Noctilucent Clouds

The recent sightings of noctilucent clouds from parts of Scotland are a beautiful natural phenomenon that can be both scientifically intriguing and visually stunning.

What Are Noctilucent Clouds?

• Appearance:
  Noctilucent clouds are thin, wispy clouds that glow with a blue or silvery hue at night. The glow is caused by sunlight illuminating the clouds from below the horizon, even when the rest of the surface is in darkness. This makes them appear to "shine" brightly in the night sky.

• Location in the Atmosphere:
  Unlike most clouds that form in the troposphere, noctilucent clouds are found in the mesosphere, the third layer of Earth's atmosphere. This makes them the highest clouds in the Earth's atmosphere, typically occurring at altitudes between 76 km to 85 km (47 to 53 miles).

• Formation:
  These clouds are primarily made of ice crystals. During the summer months, the mesosphere cools enough for ice to form on dust particles suspended in the atmosphere. The dust particles may come from micrometeorites falling from space, remnants of volcanic eruptions, or even human activity such as rocket exhaust.

• Unique Characteristics:
  The name noctilucent comes from the Latin words nocto (night) and lucent (shining), reflecting how they appear to "shine" at night. They are often referred to as polar mesospheric clouds because they are typically visible at latitudes between 45 and 80 degrees north or south of the equator.

When and Where Do Noctilucent Clouds Appear?

• Seasonal Timing:
  Noctilucent clouds are primarily a summer phenomenon. In the Northern Hemisphere, they can be seen from the end of May to early August, with sightings being most frequent in June and July. In the Southern Hemisphere, sightings are rarer, but they can occur from November to February, peaking in December and January.

• Why Summer?
  The mesosphere cools during summer, allowing the ice crystals to form on suspended particles. The long days and extended twilight at these latitudes make it possible for sunlight to reach these high-altitude clouds at night, causing them to glow. The further north you go, the longer the period of twilight, and thus the longer the potential for noctilucent clouds to be visible.

Formation Process

Noctilucent clouds are primarily composed of ice crystals.

1. Cooler Temperatures in the Mesosphere:
   During summer, the mesosphere experiences cooler temperatures, allowing water vapor to freeze into ice crystals.

2. Suspended Dust Particles:
   These ice crystals form on dust particles, which can originate from:

   • Micrometeorites: Tiny fragments of space debris entering Earth's atmosphere.

   • Volcanic Eruptions: Ash and particles from volcanoes can travel into the upper atmosphere and become part of the clouds.

   • Human Activities: Rocket launches can also introduce particles into the mesosphere, which may seed the formation of noctilucent clouds.

3. Reflection of Sunlight:
   The ice crystals reflect sunlight, especially when the Sun is just below the horizon. This causes the clouds to glow with a bluish-silver color during the night.

Noctilucent clouds are valuable for climate and atmospheric research. Their formation is sensitive to temperature and atmospheric composition, so studying them can help scientists understand changes in the mesosphere and climate patterns. They also serve as indicators of changes in the upper atmosphere.