Unusual clouds decorate evening sky

Anyone driving west toward Sisters last Thursday saw a light show that is a rare sight to witness: magnificent noctilucent clouds shimmering high over the Cascades.

You don't hear that term very often, but it's been around since 1887, when Otto Jesse of Germany first photographed the phenomena (and seems to have been the one to coin the term "noctilucent cloud" which means "night-shining cloud").

It takes very cold, thin air and just the right amount of moisture to get noctilucent clouds (known by atmospheric scientists as NLC) started, and one of the sources for all that moisture is rocket exhaust of high-flying missiles.

NLCs are a relative rarity at our latitude, visible occasionally in May and June. The real activity, however, is observed in July and in early August. During those months our atmosphere experiences temperature minimums low enough (coupled with extremely low pressure) for water vapor to condense into ice clouds at the height of between 47 to 53 miles above the Earth.

The reason these spectacular clouds are so vivid and visible is the altitude where they form, and reflection and refraction of sunlight off and through ice crystals. It was almost completely dark when the accompanying photo was shot from the viewpoint on Highway 20, but 50 miles up it was still daylight, and even at 10:15 p.m., the clouds were still glowing.

This phenomena is one of those "good news/bad news" things. Noctilucent clouds are very beautiful to witness (and photograph), but they form only under very restrictive atmospheric conditions; therefore their occurrence can be used as a sensitive guide to changes in the upper atmosphere.

Since their relatively recent classification and record-keeping, the occurrence of NLCs appears to be increasing in frequency, brightness and extent. That's the "bad news;" like ice flows melting in polar bear habitat, it is theorized that this increase is connected to climate change.

We have seen some drastic changes in climate in the past however, if only short-term, due to volcanic eruptions. 1816 was the "year of no summer" in northern latitudes because of the eruption in 1815 of Mt. Tambora of Indonesia that spewed over 20 square miles of ash into our upper atmosphere.

The death toll was at least 71,000 people (perhaps the most deadly eruption in history), and created horrific global climate anomalies, affecting North American and European weather, resulting in the worst famine of the 19th century. The volcanic junk circled the globe for years, causing wide-spread crop failure in Europe and North America.

It was thought by scientists of the 1800s that volcanic debris carried moisture and debris (which it does) to those high altitudes and that was the reason for NLCs, but after the ash had settled out of the atmosphere, the noctilucent clouds still persisted.

Noctilucent clouds were first detected from space by an instrument on the OGO-6 satellite in 1972. A later satellite, the Solar Mesosphere Explorer, mapped the distribution of the clouds between 1981 and 1986, using its ultraviolet spectrometer, proving that water ice is the primary component of noctilucent clouds.

In 2001 the Swedish Odin satellite performed spectral analyses on the clouds and produced daily global maps that revealed large patterns in their distribution.