Rain comes from a continuous natural process called the water cycle. This cycle involves the constant movement of water between our oceans, the atmosphere, and land surfaces. The sun provides energy that drives this cycle through evaporation, where water transforms into vapor and rises into the atmosphere. This vapor then condenses to form clouds, which eventually release precipitation as rain, completing the cycle.
The evaporation process begins when solar energy heats water surfaces. As water molecules absorb this thermal energy, they move faster and faster until they have enough energy to escape from the liquid surface as invisible water vapor. This happens continuously in oceans, lakes, and rivers. Plants also contribute through transpiration, where water absorbed by roots travels through the plant and evaporates from leaf surfaces into the atmosphere.
As water vapor rises into the atmosphere, it encounters cooler temperatures at higher altitudes. When the rising air cools to its dew point temperature, the invisible water vapor begins to condense. However, pure water vapor cannot condense on its own. It needs tiny particles called condensation nuclei, such as dust, pollen, or salt particles, to provide surfaces for condensation. Water molecules cluster around these microscopic particles, forming tiny water droplets that eventually become visible as clouds.
For cloud droplets to become raindrops, they must grow from tiny 0.01 millimeter droplets to at least 0.5 millimeters in diameter. Two main processes accomplish this growth. In warm clouds, collision-coalescence occurs when droplets of different sizes move at different speeds, causing them to collide and merge into larger droplets. In cold clouds above freezing level, the Bergeron-Findeisen process takes place, where ice crystals grow by absorbing water vapor from surrounding droplets, eventually becoming large enough to fall as precipitation.
The final stage of rain formation occurs when water droplets become heavy enough to overcome the upward air currents within clouds. Droplets must reach a critical size of about 0.5 millimeters to fall successfully. As they descend through different atmospheric layers, some droplets encounter dry air and evaporate before reaching the ground, creating a phenomenon called virga. However, droplets that maintain their size and mass throughout the journey successfully reach the surface as precipitation, completing the transformation from invisible water vapor to falling rain.