SCIENCE

The season for rare and mysterious noctilucent clouds is here. These beautiful, rare and mysterious clouds only appear in summer, and could be warning us about climate change. (Guardian)

Learn more about noctilucent clouds (NLCs) and other oddities of the atmosphere with our encyclopedic entry.

Teachers, scroll down for a quick list of key resources, including today’s MapMaker Interactive map.

Discussion Ideas

• Read through the Guardian article, or this short article from the good folks at the Australian Antarctic Division. What are noctilucent clouds?
  • They’re just what their name sounds like—nocturnal (night-time) and luminescent (glowing). These rare, wispy clouds appear just after sunset and usually seem to glow pale blue.

• Aren’t noctilucent clouds just like auroras?
  • No! Noctilucent clouds are just a special type of cloud, made of water vapor in the atmosphere. Auroras are not clouds at all. They are high-energy particles from the solar wind or cosmic rays that interact with the very top of Earth’s atmosphere.

• How do noctilucent clouds form?
  • Noctilucent clouds form from the same basic ingredients as all other clouds, from low-atmosphere stratus to high-atmosphere cirrus: water vapor, cloud condensation nuclei (CCN), and just the right temperature.
    • 1. water vapor. Noctilucent clouds form in very, very thin and very, very dry air—according to NASA, “one hundred million times dryer than air from the Sahara desert.”
    • 2. CCN: CCN are microscopic bits of dust, salt, or even pollution around which water vapor condenses. For high-flying noctilucent clouds, CCN may be exhaust from spacecraft exiting the atmosphere, dust from meteors entering the atmosphere, or volcanic ash from powerful volcanoes!
    • 3. temperature. The “magic ingredient” for noctilucent clouds is temperature. The atmosphere needs to be very, very cold for noctilucent clouds to form—below -123° Celsius (-189° Fahrenheit).

• If noctilucent clouds form just like other clouds, what makes them so special?
  • Three major characteristics make NLCs pretty unusual.
    • 1. Noctilucent clouds form in the mesosphere, higher in the atmosphere than any other type of cloud. Read more about noctilucent clouds and the mesosphere here.
    • 2. Noctilucent clouds are not actually made of wet water vapor. They’re made of ice. The ice crystals that form noctilucent clouds are tiny—between .1 and 100 microns in diameter, “about the size of particles in cigarette smoke.”
    • 3. Noctilucent clouds seem to sparkle. This is a result of optics, and the position of the sun, the observer, and the cloud.

      

• Why are noctilucent clouds only visible in the summer months?
  • Summer (May-August in the Northern Hemisphere, and November-February in the Southern Hemisphere) increases two key conditions necessary for the formation of NLCs.
    • temperature: As lower levels of the atmosphere grow warmer, the mesosphere gets cooler.
    • water vapor: Water vapor is more abundant in the mesosphere in the summer. “Upwelling winds in the summertime carry water vapor from the moist lower atmosphere toward the mesosphere,” says Gary Thomas, a professor who studies NLCs.

• How do scientists think noctilucent clouds might reflect climate change and global warming?
  • There is a correlation between noctilucent clouds and industrial development. Noctilucent clouds were first observed in 1885, the height of the Industrial Revolution and after the giant eruption of the Krakatoa volcano spewed millions of tons of dust and gas into the atmosphere. Sightings have increased in frequency since then.
  • As lower levels of the atmosphere grow warmer, the mesosphere gets cooler, creating the right conditions for noctilucent clouds to form. Earlier and earlier sightings of noctilucent clouds may indicate a warmer lower atmosphere.
  • Industrial farming has been linked to increased methane (a greenhouse gas) in the atmosphere, which increases the amount of water vapor in the mesosphere. Water vapor helps create the right conditions for noctilucent clouds to form. More frequent sightings of noctilucent clouds may indicate a more methane-infused lower atmosphere.

TEACHERS’ TOOLKIT

The Guardian: Watch the skies: the season for rare and mysterious noctilucent clouds is here

Nat Geo: What is an atmosphere?

NASA: AIM: Aeronomy of Ice in the Mesosphere mission

Nat Geo: Noctilucent Cloud Season map (warning: You may have to zoom  to the Southern Hemisphere on the second bookmark!)

NASA Science News: Strange Clouds

Atmospheric Optics: About NLCs, Polar Mesospheric Clouds

Australian Antarctic Division: Noctilucent Clouds