Water molecules at the surface of oceans can pick up energy from the environment and break free of the other water molecules in a process known as evaporation. The major driving force of this process is the Sun. The energy can also come from any heat source such as man made when you boil water on a stove or natural in areas of geothermal activity like volcanoes and hot springs.
The major difference between the process of evaporation and boiling is where the water molecules are breaking free of their neighboring water molecules. With evaporation, water molecules only break free at the surface, but with boiling the process occurs throughout the entire volume of water. The reason bubbles rise from boiling water is because the water vapor forming below the surface is more bouyant than liquid water and it rises.
A similar process to evaporation occurs in trees and other plants. Water that plant roots draw up from the ground is transported to the leaves and evaporates out of the stomas (pore-like holes on the underside of leaves used for gas exchange) in a process called transpiration.
There is a final way that water can get to a gaseous state called sublimation, the process of going from solid to a gaseous form without passing through a liquid intermediate stage. This occurs as winds strike the surface of ice and snow and transfer enough energy to the surface molecules for them to break free of the other water molecules.
Evaporation and transpiration form the source of almost all water vapor. This water vapor typically rises up into the atmosphere where it will condense to form clouds.
Learn the three methods by which water goes from liquid or solid form to gas form and be prepared to answer questions about these methods.
