Scientists study noctilucent clouds because, it's believed, these polar mesopheric clouds are highly sensitive to changes in the atmosphere. "They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator." A Universe Today article shows recent pictures from space of the mysterious clouds. "They used to be considered rare, but now the clouds are growing brighter, are seen more frequently, are visible at lower and lower latitudes than ever before, and... they are now even appearing during the day." The article suggests this is evidence the upper atmosphere is getter colder and more humid, a possible cause are greenhouse gases. Noctilucent clouds are composed of ice and reflect solar radiation much longer than clouds at lower altitudes, making them visible well into twilight--enjoy the view!