A drop is a small volume of liquid, bounded completely or almost completely by free surfaces. The simplest way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter. When the pendant drop exceeds a certain size it is no longer stable and detaches itself. Drops may also be formed by the condensation of a supercooled vapor or by atomization of a larger mass of liquid. The weight of the largest drop that can hang from the end of a tube of radius a is nearly

mg=2πaλ cos α

where λ is the surface tension of the liquid and α is the angle of contact with the tube. This relationship is the basis of a convenient method of measuring surface tension.

Due to the different refractive index of water and air, refraction and reflection occur on the surfaces of raindrops, leading to rainbow formation.