A Water Drop is Usually a Sphere but Never Shaped like a Tear-Drop

The shape of a "free" drop (or droplet) of water varies with the volume contained in the drop but is never shaped like a "tear-drop" that we are all familiar with and use to represent water.

Possible Source of the Tear-drop Shape

Common Representation of a water drop
Source - free.clipartof.com

The widely recognized representation of a water drop, in the shape of a tear-drop (shown on the right) simply does not exist except in our mind's eye. This shape probably originated from our observation of the appearance of water clinging to a surface or immediately before dropping from a surface (like a dripping tap). 

Water dripping from a Tap
Source - head-first.co.uk

Tiny drops of water
Source- Photography-on-the-net

The Primary Shape of a Water Drop is a Sphere

Water drops, smaller than 2 mm in size, take the shape of a perfect sphere. 

The drop takes this shape as a sphere is the geometric shape that has the smallest surface area for a given volume (see April 9, 2011 post). 

Also, the mass of water contained in a drop of this small size does not experience enough gravitational force for the shape to be impacted significantly. Simply speaking, the gravitational force is much less than the force of the Hydrogen Bonds in a drop of water.

The Shape of Raindrops depends on the size of the drop

Shapes of Raindrops of fifferent size
Source - http://en.wikipedia.org/wiki/Drop_(liquid)

As raindrops fall through the air they experience resistance from the air. This resistance is larger for larger drops and is insignificant, like the force of gravity, on drops less than 2mm in size.

As raindrop size grows,however, from the  combination of individual raindrops, the air resistance and gravity increase till they force the breakup of the large raindrops into smaller raindrops. These smaller raindrops again start exhibiting the spherical shape and combine into larger drops once again. This process of starting with tiny drops that combine to form larger drops which then break up into smaller drops, only stops when  raindrops reach the ground or some structure on the ground.