# Composition, extent, vertical division

> Explore the Earth's atmosphere components: Nitrogen and Oxygen. Learn about its vertical structure including Troposphere, Stratosphere and Tropopause.

- **Subject:** 050 — Meteorology
- **Section:** 050-01 — The Atmosphere
- **Topic code:** 050-01-01
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/the-atmosphere/composition-extent-vertical-division/

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## Layers of the Atmosphere

![Atmosphere Layers](https://upload.wikimedia.org/wikipedia/commons/0/02/Atmosphere_layers-es.svg)

The atmosphere is divided into several layers based on the variation of temperature with altitude:

1.  **Troposphere**:
    *   It is the lowest layer, extending from the surface to the tropopause.
    *   It contains approximately **75% of the mass** of the atmosphere and **99% of the water vapor**.
    *   Almost all **weather** (clouds, precipitation) occurs here.
    *   Temperature **decreases with height** (approx. 2°C/1000 ft or 0.65°C/100m).
    *   Dry air composition: **78% Nitrogen, 21% Oxygen**, 1% others (Argon, CO2, etc.). Water vapor varies between 0% and 5%.

2.  **Stratosphere**:
    *   Extends from the tropopause to the stratopause (approx. 50 km).
    *   Temperature **increases with height** (thermal inversion) due to the absorption of UV radiation by ozone.
    *   It is a very **stable** layer, with little vertical turbulence, ideal for commercial jet flight in its lower part.

3.  **Mesosphere**:
    *   From the stratopause to the mesopause (approx. 85 km).
    *   Temperature decreases with height, reaching the coldest values in the atmosphere.

4.  **Thermosphere**:
    *   From the mesopause to approx. 600 km.
    *   Temperature increases drastically.

5.  **Exosphere**:
    *   The outermost layer, transitioning into space.

## The Tropopause

It is the boundary between the troposphere and the stratosphere. It is defined as the point where the temperature stops decreasing with height (becomes isothermal or increases).

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

*   **Height Variation**: The height of the tropopause is not constant; it varies with **latitude** and **season** (surface temperature).
    *   **Equator**: Higher (~16-18 km / 56,000 ft) due to convection and expansion of warm air.
    *   **Poles**: Lower (~8-10 km / 26,000 ft) due to contraction of cold air.
    *   **Seasons**: Higher in summer, lower in winter.
*   **Height/Temperature Relationship**:
    *   **High** Tropopause (Equator) -> **Colder** Temperature (approx. -80°C).
    *   **Low** Tropopause (Poles) -> **Warmer** Temperature (approx. -40°C).

![Atmosphere Temperature Profile](https://upload.wikimedia.org/wikipedia/commons/9/9d/Comparison_US_standard_atmosphere_1962.svg)

*   **ISA (International Standard Atmosphere)**: Assumes a tropopause at **11 km (36,090 ft)** with a constant temperature of **-56.5°C**.
*   **Tropopause Breaks**: It is not a continuous surface; there are "breaks" where Jet Streams occur and air exchange between the troposphere and stratosphere takes place.

## Ozone (O3)

*   **Location**: 90% is found in the **stratosphere** (Ozone Layer, between 15-35 km). The remaining 10% is in the troposphere.
*   **Function**: Absorbs harmful ultraviolet (UV) radiation from the sun, which heats the stratosphere.
*   **Hazards for Aviation**:
    *   Ozone is **toxic**.
    *   On high-altitude flights (jet cruise in the lower stratosphere), the concentration can be dangerous.
    *   Symptoms: Irritation of eyes, nose, and throat, chest pain, cough, difficulty breathing.
    *   Modern aircraft have catalytic converters to break down ozone before it enters the cabin.

## Water Vapor

*   It is the most important gaseous component for meteorology.
*   It is the main engine of weather patterns (energy transport, latent heat).
*   Necessary for the formation of clouds and precipitation.

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