in UV radiation influence chemical reactions and the temperature also undergoes systematic changes. However, it is not clearly proven that the frequency of Noctilucent Clouds really increases during the period of solar maximum.

The increase in methane and CO2 could also be responsible for higher NLC activity, as this could cause the temperature in the mesopause to drop low enough more frequently to allow Noctilucent Clouds to form.

Another connection is suspected between NLC and Polar Mesospheric Clouds (PMC), which linger over the poles throughout the summer. Assuming the PMC's shift southward, the NLC might be "frayed" ends of the PMC layer.

Long-term trends can only be derived from observation series spanning several years or even decades. Therefore, a call to all observers to participate in this program.

Observation

When NLCs are observed in Germany, not only should the observation itself be recorded, but all observations should be posted in the AKM Forum.

Location & Direction

The observation location should be noted as well as the azimuth of the entire phenomenon:

North: 0°
East: 90°
South: 180°
West: 270°

Height

The height of the top part of the NLC should be recorded. For precise height determination, the free program Stellarium is helpful. Here, one can display the current night sky for a specific time and location and then determine the height of the NLC using the stars.

Brightness

Brightness on a five-level scale:

very faint, NLC barely visible
NLC clearly identifiable, but low brightness
NLC clearly visible, standing out against the twilight sky
NLC very bright, attracting the attention of casual observers
Extremely bright, noticeably illuminating objects

Additional Helpful Information

Forms in four basic types with subgroups, as well as classes for complex structures:

Type I: Veil - structureless "curtain", sometimes a background for more complex types
Type II: Bands - bands/stripes that are parallel or only slightly tilted against each other

IIa: Bands with diffuse blurred edges
IIb: Bands with sharp edges

Type III: Waves - herringbone patterns like sand ripples in shallow water
- IIIa: short, straight, narrow "strokes"
- IIIb: wave structures with multiple waves
Type IV: Vortices - arcs or intertwined structures

IVa: Vortices with small radius (0.1°...0.5°)
IVb: simple arcs with a radius of 3°...5°
IVc: large-scale vortices

Complex Structures