# Non-frontal Depressions: Secondary Depression and Low-Pressure Systems

> ATPL meteorology guide to non-frontal depressions, including secondary depression development, thermal lows, and orographic lows.

- **Subject:** 050 — Meteorology
- **Section:** 050-07 — Pressure Systems
- **Topic code:** 050-07-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/pressure-systems/non-frontal-depressions/

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A **depression** (or low-pressure system) is a region where the atmospheric pressure is lower than in its surroundings. It is characterized by closed isobars, where the pressure value decreases towards the center.

![3D Structure of an Extratropical Cyclone](https://upload.wikimedia.org/wikipedia/commons/2/2b/Extratropical_cyclone_airstreams_3D_structure.jpg)
*3D structure of airstreams in an extratropical cyclone or non-frontal depression.*

## General Dynamics

The movement of air in a depression is opposite to that of an anticyclone:

1.  **Surface (Convergence)**: Air flows towards the center of low pressure (cyclonic).
2.  **Vertical Motion (Ascent)**: Surface convergence forces the air to rise. As it rises, the air expands and cools adiabatically.
3.  **Aloft (Divergence)**: Air diverges at upper levels to allow the ascent from below to continue.

**Consequences**: Adiabatic cooling of the rising air favors condensation, cloud formation, and precipitation. If the air is unstable, clouds of vertical development (Cumulonimbus) will form.

## Specific Types

### 1. Orographic Depression (Lee Low / Trough)
It is a small-scale low-pressure system that forms on the **leeward** side of a mountain barrier.
*   **Mechanism**: When an airflow crosses a mountain, it compresses and warms as it descends on the other side. The atmosphere responds by generating surface convergence, which reduces pressure and eventually forms a low or trough.
*   **Favorable Conditions**: Strong wind blowing **perpendicular** to the mountain range.

### 2. Secondary Depression
It is a smaller satellite depression that forms within the circulation of a larger primary depression.
*   **Origin**: Often born as waves on the "tail" of a primary cold front.
*   **Movement**: The secondary orbits around the primary in a **cyclonic** direction (counter-clockwise in the Northern Hemisphere).
*   **Weather**: They are usually very active systems and are frequently associated with strong winds (gales).

### 3. Polar Low
It is a small-scale, short-lived depression that forms over oceans in high latitudes (on the polar side of the polar front).
*   **Origin**: Advection of very cold air (arctic/polar) over a relatively warmer water surface. This creates strong thermal instability.
*   **Weather**: Severe weather, strong winds, and intense precipitation, usually in the form of snow.

### 4. Trough
It is an elongated extension of low pressure, indicated by isobars extending outwards from a low.
*   **Shape**: Isobars are **"V"** shaped (never "U" shaped like ridges).
*   **Dynamics**: There is convergence and rising air along the axis of the trough.
*   **Weather**: Similar to that of an active depression. Clouds of vertical development, showers, squall lines, and cold fronts are often associated with troughs.

## Associated Weather

Weather in a low-pressure zone is generally unstable:

*   **Clouds**: Extensive coverage and vertically developed. If the air is unstable, CBs and thunderstorms form.
*   **Precipitation**: Can be continuous or in the form of heavy showers.
*   **Visibility**: Generally good outside of precipitation (rising air disperses haze), but poor within rain or snow.
*   **Wind**: Strong and gusty, proportional to the pressure gradient (closely spaced isobars).

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