What Is Evaporation?

Evaporation is the phase change in which molecules leave the surface of a liquid and enter the gas phase at temperatures below the boiling point. It differs from boiling in that it occurs only at the interface rather than throughout the bulk, and it proceeds whenever the partial pressure of the vapor above the surface is lower than the equilibrium vapor pressure of the liquid. Because the escaping molecules carry away more than the average kinetic energy, the remaining liquid cools, which is the physical basis for every evaporative cooling device.

The quantity that governs the energy accounting is the latent heat of vaporization, the enthalpy required to convert unit mass of liquid to vapor at constant temperature and pressure. For water at 100 degrees Celsius this is about 2,257 kilojoules per kilogram, roughly five times the energy needed to heat the same mass from freezing to boiling, and it rises somewhat at lower temperatures before falling to zero at the critical point. That large value is why evaporation dominates surface energy budgets in meteorology and why water remains the working fluid of choice in many thermal systems.

Thermodynamics and Vapor Pressure

Equilibrium between a liquid and its vapor is described by the saturation vapor pressure, which increases steeply with temperature according to the Clausius-Clapeyron relation. Engineering correlations such as the Antoine equation fit this dependence with three empirical constants over a stated temperature range, and tabulated coefficients for thousands of compounds are available through the NIST Chemistry WebBook. Precise measurement of these properties has a long history in metrology: National Bureau of Standards measurements of the heat capacity and heat of vaporization of water between 0 and 100 degrees Celsius, published in 1939, supplied reference values that anchored steam tables for decades. Net evaporation ceases when the surrounding gas reaches saturation, which is why relative humidity, not temperature alone, determines whether a surface dries.

Evaporation Rate and Mass Transfer

The rate at which mass leaves a surface is set by the difference between the saturation vapor concentration at the interface and the concentration in the surrounding gas, divided by the resistance of the boundary layer. Wind raises the rate by thinning that layer and sweeping away saturated air, while still conditions allow a stagnant vapor blanket to form. Reduced ambient pressure also raises the rate, which is why liquids evaporate faster at altitude and why vacuum is used to accelerate drying. In environmental settings, evaporation is estimated either from an energy balance, from mass transfer relations driven by wind speed and humidity deficit, or from combination methods such as the Penman equation, which merges both approaches. Pan evaporation measurements provide the empirical reference against which these estimates are calibrated.

Role in the Water Cycle

Evaporation is the principal upward flux in the hydrologic cycle. According to the USGS Water Science School, roughly 90 percent of atmospheric moisture originates from evaporation of oceans, seas, lakes, and rivers, with the remainder from plant transpiration. Globally, evaporation and precipitation balance, but the geography differs: evaporation exceeds precipitation over the oceans, and precipitation exceeds evaporation over land, with river runoff closing the budget. In hydrology and agriculture the combined loss from soil evaporation and plant transpiration is treated as a single term, evapotranspiration, because the two are difficult to separate in field measurement.

Vacuum Evaporation in Materials Processing

In materials and device fabrication, evaporation refers to a physical vapor deposition technique in which a source material is heated under high vacuum until its vapor pressure is sufficient for atoms to travel in a straight line to a substrate. Resistive heating of a boat or filament serves for low-melting metals such as aluminum and gold, while electron beam evaporation reaches the higher temperatures required for refractory materials and oxides. The long mean free path in vacuum produces a directional flux useful for lift-off patterning, though it gives poorer step coverage than sputtering.

Applications

Evaporation is applied in fields including:

• Evaporative cooling towers and air conditioning
• Desalination and industrial evaporator design
• Thin-film deposition for microelectronics and optical coatings
• Irrigation scheduling and reservoir water balance
• Drying of foods, pharmaceuticals, and coatings
• Weather and climate modeling