Evaporation, condensation and humidity

A T-shirt dries even in a cool room, while drops appear on a cold glass. How do water molecules enter the air without boiling, and why do they sometimes return?

Molecules at a liquid surface move at different speeds. Some have enough energy to overcome attraction to their neighbours and escape into the gas. Evaporation therefore occurs at the surface at any ordinary temperature at which the liquid exists; boiling is unnecessary. Imagine equal portions in a narrow glass and a wide saucer. The saucer has a larger surface from which molecules can escape, so, with other conditions equal, its water usually disappears faster.

Why can wet skin feel cold? On average, escaping molecules carry away more energy than those remaining, reducing the liquid's internal energy. If the surroundings do not replace this energy, the liquid cools. Evaporation can also occur at constant temperature, with energy supplied by the air or a heater, for example. A lack of cooling therefore does not prove that evaporation has stopped.

Three changes help fabric dry: warming it, spreading it out and blowing air over it. Warming allows more molecules to escape; spreading the fabric exposes a larger surface. Moving air carries vapour away and reduces the number of molecules returning to the surface. To test one effect, keep the others unchanged. Wind alone does not make the water hotter: it changes the exchange of vapour between the surface and the surrounding air.

Vapour molecules also move continuously and sometimes return to the liquid. This reverse change from gas to liquid is called condensation. Close a vessel containing water at constant temperature: as vapour accumulates, returns become more frequent. If there is enough liquid and some remains, the average rates of evaporation and condensation become equal. The vapour is then saturated. If all the water disappears first, saturation may never be reached.

At saturation, the amounts of liquid and vapour remain constant on average, but individual molecules keep crossing the boundary in both directions. This is dynamic equilibrium, not motion coming to a halt. The saturated vapour density ρнас is set by temperature; for water it increases on heating. After a temperature rise, the previous amount of vapour is therefore usually insufficient for saturation, and some additional liquid evaporates if liquid is available.

Two rooms can contain the same mass of water vapour per cubic metre yet be at different distances from saturation if their temperatures differ. Vapour density ρ is its mass per unit volume of air, in kg/m³ or g/m³. To assess the possibility of further evaporation and condensation, compare ρ with the saturated vapour density ρнас at the same temperature. This comparison defines relative humidity.

If the actual density is half the saturated density, relative humidity is one half, or 50%. In general, φ = ρ/ρнас; multiply this ratio by 100% to express it as a percentage. The density units must match so that they cancel. At constant temperature, doubling ρ doubles φ until saturation is reached. At ρ = 0 humidity is zero; at ρ = ρнас it is 100%. The latter means saturated vapour, not that all the air consists of water.

Cool air in a closed, rigid vessel while no droplets have yet formed. Vapour mass and volume stay constant, so ρ is unchanged while ρнас decreases. The ratio φ rises to one. This is the temperature at which condensation begins in this constant-volume experiment. The usual dew point is defined by cooling air at constant pressure and water-vapour content. With further cooling, some vapour can form droplets. Water on a cold glass thus comes from the surrounding air; it has not seeped through the glass.

A psychrometer has one dry thermometer and another wetted with water. In humid air, evaporation cools the wet thermometer less; in dry air, it cools it more. Humidity is found from both readings and a table appropriate to the instrument's operating conditions. At saturation, the readings are almost equal if the thermometers are correctly installed. Both the dry-bulb temperature and the difference between readings matter: the difference alone does not give a universal percentage.

The vapour density is 8 g/m³, and the saturated vapour density at the same temperature is 20 g/m³. Find the relative humidity.

Compare the vapour with saturated vapour at the same temperature. Both densities are in g/m³, so no unit conversion is needed in the ratio.

φ = 8/20 = 0,4; this is the fraction of the saturated density. Multiply by 100% to obtain 40%.

At unchanged temperature, the vapour is still well below saturation: its density is four tenths of 20 g/m³. On cooling, the same vapour could have a different relative humidity.

A relative humidity of 40% means that, at this temperature, the vapour density is 40% of its saturated value, not that vapour makes up 40% of the total air mass.

Evaporation and condensation occur simultaneously; the visible result depends on the difference between the molecular flows.

Saturation means equal average rates of the two processes with a liquid phase present.

Humidity depends on both vapour content and temperature; cooling can bring air to its dew point.

During boiling, vapour also forms inside the liquid. Next, we will see how pressure affects this.