Illustration showing the layers of Earth's upper atmosphere and the phenomena which occur in them. Starting from the top, red airglow, aurorae, meteors, green airglow, red sprites, noctilucent, cumulonimbus and cirrus clouds, are shown. Airglow occurs when atoms and molecules in the upper atmosphere, excited by sunlight, emit light to shed their excess energy. Aurorae occur when the electrically charged particles in solar wind collide with atoms of the Earth's upper atmosphere at a very high speed. A meteor shower occurs when the Earth passes through the trail of debris left by a comet or asteroid. Sprites are often triggered by a strong, bolt of ordinary lightning near the ground. They are thought to be a balancing mechanism that the atmosphere uses to dispense charges. Noctilucent clouds are rare high-altitude clouds that can only be seen under specific conditions. Cumulonimbus clouds are dense multi-level clouds, extending high into the sky in towers or plumes. Cirrus clouds are wispy, feathery and composed entirely of ice crystals.