# Humidity

> Understand humidity concepts for aviation: relative humidity (RH), dew point, mixing ratio, and their formulas for meteorology exams.

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

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Humidity is the measure of the amount of water vapor present in the air.

## Key Concepts

### 1. Relative Humidity (RH)
It is the ratio between the amount of water vapor the air contains and the maximum amount it could hold at that temperature, expressed as a percentage.

*   **Approximate Formula:**
    $$RH \approx 100 - 5 \times (T - T_d)$$
    Where $T$ is the air temperature and $T_d$ is the dew point.
*   **Exact Formula (based on Mixing Ratio):**
    $$RH = \frac{\text{Humidity Mixing Ratio (HMR)}}{\text{Saturation Mixing Ratio (SMR)}} \times 100$$
*   **Relationship with Temperature:**
    *   If the temperature **increases** (and the water content remains constant), the air's capacity to hold water increases, therefore, the **Relative Humidity decreases**.
    *   If the temperature **decreases**, the capacity decreases, and the **Relative Humidity increases**.
*   **Diurnal Variation:**
    *   **Maximum RH:** At sunrise (minimum temperature).
    *   **Minimum RH:** In the early afternoon (maximum temperature).

### 2. Dew Point ($T_d$)
It is the temperature to which air must be cooled (at constant pressure) to reach saturation (RH = 100%).

*   The dew point can **never** be higher than the air temperature ($T_d \le T$).
*   If $T = T_d$, the air is **saturated**.
*   Changing the air temperature **does not change** the dew point (unless the temperature drops below the dew point, causing condensation).

### 3. Saturation
Occurs when the air contains the maximum amount of water vapor possible for its temperature and pressure.
*   **RH = 100%**.
*   **T = Td**.
*   Any further cooling or addition of water vapor will cause **condensation** (clouds, fog, dew).

**Ways to saturate the air:**
1.  **Add water vapor** (e.g., evaporation of rain).
2.  **Cool the air** (e.g., adiabatic ascent, nocturnal radiation cooling).

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## Vapor Pressure over Ice vs Water
The saturation vapor pressure over **ice** is **lower** than over liquid water at the same temperature.
*   This means that air saturates faster over ice than over water.
*   It favors the growth of ice crystals at the expense of water droplets (Bergeron-Findeisen Process).
*   The "Frost Point" is higher than the Dew Point when the temperature is below freezing.

## Global Distribution of Water Vapor
*   **Poles:** ~0 g/m³ (very cold air, low capacity).
*   **Equator:** ~25 g/m³ (warm air, high capacity).
*   Water vapor is concentrated in the lower troposphere (0-5% of air volume).

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## Practical Calculations

**Example 1:**
*   Temperature ($T$): 18°C
*   Dew Point ($T_d$): 12°C
*   $RH = 100 - 5 \times (18 - 12) = 100 - 30 = 70\%$

**Example 2:**
*   Current content (HMR): 10 g/kg
*   Maximum capacity (SMR): 15 g/kg
*   $RH = (10 / 15) \times 100 = 66.7\%$

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.