Water phase changes & latent heat

Which changes absorb energy and which release it?

Connect each physical change to a familiar atmospheric example.

Liquid, vapour and ice

Evaporation changes liquid water into vapour and absorbs latent energy. Condensation changes vapour into liquid and releases that energy. Melting changes ice into liquid and absorbs energy; freezing changes liquid into ice and releases energy. Sublimation is the direct change from ice to vapour and absorbs energy. Deposition is the direct change from vapour to ice and releases energy.

Latent heat is the energy associated with a phase change per unit mass. For pure water near 0 °C, useful approximate values are 334 kJ/kg for melting/freezing, 2,500 kJ/kg for evaporation/condensation and 2,834 kJ/kg for sublimation/deposition. The values vary with temperature. In an ideal phase change at fixed pressure, supplied or released energy changes phase rather than necessarily changing the water's temperature; the surrounding air's full energy budget determines its temperature response.

Examples include a wet runway drying by evaporation, cloud droplets forming by condensation, snowflakes melting below a sufficiently warm layer, droplets freezing onto an aircraft, snow losing mass to dry air by sublimation, and hoar frost forming by deposition on a cold surface. None requires the air temperature to rise or fall by a fixed universal number of degrees per kilogram; the amount of water and the available air mass matter.

Why can a cloud form at only a small supersaturation?

Use the vapour balance and the role of suitable airborne particles.

From vapour to liquid

Net condensation is favoured when vapour pressure exceeds the saturation value for the relevant surface, commonly after air cools towards its dew point or when moisture is added. A cooler surface can also collect condensation even while nearby air is not saturated. In cloud, tiny supersaturations are sufficient when suitable particles are present. Evaporation is favoured when the air around a liquid surface is unsaturated; it can occur below the normal boiling point and can occur from supercooled liquid as well.

Cloud condensation nuclei are small airborne particles on which droplets can form, such as suitable salt, sulfate and other aerosol particles. Their size and hygroscopic properties help reduce the supersaturation needed to initiate a stable droplet compared with homogeneous formation from pure vapour. Not every aerosol acts equally well, and a condensation nucleus is not automatically an efficient ice nucleus.

Condensation creates cloud or fog droplets, reduces visibility through their scattering and releases latent energy. The release can help maintain buoyancy in rising saturated air by reducing its rate of cooling. Ordinary cloud droplets are too small to fall rapidly as rain; collisions, coalescence and ice-phase processes must grow precipitation-sized particles. Condensation therefore does not automatically imply immediate rainfall.

Why does a cloud not freeze completely at 0 °C?

Distinguish the equilibrium melting temperature from the initiation of ice.

Freezing needs a beginning

At ordinary atmospheric pressure, the melting point of pure water is near 0 °C. Ice melts when it receives enough energy under conditions favouring liquid water. Liquid cooled below 0 °C can remain metastable until an ice embryo forms. A suitable ice-nucleating particle or contact with an ice surface can initiate heterogeneous freezing; ice nuclei are generally much rarer and have different properties from ordinary cloud condensation nuclei.

Supercooled water is liquid water below its normal freezing temperature. Many clouds contain supercooled droplets at negative temperatures, sometimes mixed with ice. As temperature falls, ice nucleation becomes more effective, and small cloud droplets generally freeze homogeneously by around −38 to −40 °C. This is an approximate microphysical range, not a guarantee that every warmer cloud is liquid or every observed icing boundary sits at exactly one value.

A supercooled droplet can freeze when it strikes an aircraft or an existing ice surface, releasing latent heat in the process. The potential for airframe icing therefore depends on liquid water, temperature, drop size and the aircraft encounter, not just whether the temperature is below zero. The presence of sub-zero cloud without visible falling snow is not evidence that all the water is already ice.

How can frost form without liquid dew first?

Make the direction explicit when older meteorological terminology uses the same word broadly.

Ice to vapour and vapour to ice

Sublimation is net transfer from ice directly to vapour. It is favoured when the air is unsaturated with respect to the ice surface and sufficient energy is available; snow can therefore lose mass on a cold dry day without melting. Deposition is the reverse: vapour transfers directly to ice when conditions favour growth relative to ice saturation, releasing latent energy. Hoar frost can form on a sufficiently cold surface by this route without an intervening liquid layer.

Some older meteorological material uses 'sublimation' more broadly for direct exchange between vapour and ice, including the vapour-to-ice direction. In that usage, 'sublimation nuclei' means ice-active particles supporting direct ice formation from vapour. Here the term deposition is used for that direction to avoid an energy-sign ambiguity. An existing ice crystal or a suitable ice-nucleating surface helps deposition begin; sublimation from an existing snow crystal does not require a new nucleation particle.

Below freezing, saturation vapour pressure over ice is lower than over supercooled liquid water. Air can therefore be supersaturated relative to ice while still below liquid-water saturation. This difference allows ice crystals to grow at the expense of nearby supercooled droplets under suitable conditions, an important precipitation mechanism. '100% humidity' must consequently be interpreted with its liquid-water or ice reference stated.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.