Certain cloud formations produce some of the most awe-inspiring and spectacular displays in nature. One such meteorological phenomenon is 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, no less important or 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, and the characteristics that define them.

To understand how and why noctilucent clouds form and exhibit their defining characteristics, it is important to first look at a more detailed definition of this cloud formation:

Noctilucent Cloud Definition

Noctilucent clouds are high-level clouds that form after sunset (during astronomical twilight) at an altitude of approximately 76 – 85 km (47 – 53 miles) in the mesosphere.

Also known as night shining or polar mesospheric clouds, they occur when sunlight illuminates and reflects off ice crystals in the upper atmosphere, resulting in their characteristic blue or silver color.

They can mostly be observed at high latitudes during the summer months (May-August in the Northern Hemisphere and November-February in the Southern Hemisphere).

The name “noctilucent” is derived from the 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 believed to be meteor debris (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 dark. This allows ice crystals in the high-level noctilucent clouds that are still exposed to 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 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 primarily the result of sunlight scattering off the tiny ice crystals in the upper atmosphere.)

As the definition already alluded 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.

1. 1. Temperatures in the mesosphere need to be below -120° Celsius (-184° Fahrenheit) for noctilucent clouds to form.
   2. This means the clouds can only form during the summer (when solar radiation forces warm air to rise higher into the atmosphere and cool further, resulting in colder air in the mesosphere).
   3. For the same reason, an observer must also be located at a high latitude (approximately 50 to 70 degrees north or south), where temperatures in the mesosphere drop low enough.
   4. From an observer’s perspective, the sun also needs to be below the horizon to allow a large portion of the sky to be covered in darkness. (Noctilucent clouds appear very faint and are impossible to view in daylight.) 
   5. If enough water vapor and dust particles are present in the air, noctilucent clouds form at subzero temperatures.
   6. Although the observer is already in darkness, sunlight from below the horizon still reaches the high-level noctilucent clouds situated at an altitude of approximately 80 km (50 miles).
   7. As a result, the sunlight illuminates the ice crystals in the clouds, and the light is reflected back to the observer at the surface.
   8. The observer at the surface views the noctilucent clouds as cirrus-like clouds with a blue or silver hue. 

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

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:

1. 1. Noctilucent clouds are high-level clouds that form during astronomical twilight (after sunset).
   2. They occur in the mesosphere at an altitude of approximately 76–85 km (47–53 miles).
   3. They have an illuminated blue or silver color as a result of sunlight reflecting off ice crystals.
   4. They consist of ice crystals that form on small dust particles believed to be remnants of meteors (which typically burn up in the mesosphere).
   5. The clouds only occur during the summer when temperatures in the mesosphere drop below -120° Celsius (-184° Fahrenheit), which is a requirement for the formation of noctilucent clouds.
   6. Noctilucent clouds are typically observed at high latitudes (approximately 50 to 70 degrees north and south).
   7. They are the highest clouds that occur in the Earth’s atmosphere.
   8. In 2014, water vapor from the rocket boosters of a SpaceX Falcon 9 rocket resulted in the formation of noctilucent clouds over Florida, USA.
   9. The air in the mesosphere, where noctilucent clouds occur, is one hundred million times drier than the air found in the Sahara Desert.
   10. Noctilucent clients were first observed in 1885.
   11. Their blue color is primarily the result of sunlight scattering off the tiny ice crystals in the upper atmosphere.
   12. Although they occur in both hemispheres, noctilucent clouds have been observed thousands of times in the Northern Hemisphere compared to less than 100 in the Southern Hemisphere.
   13. Noctilucent clouds are also known as night shining clouds or polar mesospheric clouds.

This list does not cover every possible fact that is known about noctilucent clouds, but highlights the most important ones.

Noctilucent clouds have a unique appearance, but knowing when and where to look is just as important when trying to identify them.

These clouds typically appear as thin, wispy, and ripple-like formations (often resembling cirrus clouds). Unlike ordinary high-level clouds, noctilucent clouds have a striking blue or silver glow that makes them stand out against the darker sky.

They are only visible during twilight, though, which means they can be observed shortly after sunset or just before sunrise. During this time, the sky near the surface is already in darkness, while sunlight from below the horizon still illuminates the high-altitude ice crystals in the mesosphere.

For the best chance of observing these cloud formations, an observer should be located at high latitudes during the summer months. Looking toward the horizon in the sun’s direction, under clear sky conditions, will significantly increase the chances of spotting noctilucent clouds.

Noctilucent clouds are not just some of the rarest meteorological phenomena on the planet, but their location in the mesosphere also makes them difficult for meteorologists to study. (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 explained what noctilucent clouds are, how they develop, and the characteristics that distinguish them.

Until next time, keep your eye on the weather!