# Fog, mist, haze

> Understand the differences between fog, mist, and haze. Formation processes like radiation and advection fog for pilots.

- **Subject:** 050 — Meteorology
- **Section:** 050-04 — Clouds and Fog
- **Topic code:** 050-04-02
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/clouds-and-fog/fog-mist-haze/

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The formation of fog, mist, or clouds occurs when invisible water vapor condenses into visible liquid water. This happens when the air becomes saturated (Relative Humidity = 100%), either by cooling to the dew point or by the addition of moisture.

![Fog Types](https://upload.wikimedia.org/wikipedia/commons/4/46/Vallee-brouillard.jpg)

## Definitions and Differences

| Phenomenon | METAR Code | Visibility | Obscuring Agent | Relative Humidity |
| :--- | :--- | :--- | :--- | :--- |
| **Fog** | **FG** | < 1,000 m | Water droplets | ~100% |
| **Mist** | **BR** | 1,000 m - 5,000 m | Water droplets | > 95% |
| **Haze** | **HZ** | < 5,000 m | Solid particles (dust, smoke) | Low (dry air) |

## Types of Fog

### 1. Radiation Fog
Formed by the cooling of the earth's surface at night due to terrestrial radiation, cooling the air in contact with the ground by conduction.
*   **Ideal Conditions:**
    *   **Clear sky:** Maximizes nocturnal cooling.
    *   **High relative humidity:** Requires little cooling to reach saturation.
    *   **Calm or very light wind (2-8 kt):** Allows some mixing to thicken the layer, but if too strong, it lifts it forming Stratus.
*   **Characteristics:** Occurs only over **land**, typically in autumn/winter, at night or at dawn. Usual thickness up to 500 ft.
*   **Dissipation:** Insolation (solar heating) or increased wind.

### 2. Advection Fog
Formed when a warm, moist air mass moves horizontally (advection) over a colder surface. The air is cooled from below.
*   **Ideal Conditions:**
    *   **Moderate wind (up to 15 kt):** To move the air and allow vertical development.
    *   **Cold surface:** Cold land or cold sea (cold currents).
    *   **High humidity.**
*   **Characteristics:** Can occur over land or sea, day or night. Common in coastal areas (e.g., sea air over cold land in winter) or at sea (e.g., Grand Banks). It is persistent.
*   **Dissipation:** Change of air mass (passage of a cold front) or change in wind direction.

### 3. Steaming Fog / Arctic Sea Smoke
Formed when **very cold** air (from land) moves over a comparatively **warmer** water surface.
*   **Process:** Rapid evaporation from the "warm" water saturates the cold air immediately above.
*   **Conditions:** Large temperature difference (>10°C) between the air (very cold) and the water. Light wind.
*   **Characteristics:** Common in polar regions. Appearance of "smoke" rising from the water. Unstable in very low layers but often limited by an inversion aloft.

### 4. Frontal Fog
Associated mainly with **warm fronts**.
*   **Cause:** Warm rain falls through a layer of cold air (cold sector) beneath the front. Evaporation of raindrops saturates the cold air, and additionally, the cloud base lowers to the ground.
*   **Dissipation:** Upon passage of the warm front (cold air is replaced by warm air).

### 5. Hill Fog (Orographic Fog)
It is simply a cloud (Stratus or Stratocumulus) covering the top of a hill or mountain.
*   **Cause:** Forced ascent of air up the slope (windward), adiabatic cooling to condensation.

### 6. Freezing Fog
Fog composed of **supercooled** water droplets at temperatures below freezing.
*   **Hazard:** Upon impact with a surface (aircraft, ground), the droplets freeze instantly forming **rime ice**.

## Comparative Summary

| Type | Main Location | Wind | Main Cause | Dissipation |
| :--- | :--- | :--- | :--- | :--- |
| **Radiation** | Land | Calm/Light | Ground cooling (Night) | Sun / Strong wind |
| **Advection** | Sea / Coast | Moderate (15kt) | Warm air over cold surf. | Air mass change |
| **Steaming** | Sea (Polar) | Light | Very cold air over warm water | Strong wind / Dir. change |
| **Frontal** | Warm Front | Variable | Warm rain in cold air | Front passage |

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