Noctilucent clouds (NLCs) are a rare atmospheric phenomenon, appearing as delicate, wispy structures high in the sky. The name is derived from Latin, meaning “night shining,” which describes these luminous formations that glow with a brilliant silvery-blue or electric-blue color against the twilight sky. Unlike common weather clouds, these veils are only visible when the Sun is below the horizon, illuminating them from below. They are a captivating sight for sky-watchers in the summer months at higher latitudes.
The Location and Conditions for Formation
Noctilucent clouds form in the mesosphere, the third layer of Earth’s atmosphere, at an altitude of approximately 76 to 85 kilometers. This makes them the highest clouds on our planet, residing far above the troposphere where nearly all other cloud types exist. Due to their extreme height, they can catch the Sun’s light even when the Sun has set below the observer’s horizon.
The formation of NLCs requires a specific and extreme set of conditions, most notably incredibly frigid temperatures. The mesosphere is the coldest region of Earth’s atmosphere, with temperatures plummeting below about -130°C during the summer. Paradoxically, the mesosphere is coldest in the summer due to seasonal atmospheric circulation patterns that cause air to rise and cool adiabatically. This combination of low atmospheric pressure and deep cold creates the unique environment necessary for ice crystals to condense.
The Primary Components of Noctilucent Clouds
The composition of noctilucent clouds is simple: they are primarily made of minute crystals of water ice. The formation process is intricate, requiring both water vapor and solid particles to act as a foundation for the ice. These ice crystals are minuscule, measuring up to about 100 nanometers in diameter.
To form a cloud, the sparse water vapor present at this altitude must condense onto something solid, a process called nucleation. The required nucleation sites are thought to be “meteor smoke,” which are tiny dust particles left behind as meteoroids burn up upon entering the upper atmosphere. As Earth constantly sweeps up this cosmic material, it leaves a layer of dust that floats at the altitude where NLCs form.
Scientists have confirmed this extraterrestrial link, with satellite data indicating that roughly 3% of each ice crystal is meteoritic. The concentration of these airborne particles, combined with the extreme cold, allows the water molecules to freeze and grow into the ice crystals that scatter sunlight to create the cloud’s distinctive glow.
Why Noctilucent Clouds are Changing
Observations over the past few decades show that noctilucent clouds are becoming more frequent, brighter, and are being sighted at lower latitudes than ever before. This change is considered a long-term indicator of shifts in the upper atmosphere. The primary scientific explanation for this trend involves an increase in the amount of water vapor reaching the mesosphere.
The driver of this increased water vapor is the rise in methane concentrations in the lower atmosphere. Methane, a potent greenhouse gas, makes its way into the upper atmosphere. Once there, it undergoes a series of chemical reactions, primarily oxidation, which produces additional water vapor.
This mechanism results in more available moisture at the altitude where NLCs form. The increased water vapor concentration in the mesosphere is estimated to have risen by 40% since the late 1800s, effectively more than doubling the amount of ice that can form. This excess moisture allows a greater number of ice crystals to form, making the clouds more visible, more widespread, and more persistent.