What is the primary function of the fill in a cooling tower?

The fill in a cooling tower is designed to break the falling water into smaller droplets or thin films, thereby increasing the surface area for heat and mass transfer between the water and the air.

Step-by-step Derivation:
Step 1: Analyze the objective of a cooling tower, which is to remove heat from water through evaporation and convection.
Step 2: Evaluate the role of the 'fill' (packing material). The fill provides a large surface area for the water to spread over as it descends.
Step 3: Determine the physical effect of this increased surface area: it maximizes the contact interface between the descending water and the ascending air stream.
Step 4: Compare with other options: (A) Air speed is controlled by fans, not fill; (B) The goal is to decrease, not increase, water temperature; (D) Humidity typically increases as water evaporates into the air.