Strange Clouds

Image credit Pekka Parviainen Noctilucent clouds over Finland. The orange hues near the horizon are ordinary sunset colours, notes Gary Thomas. NLCs, on the other hand, are usually "luminous blue-white or sometimes just pale white," he says.

Astronaut Don Pettit is a long-time noctilucent cloud-watcher. As a staff scientist at the Los Alamos National Laboratory between 1984 and 1996, he studied noctilucent clouds seeded by high-flying sounding rockets. "Seeing these kinds of clouds [from space] ... is certainly a joy for us on the ISS," he said on NASA TV.

"Although NLCs look like they're in space," continues Thomas, "they're really inside Earth's atmosphere, in a layer called the mesosphere ranging from 50 to 85 km high." The mesosphere is not only very cold (-125 C), but also very dry - "one hundred million times dryer than air from the Sahara desert." Nevertheless, NLCs are made of water. The clouds consist of tiny ice crystals about the size of particles in cigarette smoke. Sunlight scattered by these crystals gives the clouds their characteristic blue colour.

How ice crystals form in the arid mesosphere is the essential mystery of noctilucent clouds.

Photo credit: Don Pettit and NASA TV. Another noctilucent cloud seen from the ISS. Earth's horizon has been deliberately overexposed to reveal the faint cloud tops."That little diaphanous line you see paralleling Earth's horizon is an NLC," said Pettit.

Ice crystals in clouds need two things to grow: water molecules and something for those molecules to stick to - dust, for example. Water gathering on dust to form droplets or ice crystals is a process called nucleation. It happens all the time in ordinary clouds.