Noctilucent Cloud

Noctilucent clouds or night-shining clouds are the highest clouds found in the Earth’s atmosphere located in the mesosphere at an altitude around 76 to 85 km (249,000 to 279,000 ft). The cloud consists of ice crystals and is only visible during astronomical twilight. In Latin, the term Noctilucent means “night-shining”. The clouds are most visible during the summer month and when the Sun is below the horizon from latitude between ±50° and ±70°.

The clouds are too faint to be observed during daylight and are visible only when illuminated by sunlight from below the horizon while lower levels of the atmosphere are in Earth’s shadow.

Noctilucent Meaning

The term Noctilucent means “night-shining” and is an indicator as to what makes them special.  Most clouds are dark at night, but night-shining clouds appear to glow a brilliant white or electric blue. Nlc clouds can reflect the light of the sun as shown in the figure given below as they are the highest clouds in the Earth’s atmosphere. The Nlc clouds are located in the mesosphere at an altitude of around 76 to 85 km (249,000 to 279,000 ft).

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Noctilucent Clouds Definition

Noctilucent clouds, also known as polar mesospheric clouds, are very thin clouds of water ice crystals located in the Mesosphere at a height of about 75 km to 90 km.

Noctilucent Cloud Discovery 

Noctilucent or night-shining clouds were first observed in 1885, two years after the 1883 eruption of Krakatoa in the straits of Java. 

It is not confirmed whether their appearance had anything to do with the volcanic eruptions or whether their occurrence was due to more people observing the incredibly vibrant sunset caused by volcanic debris into the upper atmosphere by eruptions and contributing to their formation.  

Subsequent studies have shown that noctilucent clouds are not solely caused by volcanic activity, although dust and water vapour could be inserted in the Earth’s atmosphere by a volcanic eruption and contributed to their formation.

The assumption was made by the scientist at that time that clouds were another phenomenon of volcanic ash, but the cloud persisted after the ash liquidated out of the atmosphere.

In 1972, Nlc clouds were first observed from space by an instrument on the OGO - satellite. 

Noctilucent Cloud Formation

The night-shining clouds are made up of small crystals of water ice up to 10 mm in diameter and exist at a height of about 76 to 85 km, higher than any other cloud in the Earth’s atmosphere. Similar to the other clouds. Clouds in the Earth’s lower atmosphere form when water collects on particles, but mesospheric clouds or clouds in the mesosphere may come directly from the water vapour as well as forming on dust particles. 

Nlc clouds required water vapour dust, and very cold temperature to form. The source of both dust and water vapour in Earth’s atmosphere is not known with certainty. The dust was expected to come from micrometres, although particles from volcanoes and dust from the troposphere are also possibilities. Scientists believe that moisture could be lifted through the gas in the tropopause, as well as forming the reaction of methane with hydroxyl radicals in the stratosphere. Noctilucent clouds form mostly in the polar regions because the mesosphere is the coldest in that region. 

Noctilucent Cloud Observation

Nlc clouds are mostly found colourless or pale blue, but occasionally other colours such as red and green are also observed. The attributes of blue colour come from absorption by ozone in the path of sunlight brightening the Nlc clouds. The Nlc clouds appear as featureless bands, but frequently show different patterns such as streaks,  waves, and wavelike undulations.

The Noctilucent occurs during summer from mid-May to mid-August in the northern hemisphere and between mid-November to mid-February in the southern hemisphere. The clouds are very faint and tenuous and may be observed only in twilight around sunrise and sunset when the clouds of the lower atmosphere are in the shadow, but the Nlc clouds are brightened by the Sun.  The clouds are best observed when the Sun is between 6° and 16° below the horizon.

Although night-shining clouds are found in both the hemisphere, they have been observed thousands of times during the northern hemisphere and fewer than 100 times during the southern hemisphere. 

Also, Nlc clouds are made up of small crystals which are not noticeable to the viewer on the ground because they cannot disperse enough light. The size of the crystals may be 30 mm or less.

Noctilucent Cloud  Forms

The noctilucent cloud comes in different forms and patterns. As per the identification scheme introduced by Fogle in 1970, the clouds are classified in five different terms. This classification is further modified and subdivided. In view of the recent research, the World Meteorological Organization now observed four major forms that can be subdivided. The four major forms are:

• Type 1 veils are very fragile and lack definite structure, similar to cirrostratus or inadequately defined cirrus.  

• Type 2 bands are long streaks that rarely appear in roughly parallel groups, usually more widely spaced than the bands or elements seen with cirrocumulus clouds. 

• Type 3 billows are arranged in a closed-space, roughly parallel short streaks that most often look like cirrus. 

• Type 4 whorls are more partial, or more often complete rings of cloud with dark centres. 

Did You Know?

• NLC clouds are the highest cloud in the sky, growing at the edge of space 50 miles above the Earth's surface.

• Nlc clouds exist in the atmosphere mesosphere layer.

• Nlc clouds are the newest clouds that we all have observed. Scientists first observed the Nlc clouds in 1885 after the same famous explosion of the Krakatoa volcano.

• Nlc clouds occur during summer from mid-May to mid-August in the northern hemisphere and between mid-November and mid - February in the southern hemisphere.