When warm wet air rises, it cools and water vapour condenses to form clouds. A cloud is made of small drops of water or ice crystals, depending on its height and surrounding cold air. Most rain originates in towering clouds.

To form rain, water vapour needs condensation nucleus. It can be tiny particles of dust, or pollen, swept up high into the atmosphere. When the condensing droplets that form the cloud get large and heavy enough to overcome the upward pressure, they begin to fall. All clouds contain water. But some produce rain and others flow away without giving rain. Cloud contains many particles and normally drops are formed by absorbing moisture by these particles.

The drops of water that fall from the cloud are the rain.  The rainfall begins as snow crystals or other solid forms. When they enter the warmer air below the cloud, the ice particles melt and fall the ground as raindrops. The raindrops have a typically diameter of at least 0.5 mm. The size of the drop rarely exceeds about 5 mm, no matter what the force of the rain is.

The raindrops after falling become speedier due to gravity.  One drop falls, giving clearance to another.  Drops that pick up speed are slowed down by the pressure of the surrounding air. If the cloud droplet is larger, the better is the chance of its colliding with a giant droplet. So each rain drop falls at a different speed as their sizes are different. Because of the collisions between raindrops, they come together, forming a large drop and breaking into smaller ones. As the number of drops grow the intensity of rain increases.

Collision does not guarantee coalescence. The presence of atmospheric electricity may be the key to what keeps the drops together as they collide. Thus when a droplet with a negative charge collides with another with a positive charge their electrical attraction may hold them together.

The rate at which the rain drops fall depends on their size.  Large droplets fall rapidly. Thus drops keep on falling side by side and not in a continuous stream.