# Change of state of water

> Learn about the changes of state of water in the atmosphere: evaporation, condensation, melting, freezing, sublimation, and the role of latent heat in aviation meteorology.

- **Subject:** 050 — Meteorology
- **Section:** 050-03 — Thermodynamics
- **Topic code:** 050-03-02
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/thermodynamics/change-of-state-of-water/

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Water exists in the atmosphere in three states: solid (ice), liquid (water), and gas (water vapor). Changes between these states involve the absorption or release of energy in the form of **Latent Heat**.

![Water Phase Change Diagram](https://upload.wikimedia.org/wikipedia/commons/e/e5/Water_Phase_Change_Diagram.png)

## Phase Change Processes

### 1. Processes that Absorb Latent Heat
These processes take energy from the environment, **cooling** the surrounding air.

*   **Melting:** Solid $\rightarrow$ Liquid. (~80 cal/g)
*   **Evaporation:** Liquid $\rightarrow$ Gas. (~590 cal/g)
*   **Sublimation:** Solid $\rightarrow$ Gas (directly). (~680 cal/g)

### 2. Processes that Release Latent Heat
These processes release energy to the environment, **warming** the surrounding air.

*   **Freezing:** Liquid $\rightarrow$ Solid. (~80 cal/g)
*   **Condensation:** Gas $\rightarrow$ Liquid. (~590 cal/g)
*   **Deposition:** Gas $\rightarrow$ Solid (directly). (~680 cal/g)
    *   *Note:* In meteorology (especially in the US), the term "Sublimation" is sometimes used for the Gas to Solid change as well, although the correct technical term is Deposition.

## Importance of Latent Heat
The energy involved in evaporation and condensation is enormous compared to melting/freezing.

![Cumulonimbus - Thunderstorms](https://upload.wikimedia.org/wikipedia/commons/a/a7/Anvil_shaped_cumulus_panorama_edit.jpg)

*   **Storm Engine:** When water vapor condenses to form clouds, it releases a large amount of latent heat. This heat warms the air inside the cloud, making it lighter than the surrounding air, which encourages continuous ascent (instability).
*   **Energy:** This release of latent heat is the main source of energy for the development of **thunderstorms** and **hurricanes**.

## Cloud Formation
For clouds to form, three ingredients are needed:
1.  **Water Vapor:** Moisture in the air.
2.  **Cooling:** The air must cool to its dew point (saturation).
3.  **Condensation Nuclei:** Microscopic solid particles (dust, salt, smoke) on which vapor can condense.
    *   Without nuclei, the air could become supersaturated (RH > 100%) without forming clouds.

## Supercooled Water Droplets (SCWD)
These are water droplets that remain in a **liquid** state at temperatures **below 0°C**.

![Ice on an aircraft wing](https://upload.wikimedia.org/wikipedia/commons/5/5e/Ice_on_an_aircraft_wing.jpg)

*   To freeze, they need to come into contact with a **freezing nucleus** or a solid surface (such as an aircraft).
*   In the atmosphere, there are many more water droplets than freezing nuclei, so supercooled water is very common in clouds between 0°C and -15°C (and even down to -40°C).
*   They are the main cause of **icing** on aircraft.

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