How Does the Vast Majority of Water Get From the Ocean to the Land Surface?

Evaporation

• Contrary to popular belief, water does not need to reach boiling point (100 degrees Celsius, or 212 degrees Fahrenheit) to evaporate. Because water molecules are only weakly bound, molecules near the surface can escape. This is why a glass of water left in the home for a few days will gradually decrease in volume. Water molecules with a higher vibrational energy have a higher probability of escaping, which is why higher temperatures increase the rate of evaporation. The water cycle starts with the evaporation of water from an ocean's surface. Electromagnetic energy (light) from the sun is absorbed by water molecules on the surface of the ocean, causing their bonds with surrounding molecules to break. Because water vapor is lighter than air, it rises into the lower atmosphere, where it cools and forms clouds.

Cloud Formation

• As water molecules rise within the atmosphere, their temperature decreases, and they bump into tiny particles known as aerosols. There are many different types of aerosols, and these include smoke particles, dust and salt. When cooled water vapor collides with aerosols, they stick to them, and condense into tiny liquid droplets. Individual droplets collide with one another, forming larger droplets and eventually clouds. Cloud formation typically takes place in the lowest part of the atmosphere, known as the troposphere, which extends up to 20 kilometers (12.4 miles) above ground level.

Transport of Clouds

• Clouds that have formed in the troposphere are subject to winds, which are the result of pressure differences within the atmosphere. Clouds will move from high pressure to low pressure and in doing so may traverse land. Weather forecasters will therefore often use low atmospheric pressure as a sign of cloudy weather.

Precipitation

• As clouds move through the atmosphere, the mass of their water droplets can continue to grow as a result of collisions with other water molecules or with aerosols. This has to occur millions of times before rain drops form. When the droplets acquire fall speeds larger than the updraft speed of the cloud (due to evaporated water molecules moving up), they fall to the ground in the form of rain. Rain water collects on the ground and eventually flows back into the sea via a series of streams and rivers, completing the water cycle.