Rainwater is liquid water that falls from the atmosphere. It is a part of Earth’s continuous natural process where water moves between the surface and the atmosphere. This constant movement is known as the water cycle, a system that redistributes water across the planet. Understanding rainwater’s origin involves tracing its journey through this cycle.
Water’s Journey to the Sky
The sun’s energy drives the initial phase of rainwater formation by transforming water into a gaseous state, known as water vapor. This process, called evaporation, occurs from large bodies of water such as oceans, lakes, and rivers. The sun’s warmth provides the energy for water molecules to break free from the liquid surface and ascend into the air.
Plants also contribute water vapor to the atmosphere through a process called transpiration. They absorb water through their roots and release it as vapor through tiny pores on their leaves called stomata. This vapor, along with that from evaporation, becomes less dense and rises into the atmosphere.
From Vapor to Clouds
As water vapor rises higher into the atmosphere, it encounters cooler temperatures. This cooling causes condensation, changing the vapor into tiny liquid water droplets or ice crystals. For these droplets to form, they need microscopic airborne particles, known as condensation nuclei, to condense upon. These particles can include dust, pollen, smoke, or salt crystals from ocean spray.
These minuscule water droplets are too small to fall to Earth as rain individually. Billions of these tiny droplets aggregate and become suspended in the air, forming visible clouds. Clouds are vast collections of these condensed water droplets or ice crystals, floating at various altitudes depending on atmospheric conditions. The appearance and type of cloud depend on the specific conditions under which these droplets form and collect.
The Descent as Rain
Once formed within clouds, the tiny water droplets continue to interact and grow. These small droplets collide and combine in a process known as collision-coalescence. This causes the droplets to gradually increase in size and mass. Growth continues until they become too heavy for the air currents within the cloud to keep them suspended.
When water droplets reach a sufficient size, gravity overcomes air resistance, and they fall to Earth as rain. The temperature within the cloud and surrounding atmosphere determines whether precipitation falls as liquid rain, snow, or hail. Air currents also influence how long droplets remain suspended.