Noctilucent clouds (NLCs) are a captivating atmospheric phenomenon that appear as glowing, electric blue or silvery-white streaks against the dark twilight sky. The name, derived from Latin, translates to “night-shining.” These clouds exist at the very edge of Earth’s atmosphere and are only visible under a precise set of geographical and solar conditions. Their frequency and range have been changing in recent years, making them a sensitive indicator of conditions in the upper atmosphere.
The Highest Clouds on Earth
These clouds hold the record for the highest altitude of any cloud type on our planet. They form in the mesosphere, the third major layer of the atmosphere, situated far above the stratosphere. NLCs are found at an extreme altitude, typically between 76 and 85 kilometers (47 to 53 miles) above the Earth’s surface. This height is nearly ten times higher than the highest thunderclouds. The air at this level is incredibly thin and dry. NLCs are composed of tiny ice crystals, often less than 100 nanometers in diameter. These particles form because the upper mesosphere experiences the lowest natural temperatures in the entire atmosphere, dropping to around -140 degrees Celsius (-220 degrees Fahrenheit).
Geographical Range and Optimal Viewing Times
NLCs are a polar phenomenon, primarily observed from high-latitude regions, generally between 50 and 70 degrees north or south of the equator. This latitude band includes:
- Northern Europe
- Canada and parts of Russia
- The southern tip of South America and Antarctica
These clouds are strictly seasonal, appearing only in the summer months when the mesosphere at the poles is coldest due to a unique circulation pattern. For the Northern Hemisphere, the viewing season runs from late May through August, peaking around the summer solstice. The Southern Hemisphere season spans from late November to mid-February. NLCs are only visible during astronomical twilight, which occurs after sunset or before sunrise when the Sun is between 6 and 16 degrees below the horizon. Due to their great height, they catch the sunlight that has already set for ground-level clouds, causing them to glow against the dark sky.
Why Noctilucent Clouds Form
The formation of NLCs depends on the convergence of three atmospheric ingredients: extremely cold temperatures, a small amount of water vapor, and microscopic particles to act as condensation nuclei. The necessary frigid conditions are created by summertime air currents that rise over the poles, causing the air to expand and cool adiabatically. The ice crystals form when the scarce water vapor present in the dry mesosphere freezes onto fine particles. A significant source of condensation nuclei is meteoritic dust, or “meteor smoke,” which consists of tiny debris left behind as meteors burn up upon entering the atmosphere. The increasing frequency and appearance of NLCs at lower latitudes have prompted scientific study into their connection with atmospheric changes. Rising atmospheric methane levels contribute to their formation, as methane travels to the upper atmosphere where it reacts chemically to produce additional water vapor, feeding the ice cloud formation process.
Observing and Documenting the Phenomena
For sky-watchers in high-latitude regions, viewing NLCs requires looking toward the pole—the northern horizon in the Northern Hemisphere—after the sun has set or before it rises. The clouds appear low in the sky, typically within about 20 degrees of the horizon, and can sometimes be mistaken for high-altitude cirrus clouds. They are distinguished by their shimmering, electric blue or silvery coloring, often displaying delicate, wavy, or fibrous patterns. No special equipment is needed to view NLCs, as they are often bright enough to be seen with the naked eye. Documenting the time, location, and appearance of these clouds contributes to citizen science, providing researchers with data to track how these highest clouds are changing over time.