Clouds are not usually the most enthralling part of nature. But in the late spring and summer, weather conditions conspire to create noctilucent or 'night-shining' clouds, high-altitude clouds that glow with an electric-blue hue long after the Sun has set.
It takes a very particular set of atmospheric conditions to form noctilucent clouds: a combination of lower atmostphere warming and upper atmostphere cooling causes ice crystals to form on meteor dust and other particles at high altitude, around 50 miles above the Earth's surface. Those crystals then reflect the Sun's light back at the Earth long after sunset, with the frequency of the light (somewhere between blue and white) depending on the density of the ice.
Night-shining clouds appear rarely, and only at high latitudes over the Arctic. But in recent years, scientists have been observing the clouds earlier and lower, including as far south as Utah, Colorado and Nebraska. The most recent sighting was made on June 10 by NASA's Aerology of Ice in the Mesosphere (AIM) satellite, producing the only slight-terrifying image above.