Cloud and precipitation Droplet fall speed

The terminal velocity of an object falling through a fluid is usually determined by a force balance between gravity and some drag force trying to slow the object down. The drag force depends on the velocity of the particle and therefore can be solved for when the forces are equal and opposite. The gravitational force is a so-called body force referring to the fact that the force acts on the entire body of the object. The drag force is a surface force where the force acts on the surface area of the object. There are two classes of drag force balance against the downward force of gravity:

1. 
   Diffusive or viscous
   

• 
  The surface drag force acts (more or less) along the sides of the falling object and the force is due to the fluid fighting against the velocity shear along the sides of the object
  
• 
  There are two forms of viscous force depending on the source of the viscosity:
  

1. 
   Stokes drag: molecular viscosity dominates the viscosity
   
2. 
   Eddy drag: where the drag associated with eddies dominates the viscosity
   

2. 
   Dynamic pressure
   

• 
  The drag force is the result of the molecules in front of the object colliding with the object