Certain cloud formations can often result in some of the most awe-inspiring and spectacular displays created by nature. We look at one such meteorological phenomenon known as Noctilucent Clouds.

These cloud formations are not nearly as common as more familiar cloud types and can mostly be viewed at high latitudes during the summer months (during May-August in the Northern Hemisphere and during November-February in the Southern Hemisphere).

They are, however, of no less importance or any less spectacular. In fact, noctilucent clouds are often used by meteorologists to study wind movement in the upper atmosphere.

In this article, we take a closer look at the definition of noctilucent clouds, how they form, as well as the characteristics that define them.

What Are Noctilucent Clouds?

In order to understand how and why noctilucent clouds form and why they exhibit the characteristics that define them, one needs a more expansive and detailed definition of what precisely this cloud formation is:

The name "noctilucent" is derived from Latin words "nocto" and "lucent," which translates to "night shining" (A direct reference to the radiant characteristics of this cloud formation.)

Noctilucent clouds are also more commonly known as night shining clouds or polar mesospheric clouds. Their structure closely resembles the streaky or flaky characteristics of Cirrus clouds (another high-level cloud type).

They occur at an altitude of 76 - 85 km (47 - 53 miles) in the mesosphere, where it is too cold for water to remain in its liquid state, and ice crystals form on small dust particles, which are believed to be remains of meteors. (Meteors burn up in the mesosphere.)

The clouds are too faint to be visible during the day and only become visible after sunset when the sky is mostly covered in darkness. This allows ice crystals in the high-level noctilucent clouds that are still exposed to the sunlight to be illuminated.

Noctilucent clouds are typically observed at high latitudes (approximately 50 to 70 degrees north or south) since this is where temperatures in the mesosphere drop to below -120° Celsius (-184° Fahrenheit), which is a necessary condition for the clouds to form. 

An observer located at a high latitude on the planet's surface can view the illuminated phenomenon as cirrus-like cloud formations with a blue or silver hue.

(The blue color is a result of the absorption of ozone in the atmosphere.)

As the definition already eluded to, this cloud type is a fairly rare meteorological occurrence. The reason for this is the restrictive conditions that need to be in place for them to form and also be observed in the first place. We discuss them in the next section.

How Noctilucent Clouds Form

Noctilucent clouds are rare meteorological phenomena for a reason. The strict conditions necessary for their development only occur at certain latitudes & during specific times of the year. Noctilucent cloud formation occurs as follows, starting with the necessary conditions:

There are also other processes involved in the formation of noctilucent clouds, but this section highlighted the key steps involved in the development of this rare meteorological phenomenon. 

Facts About Noctilucent Clouds

Much of the information provided in this section has already been covered elsewhere in this article. However, the following list highlights the key facts that define and characterize noctilucent clouds:

This list does not cover every possible fact there is know about noctilucent clouds but highlights the most important ones.

Conclusion

Noctilucent clouds are not just some of the rarest meteorological phenomena on the planet, but their location in the mesosphere also makes them hard to study by meteorologists. (The location of the mesosphere makes it the hardest atmospheric layer to explore.)

This does not make them any less spectacular or stop scientists from studying them. After all, they have their own dedicated satellite observing them. NASA's AIM (Aeronomy Of Ice In The Mesosphere) satellite observes noctilucent cloud characteristics and behavior.

This article highlighted and defined what exactly noctilucent clouds are, how they develop, and the characteristics that define them.

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Until next time, keep your eye on the weather!

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The Earth's atmosphere consists of five distinct layers, each with its own unique features. We examine what precisely the outermost layer, the exosphere, is and what its defining characteristics are.

When viewed from the surface or low Earth orbit, our atmosphere seems to be one continuous layer of air. However, it consists of five different layers: The troposphere, stratosphere, mesosphere, thermosphere, and exosphere.

This article focus on the exosphere, the outermost layer of the atmosphere. It not only examines what the exosphere is but also looks at the characteristics or facts that define it.

