Strange Clouds

Ordinary clouds, which are relatively close to Earth, get their dust from sources like desert wind storms. It's hard to waft wind-blown dust all the way up to the mesosphere, however. "Krakatoa may have seeded the mesosphere with dust in 1883, but that doesn't explain the clouds we see now," notes Thomas. "Perhaps," he speculates, "the source is space itself." Every day Earth sweeps up tons of meteoroids - tiny bits of debris from comets and asteroids. Most are just the right size to seed noctilucent clouds.

The source of water vapour is less controversial. "Upwelling winds in the summertime carry water vapour from the moist lower atmosphere toward the mesosphere," says Thomas. This is why NLCs appear during summer, not winter.

One reason for the recent spread of noctilucent clouds might be global warming. "Extreme cold is required to form ice in a dry environment like the mesosphere," says Thomas. Ironically, global warming helps. While greenhouse gases warm Earth's surface, they actually lower temperatures in the high atmosphere. Thomas notes that noctilucent clouds were first spotted during the Industrial Revolution - a time of rising greenhouse gas production.

Are NLCs a thermometer for climate change? An unusual sign of meteoroids? Or both? "So much about these clouds is speculative," says Thomas.

A NASA spacecraft scheduled for launch in 2006 should provide some answers. The Aeronomy of Ice in the Mesosphere satellite, or AIM for short, will orbit Earth at an altitude of 550 km. Although it's a small satellite, says Thomas, there are many sensors on board. AIM will take wide angle photos of NLCs, measure their temperatures and chemical abundances, monitor dusty aerosols, and count meteoroids raining down on Earth. "For the first time we'll be able to monitor all the crucial factors at once."

Meanwhile, all we can do is wait... and watch. There's never been a better time to see noctilucent clouds. "During the summer months, look west perhaps 30 minutes to an hour after sunset when the Sun has dipped 6^o to 16^o below the horizon," advises Thomas. If you see luminous blue-white tendrils spreading across the sky, you've probably spotted an NLC. Observing sites north of 40^o latitude are favoured.

One more thing: don't forget your camera. According to astronaut Don Pettit, "you can never have too many pictures of noctilucent clouds."