Rain occurs through the process of coalescence, where a mass of water droplets gathers around cloud condensation nuclei to saturate the atmosphere and allow condensation and precipitation to take place. This process is driven by three primary lifting mechanisms that cause the upward movement of air.
Low atmospheric temperatures and high relative humidity results in air saturation, where clouds begin to thicken and moisture particles fall to the ground as precipitation. Aside from rain, other types of precipitation include snow, sleet, freezing rain, hail and virga, which is a type of rain that vaporizes before it comes into contact with the ground.
Rain is a common meteorological phenomenon that occurs almost everywhere on Earth. The three lifting mechanisms that influence rain formation include frontal lifting, orographic lifting and convective lifting.
Frontal lifting occurs when a warm, less dense air mass meets a cold, denser air mass. The warm air is lifted up over the cold air, causing the warm air to cool down, form clouds that gather vast amounts of water vapor, which eventually fall as frontal rain. Orographic lifting occurs in mountainous regions where moist air rises to overcome the high elevation. The rain that falls via this process is known as orographic rain. Convective lifting is the result of unequal solar heating on the planet's surface. Warm air generally moves up and cold air sinks to the ground. The rising column of air expands, cools down, forms clouds, condenses and falls to the ground as convective rain.