Airglow Ripples over Tibet

Why would the sky look like a giant target? Airglow . Following a giant thunderstorm over Bangladesh in late April, giant circular ripples of glowing air appeared over Tibet , China , as pictured here . The unusual pattern is created by atmospheric gravity waves , waves of alternating air pressure that can grow with height as the air thins, in this case about 90-kilometers up. Unlike auroras powered by collisions with energetic charged particles and seen at high latitudes, airglow is due to chemiluminescence , the production of light in a chemical reaction. More typically seen near the horizon, airglow keeps the night sky from ever being completely dark .