Noctilucent Clouds. Also known as polar mesospheric clouds, these are the highest, coldest, and rarest clouds in Earth’s atmosphere. Forming in the mesosphere at an altitude of around 80 kilometers—on the very edge of outer space—they are composed of microscopic ice crystals seeded by space and meteoric dust. By day, they are completely invisible, masked by their own transparency. But in the deep twilight, as the sun dips far below the horizon and darkness blankets the earth, these ultra-high-altitude clouds remain bathed in sunlight from beneath the curve of the globe. This unique perspective causes them to glow with a brilliant, electric blue, silver, or golden light. A strictly seasonal phenomenon, they can only be observed during the summer months (from June to August in the Northern Hemisphere) at latitudes between 45° and 70°. Observers across Europe, Canada, and Russia often catch sight of them glowing low on the northern horizon, typically one to two hours after sunset or just before dawn. I have tried to capture the profound beauty, ephemeral magic, and quiet mystery of this phenomenon in my eponymous track.