Explanation: Water vapour is also a variable gas in the atmosphere, which decreases with altitude.

• In the warm and wet tropics, it may account for four per cent of the air by volume, while in the dry and cold areas of desert and Polar Regions; it may be less than one per cent of the air. Water vapour also decreases from the equator towards the poles.
• Atmosphere has a sufficient capacity to keep small solid particles, which may originate from different sources and include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors.
• Dust particles are generally concentrated in the lower layers of the atmosphere; yet, convectional air currents may transport them to great heights.
• The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and Polar Regions.
• Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds.

Source: NCERT – Fundamental of Physical Geography