# Jet Streams: Characteristics and Aviation Impact

> ATPL guide to jet stream characteristics, formation, and operational impact in aviation, including clear-air turbulence considerations.

- **Subject:** 050 — Meteorology
- **Section:** 050-02 — Wind
- **Topic code:** 050-02-07
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/wind/jet-streams/

---

A Jet Stream is a fast, ribbon-like air current in the upper troposphere.

![Jet Stream Globe](https://upload.wikimedia.org/wikipedia/commons/d/d7/Straalstroom.jpg)

## General Characteristics
*   **Minimum Speed:** 60 kt (can exceed 200-300 kt).
*   **Direction:** Generally from **West to East** (Westerly), due to the Earth's rotation (Coriolis).
*   **Typical Dimensions:**
    *   Length: Thousands of miles (~1500 NM).
    *   Width: Hundreds of miles (~200 NM).
    *   Depth: Thousands of feet (~12,000 - 18,000 ft).
    *   Height/Width Ratio: 1/100.
*   **Cause:** Strong horizontal temperature contrasts between two air masses.

## Core Location

![Jet Cross Section](https://upload.wikimedia.org/wikipedia/commons/6/64/Jetcrosssection.svg)

*   The Jet core is always found in the **warm air** mass (Tropical).
*   Vertically, it is located:
    *   Just **below** the tropopause of the warm air.
    *   **Above** the tropopause of the cold air.

## Main Types

![Main Jet Streams Types](https://media.sciencephoto.com/c0/21/63/53/c0216353-800px-wm.jpg)

### 1. Polar Front Jet
*   **Location:** Between the Polar Cell and the Ferrel Cell (over the Polar Front).
*   **Altitude:** Approx. **30,000 ft**.
*   **Latitude:** Variable (40°N - 60°N), moves south in winter and north in summer.
*   **Direction:** West (Westerly).

### 2. Subtropical Jet
*   **Location:** Between the Ferrel Cell and the Hadley Cell (over the Subtropical Highs).
*   **Altitude:** Approx. **40,000 ft** (200 hPa).
*   **Direction:** West (Westerly).
*   **Characteristics:** More constant and permanent than the polar jet.

### 3. Equatorial Jet
*   **Exception:** It is an **Easterly** jet.
*   **Seasonal:** Only appears in the Northern Hemisphere **Summer** (Jun-Aug).
*   **Location:** SE Asia, India, Central Africa (~15°N).
*   **Altitude:** Very high, **FL450 - FL500** (~50,000 ft).

## Clear Air Turbulence (CAT)
Turbulence associated with the Jet Stream is caused by strong wind shear (horizontal and vertical).

*   **Zone of Greatest Turbulence:**
    *   On the **COLD** side (Low Pressure) of the Jet.
    *   Just **below** and to the side of the core axis.
    *   In the Northern Hemisphere, it is the **North** side.
*   **Identification on Charts:** Areas where **isotachs** (lines of equal wind speed) are very close together.
*   **Visual Signs:** Striated or elongated **Cirrus** clouds (Jet streaks).

## Relationship with Temperature (Northern Hemisphere)
If you fly perpendicular to the Jet Stream (from South to North) below the core:
1.  You are in the warm sector (South of the Jet).
2.  You cross into the cold sector (North of the Jet).
3.  The temperature **decreases**.
4.  The wind (from the West) will come from your **left**.

---

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