Rain formation is part of the continuous water cycle. Solar energy heats water in oceans, lakes, and rivers, causing it to evaporate and rise as invisible water vapor into the atmosphere. This water vapor accumulates in the atmosphere and eventually forms clouds.
As water vapor rises through the atmosphere, it encounters cooler temperatures. When the vapor reaches its dew point, it begins to condense around tiny particles called condensation nuclei, such as dust, pollen, or salt crystals. These microscopic water droplets cluster together to form clouds.
Inside clouds, water droplets grow through two main processes. In the collision and coalescence process, droplets bump into each other and merge to form larger drops. In the ice crystal process, ice crystals grow at the expense of water droplets. When these droplets become too heavy to remain suspended in the air, gravity pulls them down as precipitation - rain, snow, sleet, or hail, depending on atmospheric conditions.
The complete water cycle demonstrates how rain formation fits into Earth's continuous water circulation system. Solar energy drives evaporation from oceans and water bodies. Water vapor rises, cools, and condenses to form clouds. When droplets grow large enough, they fall as precipitation. This water then flows back to oceans through rivers and groundwater, completing the cycle that sustains all life on Earth.
Water vapor in the atmosphere behaves according to temperature-dependent principles. Humidity measures the amount of water vapor present, while saturation represents the maximum amount air can hold at a given temperature. The dew point is the temperature at which condensation begins. Crucially, warm air can hold significantly more water vapor than cold air, which explains why cooling leads to condensation and cloud formation.
Cloud formation begins when water vapor reaches saturation and encounters condensation nuclei. These microscopic particles include dust, pollen, salt crystals, and smoke particles that provide surfaces for water vapor to condense upon. As millions of tiny water droplets form around these nuclei, they create visible clouds at different atmospheric levels, from low clouds near the surface to high clouds in the upper atmosphere.