Like blue-white spider webs laced across the twilight sky, noctilucent clouds form a wispy filigree in the heavens. Truly a polar phenomenon, noctilucent clouds are never seen at latitudes below 45°. Thus, in North America, noctilucent clouds are pretty much the property of Alaska and Canada.
Nor are noctilucent clouds an everyday occurrence. In 1885, they were first recognized as something strange in the sky. Since then more than a thousand sightings have been recorded in the world. Several displays occurred over central Alaska in the summer of 1979.
The characteristic that distinguishes noctilucent clouds from all others is their remarkably high altitude, 82 (plus or minus a few) kilometers (about 50 miles). Rarely do normal clouds extend as high as 15 kilometers. Noctilucent clouds are seen only in deep twilight, when the sun is 6° to 16° below the horizon. Then the sky is dark enough for the thin noctilucent clouds to be seen and yet the sun is still in position to reflect enough light from the clouds to make them visible to an observer.
In the northern hemisphere, noctilucent clouds are seen from early March to late October and are most frequently observed in midsummer. Similarly, austral summer is the time the clouds are seen in the southern hemisphere.
Though noctilucent clouds have been recognized for nearly a century, no one quite knows why they occur. Almost certainly, the clouds consist of ice-coated dust particles, the dust presumably coming from meteors striking the atmosphere. Beyond that, not much is known.