2.2: Latent Heat

Phase Change

Water vapor is an invisible gas that becomes visible when it changes into larger liquid or solid particles. This process of transformation is known as a change of state or phase change. It is the physical, reversible transition of matter among the solid, liquid, and gas states, driven by the absorption or release of heat at constant temperature. Thus, heat must be added or removed from a system during such a phase change, and this heat is called Latent Heat. In atmospheric science, we're mostly only concerned with the phase changes of water (\(\ce{H2O}\)).

When heat is added to an ice cube, its temperature increases until it reaches its melting point (0°C). After this point, any additional heat will melt the ice into liquid water, but the temperature of the ice-water system will not change until the phase change is complete. The heat absorbed by the ice-water system from the environment to facilitate the phase change is known as latent heat. You have experienced this as your ice melts in your water glass. It stays cold until all the ice is gone, then rapidly warms. The absorption of latent heat is also the reason you feel cold right after leaving your shower or a body of water. The excess water on your skin starts to evaporate, but it requires additional energy to transition from liquid to vapor. Energy is absorbed from the environment (your skin) to evaporate the remaining liquid, which cools your skin. This is known as evaporative cooling.

Processes that take heat from the environment (melting, evaporation, sublimation) are considered cooling processes. On the other hand, processes that add heat to the environment (condensation, freezing, and deposition/resublimation) are warming processes because energy is released during the phase change. This is an important distinction, so ensure it is clear before proceeding.

Figure \(\PageIndex{1}\): Phase changes of water. Melting, evaporation, and sublimation are considered cooling processes because they take heat from the environment. Condensation, freezing, and deposition are considered warming processes because, during the phase change, energy is released into the environment. (CC BY-SA 3.0; Synkizz via Wikimedia Commons).

The reason phase changes of water are so important in the atmosphere is that they act as the energy to fuel convection and thunderstorms. When water vapor condenses to form liquid water in clouds, this process releases enormous amounts of latent heat into the environment.