Raindrops in the atmosphere tend to acquire a nearly spherical shape mainly due to which physical phenomenon?

Introduction / Context:
The shape of small liquid drops, such as raindrops or water droplets from a tap, is a classic topic in basic physics. At very small sizes, gravity is relatively weak compared to molecular forces at the surface of the liquid. This question tests your understanding of why these drops become roughly spherical and which physical property of liquids governs this behaviour. It is often asked in general science and competitive exams to check conceptual clarity about surface phenomena.

Given Data / Assumptions:

• We are concerned with small raindrops or liquid droplets in air.
• External influences like strong winds or collisions are ignored.
• The main competing effects are surface tension, viscosity, friction with surrounding air, and elasticity of the liquid.
• We consider a droplet free to adjust its shape under the action of these forces.

Concept / Approach:
Surface tension is the tendency of the surface of a liquid to contract and reduce its area due to molecular attraction. For a given volume, a sphere has the minimum possible surface area. Therefore, when surface tension dominates over other forces, a liquid drop will try to become spherical to minimize its surface energy. Viscosity relates to internal friction within the fluid and affects flow, not the equilibrium shape directly. Friction with air influences motion, not shape at rest. Elasticity usually refers to solids, not to the free surface of liquids. Thus, surface tension is the primary reason for the nearly spherical shape of small drops.

Step-by-Step Solution:
Step 1: Recall that for a given volume, the sphere is the geometrical shape with minimum surface area. Step 2: Understand that surface tension tends to reduce the surface area of a liquid, pulling the surface molecules inward. Step 3: For a very small droplet, the force due to gravity is small, so it cannot distort the droplet significantly from spherical shape. Step 4: The dominating effect is surface tension, which pulls the molecules into a shape with minimum surface area, that is, a sphere. Step 5: Viscosity and friction may influence how the drop moves or falls, but not its final equilibrium shape. Step 6: Conclude that surface tension is the phenomenon responsible for raindrops becoming nearly spherical.

Verification / Alternative check:
Laboratory observations of small water drops in microgravity or floating on surfaces also show nearly spherical shapes, consistent with surface tension dominance. Mathematical models of capillary phenomena treat the shape of drops as the result of balancing surface tension and other forces. When gravitational and inertial effects are negligible, the equilibrium shape is a sphere. In contrast, large raindrops can become distorted or even break up due to air resistance, which explains why real raindrops are not perfect spheres but still originate from this basic tendency driven by surface tension.

Why Other Options Are Wrong:
Viscosity: This property measures internal resistance to flow; it affects how quickly the drop can change shape but not the preferred equilibrium shape itself.
Friction: Friction between air and the drop mainly affects the motion and path of the drop, not its tendency to be spherical.
Elasticity: Elasticity refers mainly to deformation of solids and is not the controlling factor for the free surface shape of a liquid droplet.

Common Pitfalls:
A typical mistake is to associate any resistance to deformation with viscosity or elasticity, overlooking the special role of surface tension at liquid surfaces. Another pitfall is thinking that raindrops are shaped by wind alone. While air resistance can distort larger drops, the fundamental reason small drops are nearly spherical is surface tension. Remembering that spheres minimize surface area and that surface tension seeks to reduce surface area helps overcome these misunderstandings.

Final Answer:
Raindrops acquire a nearly spherical shape mainly because of Surface tension of the liquid.