# Primary cause of wind

> Discover the primary causes of wind in aviation meteorology, including Pressure Gradient Force (PGF), Coriolis force, and geostrophic wind.

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

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Wind is the horizontal movement of air caused primarily by atmospheric pressure differences.

## Main Forces

### 1. Pressure Gradient Force (PGF)
It is the initiating force of the wind.
*   Acts from **High Pressure towards Low Pressure**.
*   Acts **perpendicular (90°)** to the isobars.
*   The closer the isobars are, the greater the gradient and the stronger the wind.

![Coriolis Effect](https://upload.wikimedia.org/wikipedia/commons/d/d3/Coriolis_effect.svg)


### 2. Coriolis Force (CF)
It is an apparent force caused by the Earth's rotation.
*   **Deflects the wind**:
    *   To the **right** in the Northern Hemisphere (NH).
    *   To the **left** in the Southern Hemisphere (SH).
*   **Magnitude**:
    *   Proportional to **wind speed** (more speed = more Coriolis).
    *   Proportional to **latitude** ($sin(\phi)$):
        *   **Zero** at the Equator ($sin(0°) = 0$).
        *   **Maximum** at the Poles ($sin(90°) = 1$).
*   Acts **perpendicular (90°)** to the wind direction.

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## Theoretical Wind Types

### Geostrophic Wind
Occurs when the **PGF** and the **Coriolis Force** are in perfect balance.

![Geostrophic Wind](https://cdns3.eltiempo.es/eltiempo/blog/noticias/2019/03/03065030/gfs_jet_300_conus_9.png)

*   Flows **parallel** to **straight** isobars.
*   Occurs above the friction layer (> 2000-3000 ft).
*   Does not exist at the Equator (needs Coriolis, latitude > 15°).

**Relationship with Latitude:**
For the same pressure gradient (same isobar spacing):
*   **Lower latitude** $\rightarrow$ Less Coriolis $\rightarrow$ Wind must be **faster** to balance the PGF.
*   **Higher latitude** $\rightarrow$ More Coriolis $\rightarrow$ Wind is **slower**.
*   *Formula:* $V \propto \frac{1}{sin(\phi)}$

### Gradient Wind
Occurs when isobars are **curved**. Involves PGF, Coriolis, and Centrifugal Force (inertia).

![Balanced Flow Speeds](https://lh5.googleusercontent.com/proxy/R3j38lQicN3EhJiiuo_SJQeyU_XZeBmAV9DGWYmM-aLu98jaGxZOHqakXDJlJgg2Oyd2HjlL-tNQDvUFCAHsiU1KaTS2fc9pmqcjxw)

1.  **Around a LOW (Cyclone):**
    *   Wind rotates counter-clockwise (NH).
    *   PGF pulls inward, Coriolis and Centrifugal pull outward.
    *   Coriolis has "help" from centrifugal, so it doesn't need to be as strong.
    *   **Sub-geostrophic Wind:** It is **slower** than the geostrophic wind for the same gradient.

2.  **Around a HIGH (Anticyclone):**
    *   Wind rotates clockwise (NH).
    *   PGF and Centrifugal pull outward (or oppose in a way that Coriolis must be larger).
    *   Coriolis must balance PGF + Centrifugal.
    *   **Super-geostrophic Wind:** It is **faster** than the geostrophic wind for the same gradient.

> **Rule:** Around a Low the wind is slower; around a High it is faster (for the same isobar spacing).

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## Effect of Friction (Surface Wind)

Friction with the terrain reduces wind speed in the lower layer (up to 2000-3000 ft).

1.  Friction **reduces speed**.
2.  Lower speed $\rightarrow$ **Less Coriolis Force**.
3.  The PGF (which does not change) becomes dominant.
4.  The wind crosses the isobars **towards Low Pressure**.

### Change in Direction and Speed (Northern Hemisphere)

| Surface | Speed (% of wind aloft) | Direction (Change) |
| :--- | :--- | :--- |
| **Sea** | ~70% | **Backs** (Turns left) ~10° |
| **Land** | ~50% (Day) / ~30% (Night) | **Backs** (Turns left) ~30° (Day) / ~40° (Night) |

*   **Descent (Aloft $\rightarrow$ Surface):** Wind decreases and **Backs** (turns counter-clockwise, e.g., 270° $\rightarrow$ 240°).
*   **Ascent (Surface $\rightarrow$ Aloft):** Wind increases and **Veers** (turns clockwise, e.g., 240° $\rightarrow$ 270°).

> **In the Southern Hemisphere:** Surface wind **Veers** (turns clockwise) relative to wind aloft, crossing isobars towards low pressure.

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## General Circulation and Weather

### Buys Ballot's Law
In the Northern Hemisphere, if you stand with your back to the wind, the **Low Pressure** is on your **left**. (In the SH it is on the right).

![Buys Ballot Law](https://www.pasionporvolar.com/wp-content/uploads/2015/08/buys-ballot-1.png)

![Circulation High Low](https://upload.wikimedia.org/wikipedia/commons/9/9b/High_and_low_pressure_N_and_S.svg)

### Convergence and Divergence

| System | Surface | Aloft | Vertical Movement | Typical Weather |
| :--- | :--- | :--- | :--- | :--- |
| **Low Pressure (Cyclone)** | **Convergence** (inward) | Divergence | **Ascent** (air rises) | Clouds, precipitation, bad weather. |
| **High Pressure (Anticyclone)** | **Divergence** (outward) | Convergence | **Subsidence** (air sinks) | Good weather, clear skies, inversions. |

### Altimetry and Wind Rules
*   If you fly with wind from the **left** (NH), you are flying towards a Low Pressure.
    *   "High to Low, Look out below" $\rightarrow$ Your true altitude is decreasing.
*   If you fly with wind from the **right** (NH), you are flying towards a High Pressure.
    *   Your true altitude is increasing.

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