In News:

Recently, rare noctilucent clouds were sighted over parts of Scotland, drawing attention due to their unique shimmering appearance in the night sky. These occurrences are significant in the context of climate studies and upper atmospheric science.

What are Noctilucent Clouds?

• Definition: Noctilucent clouds (NLCs), also known as polar mesospheric clouds, are high-altitude ice crystal clouds that appear thin, wispy, and glow with a blue or silvery hue after sunset.
• Etymology: The term “noctilucent” is derived from Latin—"nocto" (night) and "lucent" (shining)—meaning "night shining."

Atmospheric Location

• These are the highest clouds in Earth’s atmosphere, found in the mesosphere, around 76–85 km above the Earth's surface.
• In contrast, most other cloud types form in the troposphere, the lowest atmospheric layer.

Seasonal and Geographical Occurrence

• Seasonality:
  • Northern Hemisphere: Visible from late May to early August, peaking during June and July.
  • Southern Hemisphere: Much rarer; may appear from late November to early February, most commonly in December and January.
• Latitude Range: Typically occur between 45° and 80° latitude, both north and south of the equator.
• Visibility Conditions:
  • Seen only during summer months, shortly after sunset or just before sunrise.
  • The Sun remains just below the horizon, illuminating these high clouds from below, creating a glowing effect while the lower atmosphere is in darkness.

Formation Mechanism

• Composition: Made up of tiny ice crystals.
• Temperature Conditions: The mesosphere becomes extremely cold during summer, enabling the formation of ice on fine particles.
• Sources of Dust Nuclei:
  • Natural: Micrometeorites, volcanic dust.
  • Anthropogenic: Rocket exhaust particles and other upper-atmospheric pollutants.
• Optical Phenomenon: These ice crystals reflect sunlight even when the lower atmosphere is dark, giving them their luminous appearance.

Significance

• Serve as indicators of mesospheric conditions, especially temperature and humidity.
• Their increasing frequency and intensity in recent decades may be linked to climate change and human activities, including space exploration.
• Valuable for understanding upper atmospheric dynamics, particularly in the context of atmospheric chemistry and space weather.