Non-frontal Depressions: Secondary Depression and Low-Pressure Systems
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 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:
- Surface (Convergence): Air flows towards the center of low pressure (cyclonic).
- Vertical Motion (Ascent): Surface convergence forces the air to rise. As it rises, the air expands and cools adiabatically.
- 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).