As the air close to the ground or water surface of the Earth is heated, a rising current of air will be formed, carrying moisture upwards. As the air rises, it expands and cools. The water vapour indside will condense into water droplets when the temperature falls to the dew-point. A cloud then forms.

When air flows over mountain, water vapour condenses into water droplets due to decrease in temperature of the air mass with height. A cloud then forms.

Clouds will form when a mass of warm air is uplifted by a mass of cold dense air along fronts. A ‘front’ is the boundary between warm air and cold air.

When a low pressure area is formed, the surrounding air will flow in and converge near the centre of the low pressure. The air will then rise, leading to condensation and thereby formation of clouds. It is often accompanied with precipitation.