Precipitation Processes

 Rainfall occurs when raindrops become so large, gravity cannot keep them in the air.  In warm clouds (clouds having temperatures greater than 0°C), the collisioncoalescence process produces precipitation.  Collision  The largest drop (i.e., the collector drop) will collect some of the drops in its path when it falls.  Coalescence  When a ‘collector drop’ and smaller drop come together, two things can happen:  They can combine to form a single, larger drop.  They can bounce apart.  Most times, the two drops stick together.  This is known as ‘coalescence’.  Collision and Coalescence is the primary method for producing precipitation in the Tropics.  In the middle latitudes, clouds that produce precipitation usually have freezing temperatures.  As a result, precipitation is usually created by another mechanism.  This involves the coexistence of ice crystals and supercooled water droplets.  This is known as the Bergeron-Findeisen process, or just the ‘Bergeron process’ for short.  Bergeron process  In a cloud where ice crystals and supercooled droplets (liquid droplets in subfreezing environments) coexist, the rate of condensation onto the liquid droplet equals the rate of evaporation.  As a result, some of the water vapor in the air is deposited directly on the ice crystal.  Afterwards, this will result in the liquid droplet evaporating.  When the liquid droplet evaporates, even more deposition occurs on the ice crystals.  Very simply, ice crystals continue to grow as the supercooled droplets evaporate.