The atmosphere is composed of gases, water vapour, and dust particles. The proportion of gases changes in the higher layers; for instance, oxygen is almost negligible at 120 km, while carbon dioxide and water vapour are found only up to 90 km from the earth's surface.

Dust particles originate from sources such as sea salts, fine soil, smoke-soot, ash, pollen, and disintegrated particles of meteors. They are mostly concentrated in the lower layers but can be transported to great heights by convectional currents. These particles act as hygroscopic nuclei for the condensation of water vapour to produce clouds.