Summer months are traditionally considered the least favorable period for astronomical observations, and it’s hard to argue with that. The short nights leave very little time for any stargazing endeavors. However, the summer sky itself can offer a fascinating and stunning phenomenon: noctilucent clouds.
What are Noctilucent Clouds?
Noctilucent clouds are luminescent clouds visible in the northern part of the sky. They appear about 1-2 hours after sunset/before sunrise and capture attention with their “phantom-like” structure (stars can be seen through them) and distinctive silvery color, from which they derive their name.
How Noctilucent Clouds Form
They consist of water ice crystals that condense on dust particles. In the absence of such condensation nuclei, the appearance of silver clouds is impossible. Presumably, its source is micrometeorites that are slowed down in the atmosphere before they warm up to the evaporation temperature. Approximately 100 tons of meteoric material fall to the Earth’s surface during the day, which is enough to produce the required number of condensation nuclei. In condensation processes, nuclei differ in size as follows: Aitken nuclei (0.005–0.1 μm), heavy (0.1–1 μm) and superheavy (1–20 μm). The main role in the formation of tropospheric clouds is played by the last two types of nuclei.
Silver clouds occur when the temperature in the mesosphere drops to a minimum. For them to form, it must be about -120 °C. This occurs between March and October, but most often between late May and mid-August. The maximum visibility almost always falls on July 5.
Distinguishing Noctilucent Clouds from Polar Auroras
Noctilucent clouds are often confused with polar auroras. It’s not surprising since both phenomena are observed in the northern part of the sky (referring to the northern hemisphere).
If silver clouds are formed as a result of condensation of water vapor on microdust, then auroras are a product of ionization of gas molecules of the Earth’s atmosphere after interaction with charged particles of the solar wind. Silver clouds always have a constant color, while auroras are characterized by a green, red or purple glow that changes rapidly over time.
They also differ in terms of visibility. Polar auroras are challenging to observe in urban settings, whereas noctilucent clouds are easily visible even in large cities, given the opportunity to observe the northern horizon.
How Noctilucent Clouds Differ from Regular Ones
Regular clouds are located at much lower altitudes, typically up to 13 km, and have a denser structure. After sunset, regular clouds quickly fade against the backdrop of stars, while noctilucent clouds, on the contrary, start to glow only an hour or two after the Sun disappears below the horizon.
Period of Noctilucent Clouds Visibility
In the northern hemisphere, noctilucent clouds are observed from May to September, between latitudes of 45° and 77°, in the Southern Hemisphere — from -52 to -80°. During this time, even at midnight, the Sun does not dip too far below the horizon, and its rays illuminate the mesosphere, making noctilucent clouds visible. At higher latitudes, this phenomenon cannot be seen due to the polar day. At lower latitudes, the Sun descends too low, preventing the observation of noctilucent clouds.
Can Noctilucent Clouds be Predicted in Advance?
No, they cannot. While astronomers can make approximate forecasts for the appearance of polar auroras, predicting the occurrence of noctilucent clouds in advance is not possible. The only way is to rely on neighboring regions where the Sun has already set, and local observers can confirm or refute the presence of noctilucent clouds.
The Enigma of Noctilucent Clouds
One ancient mystery is associated with noctilucent clouds. The thing is, the first confirmed reports of their observation date back only to 1885. Considering the visibility of noctilucent clouds, this is very peculiar.
As a result some scientists suggest that noctilucent clouds are a relatively young phenomenon. Perhaps their appearance is related to the processes of global climate change. Some studies indicate a connection between methane emissions and the increase in water vapor concentration in the mesosphere, which contributes to the formation of clouds.
Be that as it may, noctilucent clouds are of significant scientific interest. They occur in a poorly explored region of the Earth’s atmosphere. Aircraft and balloons cannot reach an altitude of 80 km, while satellites cannot descend there. Therefore, scientists are actively studying noctilucent clouds, hoping to better understand the processes that occur in the Earth’s mesosphere. For example, they are used to determine the nature of the movement of air masses. It has been discovered that noctilucent cloud move exceptionally fast. The speed of movement of silver clouds is from 40 to 177 m/s. Further study of silvery clouds will surely bring us many more discoveries.
