By day and by night, the interaction of sunlight and moisture in our atmosphere creates sights in the sky.
A sunlight passes through the ice crystals and water droplets suspended in the air above us, it creates an endless variety of arcs, rings, dapples of color, and splashes oflight that can be seen throughout the year.
Most of these phenomena occur within Earth's lower atmosphere, which means that they are actually meteorological in nature. Noctilucent clouds, however, can claim both meteorological and astronomical ties.
Noctilucent clouds appear at night—usually an hour or so after sunset, when the Sun is more than 10 degrees below the horizon. This phenomenon is known to appear around midsummer in both the Northern and Southern hemispheres, but it can only be seen between latitudes of 45 and 60 degrees, with most sightings occurring in Canada and northern Europe.
These silvery-blue clouds form at the edge of space at an altitude of 50 miles (80 km), which is almost five times higher than most of Earth's weather-making systems. At such heights, the temperature can dip to -150 degrees Fahrenheit (-100° C). The scant amount of water vapor present at this altitude condenses onto meteoric dust and charged particles in the atmosphere, forming the clouds.
Sightings of noctilucent clouds have doubled since the 1950s. A possible explanation is that industrial pollution has increased the amount of particles at high altitudes around which the clouds can form.