Next time you are outdoors near sunset, look towards the West. If you are lucky, you might spot a rare cloud formation known as a noctilucent cloud. Noctilucent clouds look similar to a high cirrus cloud, but appear to glow iridescently. These beautiful cloud formations are seasonal, appearing most commonly in spring and summer, so now is a great time to keep your eye out for them! Location plays a role in how likely you are to see these clouds, too. Northern mid-latitudes, such as the northern US and Canada experience noctilucent clouds more often; however, they have been seen further South than this and have become more frequently sighted in Southern locations than ever before. Sometimes they are even visible from US states as far South as Virginia! So what causes these beautiful "glowing" clouds, and why have they become more common?
While noctilucent clouds are not fully understood, it is known that they form from water in the extreme upper atmosphere that condenses into ice. These cloud formations are also known to exist at altitudes of 50 miles or more, higher than any other cloud formation or meteorological phenomenon. The temperatures at these heights can be as low as -220* F (-140* C)! These extremely cold temperatures, combined with the high altitude give the noctilucent clouds their cirrus-like appearance. These clouds can be differentiated from cirrus by observing them with binoculars. While cirrus clouds can be faint and wispy, noctilucent clouds are typically very well defined.
The extreme height at which these clouds form is also the cause of their iridescent glow. Because these clouds are so high up in the atmosphere, when we are in the Earth's shadow, they are still in the sunlight. This makes it seem like the sun is still shining on these clouds, giving them an ethereal glowing appearance. There is a brief moment in time when noctilucent clouds will be visible. The best time to observe them is when the Sun has just recently set. About 30 minutes after sunset, while the horizon is still dusky, is a good time to start looking. Remember that these clouds are faint, so they won't be blindingly obvious at first, unless you get lucky enough to see a very nice noctilucent cloud! The best conditions for observing a noctilucent cloud is a clear night, with little or no low cloud cover, and no moonlight or light pollution. Late spring to Summer are the best seasons to observe these clouds.
Interestingly, these noctilucent clouds are becoming more common! While the reason for this is not fully known, some have linked the increase in noctilucent clouds to pollutants in the upper atmosphere, and to water vapor left by the space shuttle and other space exploration craft. If this trend continues, noctilucent clouds may become commonplace!
Image Courtesy of Mika Yrjölä