# Local winds

> Aviation meteorology guide on local winds: sea and land breezes, Foehn wind, Mistral, Bora, and their impact on aircraft operations.

- **Subject:** 050 — Meteorology
- **Section:** 050-02 — Wind
- **Topic code:** 050-02-04
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/wind/local-winds/

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Local winds are small-scale air circulations caused by topography or local thermal differences.

## Sea and Land Breezes
Caused by differential heating/cooling between land and sea.

![Sea and Land Breezes](https://upload.wikimedia.org/wikipedia/commons/6/6a/Sea_Land_Breeze.svg)

### Sea Breeze
*   **When:** During the **day** (especially sunny days).
*   **Mechanism:** Land heats up faster than the sea $\rightarrow$ Air over land rises (Low Pressure) $\rightarrow$ Cool air from the sea flows towards the land.
*   **Direction:** From **Sea to Land**.
*   **Intensity:** Stronger than the land breeze.

### Land Breeze
*   **When:** During the **night**.
*   **Mechanism:** Land cools down faster than the sea $\rightarrow$ High Pressure over land $\rightarrow$ Air flows towards the sea (which is relatively warmer).
*   **Direction:** From **Land to Sea**.
*   **Intensity:** Weaker (~5 kt), as the temperature difference is smaller than during the day.

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## Slope and Valley Winds

![Anabatic and Katabatic Winds](https://upload.wikimedia.org/wikipedia/commons/b/b4/Diurnal_mountain_winds_variation.jpg)

### Anabatic Wind
*   **When:** During the **day**.
*   **Direction:** **Up** the slope.
*   **Cause:** The sun heats the slope $\rightarrow$ Air in contact warms up and rises.
*   **Characteristics:** Light wind (~5 kt).

### Katabatic Wind
*   **When:** During the **night**.
*   **Direction:** **Down** the slope.
*   **Cause:** The slope cools by radiation $\rightarrow$ Air in contact cools, becomes dense, and "falls" towards the valley.
*   **Characteristics:**
    *   Stronger than the anabatic wind (~10 kt or more).
    *   Cold air accumulates at the bottom of the valley, favoring the formation of **radiation fog** or frost.
    *   More pronounced with clear skies (greater cooling) and snow-covered slopes.

### Venturi Effect in Valleys
When wind is forced through a narrow valley:
*   Speed **increases**.
*   Pressure **decreases** (Bernoulli's Principle).
*   **Hazard:** The altimeter will overread (indicate higher than actual), so the **true altitude will be lower**.

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## Low Level Jet Streams
These are fast air currents in the lower atmosphere, typically nocturnal.

*   **Formation:** Occur just **above a nocturnal inversion**.
*   **Necessary conditions:**
    *   Clear skies (to allow radiative cooling and form the inversion).
    *   Moderate geostrophic wind (if too strong it breaks the inversion, if too weak it doesn't form the jet).
*   **Location:** Common in great plains (e.g., central USA, southern Germany).
*   **Hazard:** Can generate significant wind shear near the ground.

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