Water is the only substance that lives in all three states at atmospheric temperatures, and the six changes between them carry the module's energy accounting. The definitions first:
Condensation — water vapour to liquid water.
Evaporation — liquid water to vapour.
Freezing and melting — water to ice, and ice back to water.
Sublimation — directly between ice and vapour, in either direction, with no liquid stage; the vapour-to-ice sense is also called deposition.
Latent heat — the energy absorbed or released in a change of state, without any change of temperature: the same latent heat the atmosphere module's heat-transfer lesson promised this module would weigh properly.
The energy rule of the figure is worth memorising as a single sentence: changes toward vapour absorb latent heat; changes away from vapour release it. Melting, evaporation and ice-to-vapour sublimation all take energy from the surroundings: evaporation is why wet skin chills in the wind. Condensation, freezing and vapour-to-ice sublimation give that energy back, and the release of latent heat by condensation is the fuel of convective weather: every cloud that forms warms the air around it, and the flight-hazards module's thunderstorm lessons will run entirely on that warmth.
Condensation needs two things: air brought to saturation (by cooling to the dew point or by adding vapour) and something to condense onto. Pure vapour resists condensing even beyond saturation; the atmosphere gets around this with condensation nuclei: microscopic particles of salt, dust and smoke on whose surfaces the vapour can settle. The real atmosphere is never short of them, which is why real air condenses almost exactly at its dew point. Evaporation is the mirror, and its conditions are the mirror too: unsaturated air above the water (room under the ceiling) and energy to free the molecules, supplied by sun, warm water or wind.
Freezing has the same shape with a colder twist. Melting happens dependably at 0 °C, but freezing needs freezing nuclei (particles whose structure lets ice crystals start), and suitable nuclei are far rarer than condensation nuclei. Without one, a droplet cooled below 0 °C simply stays liquid: supercooled water, liquid water at a temperature below 0 °C, common in cloud between 0 °C and about −20 °C, and the raw material of airframe icing, as the flight-hazards module will make painfully clear. The droplet freezes the instant it finds a nucleus — or an aircraft. Only near −40 °C does the waiting end on its own: there, droplets freeze spontaneously without any nucleus at all.
Sublimation follows the pattern: ice exposed to dry air can pass directly to vapour (a snowbank shrinking in clear, freezing weather without ever melting), and vapour in very cold air can pass directly to ice given sublimation nuclei to start the crystal: the frost that grows on a cold-soaked wing on a clear night skips the liquid stage entirely. The everyday examples are the exam's favourite dress for these processes:
Dew and mist — condensation.
Drying laundry — evaporation.
Hoar frost — vapour to ice.
The shrinking snowbank — ice to vapour.
Hail — freezing.
Hail melting into rain — melting.