# Stratospheric conditions

> High altitude flying, tropopause, stratospheric conditions, and ozone hazards for commercial aviation.

- **Subject:** 050 — Meteorology
- **Section:** 050-09 — Flight Hazards
- **Topic code:** 050-09-07
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/flight-hazards/stratospheric-conditions/

---

The stratosphere is the layer of the atmosphere situated immediately above the troposphere. The boundary between the two is the **tropopause**, which in the International Standard Atmosphere (ISA) is found at 36,090 feet (11 km).

![Atmosphere Layers](https://upload.wikimedia.org/wikipedia/commons/f/f9/Atmosphere_layers-en.svg)

## Characteristics of the Lower Stratosphere
1.  **Temperature**:
    *   It is an **isothermal** layer in its lower part (up to approx. 60,000 feet), meaning the temperature remains constant with height (at -56.5°C in ISA).
    *   Above that level, the temperature begins to increase gradually due to the absorption of ultraviolet radiation by ozone.
2.  **Stability**:
    *   Because the temperature does not decrease with height, the stratosphere is **extremely stable**.
    *   There is very little vertical air movement.
3.  **Humidity and Clouds**:
    *   The air is very dry.
    *   There are practically no clouds, except for the tops of violent Cumulonimbus clouds penetrating the tropopause or very rare polar stratospheric clouds.
4.  **Density**:
    *   Air density decreases rapidly with height.

## Advantages of Stratospheric Flight
Most modern commercial aircraft fly in the lower stratosphere (or near the tropopause) for several reasons:
1.  **Fuel Efficiency**: Jet engines operate more efficiently at high altitudes due to the low temperature and low air density.
2.  **Speed**:
    *   Low air density reduces parasite drag.
    *   A higher **True Airspeed (TAS)** is obtained for the same Indicated Airspeed (IAS).
3.  **Comfort (Less Turbulence)**: Being above most weather phenomena and in a stable layer, the flight is much smoother.
4.  **Weather Avoidance**: The vast majority of clouds and storms, which are confined to the troposphere, are avoided.

## Considerations
*   **Wind**: Although there is less vertical movement, horizontal winds can be very strong (Jet Streams).
*   **Ozone and Radiation**: At these altitudes, exposure to cosmic radiation and ozone is higher than at lower levels.

---

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