A portion of water from rivers, seas and rivers evaporates by the action of sunlight, and integrates the mass of air as vapor. These masses of warm air to rise and meet others colder, the vapor condenses into clouds. They are formed, then by water droplets, whose diameter varies between 0.5 and 1 mm. Within the clouds are air currents that make the droplets collide and cause his fall as rain. When the conditions of formation of clouds cause the water vapor to rise high and fast, the precipitation falls in the solid state, as snow or hail.
The rainfall maps are those in which are recorded precipitation levels. In them, lines are drawn along the points of equal rainfall, called isohietas.
Calls relief or orographic rains occur when the mass of air rises in the presence of a mountain and cools to gain altitude. The steam condenses and water droplets precipitate.