Which energy source drives the evaporation step of the Hydrologic Cycle?

• A. The Moon
• B. The Sun
• C. Electrical energy from lightning
• D. Internal geothermal energy

Answer: (B)

Evaporation needs energy to break the bonds holding water in liquid form, and the main heat source for that process on Earth is sunlight. When water absorbs solar radiation, its molecules gain energy and some gain enough to overcome intermolecular forces and escape as water vapor. This solar heating at oceans, lakes, and rivers drives the global hydrologic cycle. The Moon doesn’t provide heat, electrical energy from lightning can cause only localized, brief vaporization, and internal geothermal energy supplies only a tiny amount of heat at the surface compared with the Sun, so it doesn’t drive the evaporation step.