Noctilucent Clouds Formation: How Night-Shining Clouds Form in the Mesosphere

Noctilucent clouds formation occurs when water vapour freezes around microscopic meteoric smoke particles in the mesosphere. This process needs very cold temperatures, usually near the mesopause during summer. The resulting clouds sit so high that they can still catch sunlight long after sunset.

Key takeaways

• Noctilucent clouds, also called polar mesospheric clouds, form about 76 to 85 kilometres above the Earth.
• They consist of tiny ice crystals that grow around meteoric smoke and other fine particles.
• Their silver-blue colour comes from sunlight reaching the clouds while the lower atmosphere is in shadow.
• The main viewing season is local summer, when the mesopause is at its coldest.
• Changes in upper atmosphere water vapour, linked in part to methane oxidation, can affect how often these clouds appear.

What are noctilucent clouds?

Noctilucent clouds are high-altitude, pale blue or silver clouds that shine during twilight. The name means night-shining clouds, which matches their look. You can see them after sunset or before sunrise, when the Sun is below the horizon but still lights the upper atmosphere. Scientists also call them polar mesospheric clouds, and they are part of cloud types and weather forecasting research.

The highest clouds on Earth

Most weather happens in the troposphere, which reaches to about 15 kilometres above the surface. Noctilucent clouds form far above that, in the mesosphere at around 76 to 85 kilometres. At that height, the air is very thin and very dry, so these clouds stay well away from fronts, pressure systems and rain-bearing cloud near the ground.

Because they form near the edge of space, they do not produce rain. They also sit outside the convection that builds thunderstorms, so they depend on the temperature and moisture conditions near the mesopause.

Night-shining clouds explained

The glow from these clouds comes from basic geometry. They are easiest to see during civil and nautical twilight, when the lower atmosphere has gone dark but sunlight still reaches the mesosphere from below the horizon. The World Meteorological Organization describes this as the Sun being between 6 and 16 degrees below the horizon. The ice crystals scatter that light back towards the ground, creating a bright blue-white edge against the dark sky and one of the more striking cloud optical phenomena Australia and other places can see.

The mechanics of noctilucent clouds formation

The creation of these upper-atmospheric clouds needs three things at once: very cold air, enough water vapour, and tiny particles for ice to form on.

How do noctilucent clouds form in the mesosphere?

Noctilucent clouds formation relies on extreme cold. When temperatures fall low enough, water vapour can freeze in the mesosphere instead of staying as gas. At the mesopause, the air is so thin that ordinary condensation does not work well. The water molecules need a surface to stick to, so they deposit onto fine particles floating in the upper atmosphere. The ice crystals that form are tiny, but they are good at scattering short-wavelength sunlight, which gives the clouds their blue-white colour.

The Role of Meteor Smoke in Cloud Nucleation

Near the surface, water droplets usually form around sea salt, dust or pollution. In the mesosphere, the particles are different. Each day, micrometeoroids burn up as they enter Earth's atmosphere and leave behind meteoric smoke. These tiny particles act as nucleation sites for ice crystal growth high above the surface. Without a steady supply of this material, water vapour has far fewer places to freeze.

Ice crystals and sublimation

The ice crystals in polar mesospheric clouds exist in a narrow balance. If air currents move them into slightly warmer parts of the upper atmosphere, they do not melt into liquid droplets. Instead, the ice changes straight from solid to vapour. That process is called sublimation. The link between temperature, pressure and moisture at this height shows how dew point vs humidity works very differently near the edge of space than it does at ground level.

How Rising Methane Levels are Making NLCs Brighter

Human activity can also affect the upper atmosphere by changing the amount of water vapour available there. Researchers study methane oxidation because it can add water vapour high in the atmosphere, which may help noctilucent clouds form more often or appear brighter. Scientists also examine solar cycle influence and changes in thermal structure of the atmosphere, since both can alter temperatures near the mesopause. The AIM satellite has been used to observe these clouds and track how they change through the noctilucent cloud season.

Southern Hemisphere vs Northern Hemisphere NLC Visibility

Noctilucent clouds are seen most often in the Northern Hemisphere, especially in summer at mid to high latitudes. They are much less common and more difficult to spot in the Southern Hemisphere, mainly because there is less land at the right latitudes and fewer long-running observing records. That difference matters if you're in Australia, since sightings here are rarer and usually brief.

FAQ

What causes noctilucent clouds formation?

It happens when very cold mesosphere air lets water vapour freeze around tiny meteoric smoke particles. The clouds form near the mesopause, where temperatures are low enough for ice crystals to survive.

Why do noctilucent clouds glow after sunset?

They sit high enough that sunlight can still reach them while the ground is already dark. Their ice crystals scatter that light back to observers below.

Are noctilucent clouds the same as polar mesospheric clouds?

Yes. Polar mesospheric clouds is the formal name used for noctilucent clouds.

When is the noctilucent cloud season?

The noctilucent cloud season runs through local summer, when the mesopause is coldest and the clouds are most likely to form and remain visible.

Why are they less common in the Southern Hemisphere?

They are still part of the same upper-atmosphere process, but sightings are less common in the Southern Hemisphere than in the Northern Hemisphere. Observing records are also thinner in the south.

What are the main ingredients for noctilucent clouds?

You need upper atmosphere water vapour, very cold air, and nucleation sites such as meteoric smoke for mesosphere ice crystals to form.

Sources

1. International Cloud Atlas reference (cloudatlas.wmo.int)
2. Upper atmospheric clouds (cloudatlas.wmo.int)
3. Noctilucent clouds – Australian Antarctic Program (antarctica.gov.au)
4. International Cloud Atlas (cloudatlas.wmo.int)
5. First Antarctic ground-satellite view of ice aerosol clouds at the edge of space – Magazine Issue 14: 2008 – Australian Antar… (antarctica.gov.au)
6. World Meteorological Organization cloud atlas reference (cloudatlas.wmo.int)
7. International Cloud Atlas reference (cloudatlas.wmo.int)
8. Nubes noctilucentes (nubes mesosféricas polares) (cloudatlas.wmo.int)

Last verified: 2026-08-04