The Collision Coalescence and the Bergeron Findeisen theory tries to explain how water droplets when formed in the cloud actually fall to the ground as rainfall. For rain to fall down, 3 things are needed:

  1. Uplift mechanism which lifts air parcels.
  2. Condensation nuclei where water vapour accumulates to form tiny droplets
  3. … and the Collision coalescence or Bergeron Findeisen process that actually makes the droplets fall as rain.

Collision Coalescence

This theory explains that water droplets big and small in a cloud collide against each caused by convectional currents in the cloud. The initial small droplets are not dropped but kept suspended in the cloud until they are large enough to be dropped.

As other large water droplets (that are still suspended in the cloud) collide with small water droplets, they coalesce (join) causing them to become more big. Eventually, they get larger that they are unable to be suspended any longer in the cloud and falls down.

Upon falling, the large droplets are ripped apart by convectional currents and air friction which disintegrates them into new droplets which fall as rainfall.

Collision Coalescence mostly applies to warm convectional clouds in tropical areas.

Collision Coalescence Diagram
Collision coalescence Process

Animation showing collision coalescence process

Bergeron Findeisen Theory

Supercooled water
is water that is very cold (below freezing), but still in its liquid state. It probably exists in the cloud when the droplets are too small to be dropped or no ice nuclei.
Ice crystals
form high up in the cloud where temperatures are low(-40oC) given ice nuclei is present.
Ice nuclei
are minute particles around which droplets accumulate to form ice crystals.

Also known as WBS after first discovered by Alfred Wegener, modified by Tor Bergeron and then finally fully elaborated by Walter Findeisen.

This process occurs when supercooled water and ice crystals exist together. Supercooled water surfaces have a high vapour pressure (pressure generated by water vapour on a water body or air) than ice surfaces. Water vapour around ice crystals have no time to condense thus go straight for deposition (solid).

Water vapour moves from high vapour pressure (supercooled water) and deposited to the ice crystals where they freeze and merge with other ice crystals. The ice crystals becomes heavy therefore fall, where upon falling experience warm temperatures and melt as rainfall.

This process mainly occurs in towering cumulonimbus clouds where ice crystals are mainly located around the anvil (uppermost part of cumulonimbus clouds) because of very low temperatures and in temperate and mid-latitude regions where temperatures are low.

However, this theory fails to apply to tropical areas where rain can form in low level clouds without ice crystals. Another assumption is that the collision coalescence process may take place on the melted droplets and rain then falls to the ground.

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