Evaporation happens at the surface, not throughout the liquid
Water molecules are always jiggling, and the fastest ones at the surface can break free into the air even when the water is cool. Sunlight adds energy, warming the puddle and speeding up this escape, so the puddle shrinks steadily. This is evaporation, and it happens at the surface at any temperature below boiling. [1][2]
Boiling is different. It occurs when the liquid as a whole gets hot enough that vapor bubbles form inside it, not just at the surface. A puddle under the sun rarely reaches that point because it is spread thin and loses heat to the air and ground, so it dries by surface evaporation long before it could boil. [1]
Sunlight supplies the energy, and the surface does the work
Solar vapor generators illustrate the principle: an absorber captures sunlight and converts it to heat, which drives liquid to evaporate into vapor without pressurization. [1] The energy goes into loosening molecules at the surface rather than raising the whole puddle to boiling. [1][2]
In solar-driven interfacial evaporation, the heat is concentrated at the evaporating surface, which lets even high-boiling solvents evaporate under ambient conditions. [4] The same idea applies to a puddle: the sun heats the top layer, and molecules leave from there, so the bulk of the water never needs to boil. [4]
Why a puddle dries instead of boiling
A puddle is a thin, open layer of water, so it loses heat easily and never builds up the uniform high temperature that boiling requires. Sunlight keeps feeding energy to the surface, where molecules escape as vapor, and the puddle gradually disappears. [1][2]
Solar evaporation systems also show that the process can be efficient and can even superheat the resulting vapor, but that is a designed effect, not what happens in an ordinary puddle. [1] For a puddle, the practical answer is simple: the sun warms the surface, molecules evaporate at ordinary temperatures, and boiling is never needed. [1][2]
Sources used in this answer
Methods, apparatus and systems for generating and superheating vapor under sunlight
A solar vapor generator absorbs sunlight and radiatively evaporates liquid under less than one sun illumination without pressurization, and can heat components above the liquid's boiling point to superheat vapor.
Contactless steam generation and superheating under one sun illumination
Solar-driven evaporation can be achieved under one sun illumination, with contact between water and the structure affecting temperatures near the boiling point.
Investigation of near‑wall thermal transport in a supercritical pool via a two-phase model and pseudo-boiling theory
A two-phase model and pseudo-boiling theory were used to study near-wall thermal transport and evaporation in a supercritical pool, without addressing solar heating.
Ambient-temperature recovery of high-boiling solvent by solar-driven interfacial evaporation
Solar-driven interfacial evaporation recovers high-boiling solvents under ambient conditions, with thermal localization at the interface aiding the process.
Multi-Synergistic Removal of Low-Boiling-Point Contaminants with Efficient Carbon Aerogel-Based Solar Purifier
A carbon aerogel solar purifier achieves a maximum solar evaporation rate of 2.1 kg m-2 h-1 under one sun and suppresses evaporation of low-boiling-point contaminants through adsorption.