Exosphere Definition

It was already briefly described during the introduction, but in order to examine the exosphere in more detail, a more comprehensive definition is required.

The name "exosphere" is derived from Ancient Greek, with "ἔξω éxō" meaning "outside," and "σφαῖρα sphaĩra" meaning "sphere." Directly translated "outside sphere" perfectly describes the exosphere since it is the Earth's outermost layer in a spherical shape.

The exosphere is situated just above the thermosphere, with a thin layer of air called the thermopause separating the two layers. It reaches up for thousands of miles and seamlessly merges with the vacuum of space, where all Earth's atmospheric characteristics disappear. 

It does not have a fixed lower boundary but starts at an altitude of approximately 500 - 1 000 km(311 - 620 miles), extending up to a height of roughly 10 000 km (6 200 miles). It also has no clear upper boundary.

The lower boundary is also called the exobase. The exobase is also known as the "critical altitude." Above this height, temperatures remain relatively stable, and barometric conditions are no longer relevant.

Compared to the four layers below it, the exosphere contains little to almost no traces of the characteristics that define the Earth's atmosphere. In fact, the "air" so thin that it resembles interplanetary space more than it does the rest of Earth's atmosphere.

(Gas molecules can travel for hundreds of miles without ever coming into contact or colliding with another particle.)

What sets the exosphere really apart, though, is the fact that small particles in the layer do not react the same as in other atmospheric layers where gas molecules mix and collide with each other. 

The few molecules present in the layer basically never collide. Instead, they enter the exosphere and typically follow a ballistic trajectory, which after reaching their apex (maximum altitude), curve back down and reenter the lower atmosphere.

(Some small particles travel too fast, though, and manage to break free from the Earth's gravitational pull and escape into space. As a result, smaller particles are continuously escaping the planet's atmosphere.)  

The small number of gas molecules present in the exosphere is mostly light gases such as hydrogen and helium. Heavier gas molecules like carbon dioxide and oxygen can be found closer to the exobase.

Temperatures can easily exceed 2 000° Celsius (3 632° Fahrenheit) in the exosphere, but there are no particles to transfer the heat, so one will never experience it. In fact, if exposed, one will experience the typically freezing cold temperatures of outer space in the exosphere.

As a result of its vastly different characteristics compared to the other atmospheric layers, it should come as no surprise that some scientists don't regard the exosphere as part of the atmosphere at all and see the thermosphere as Earth's uppermost layer.

Due to its altitude and composition, the exosphere is home to a large number of navigational, communication, and weather satellites. Its most famous inhabitant, though, is the Hubble Space Telescope.

Earth is not the only planet in our solar system that has an exosphere. Jupiter, Mars, and Saturn are just two examples of other planets also having exospheres.

(Even smaller satellites that do not have atmospheres containing a mix of gases, like the moon and three of Jupiter's moons, have what is called a "surface boundary exosphere.")

Facts About The Exosphere

The following list highlights the characteristics and facts of the exosphere in more detail.

This is by no means a comprehensive and exhaustive list that contains all the data available about the exosphere but highlights the key facts and characteristics that define this layer.

Conclusion

As this article highlighted, the exosphere still contains small of the gases present in the other four atmospheric layers, but these quickly disappear as altitude increase and the last traces of the atmosphere transition into the vacuum of space.

It is not surprising, then, that many scientists argue that the exosphere cannot be considered part of Earth's atmosphere and that the thermosphere constitutes the atmosphere's upper boundary.

It is technically still part of the atmosphere, though, and contains a small percentage of the gases found closer to the planet's surface. And, yes, it still has an important role to play, as just illustrated. 

If you are interested in the complete structure and make-up of the atmosphere, this article covers all five atmospheric layers and their relation to each other in more detail.

Never miss out again when another interesting and helpful article is released and stay updated, while also receiving helpful tips & information by simply  clicking on this link .

Until next time, keep your eye on the weather!

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