Temperature inversion

A temperature inversion is an atmospheric layer in which temperature increases with altitude, reversing the normal lapse rate. Inversions act as lids on vertical mixing, trapping moisture, aerosols, and pollutants below the inversion base. They are fundamental to understanding Precipitation regimes, air quality, and boundary layer dynamics.

Types

1. Radiation inversion: Forms near the surface at night due to radiative cooling. Common in clear-sky, low-wind conditions. Typically shallow (tens to hundreds of meters) and diurnal.

2. Subsidence inversion: Forms aloft when large-scale descending air (subsidence) in subtropical high-pressure systems compresses and warms adiabatically. The trade wind inversion in the subtropics is a prominent example.

3. Marine inversion: A subsidence inversion that caps the marine boundary layer. Cool SSTs stabilize the lower atmosphere, while subsidence warms the air above. The resulting inversion traps moisture in a shallow layer, promoting Stratocumulus cloud formation.

4. Frontal inversion: Occurs at the boundary between warm and cold air masses along a frontal surface.

The Galapagos marine inversion

In the Galapagos, a persistent marine inversion forms during the Garua season at approximately 400-800 m altitude. It is driven by:

• Cool SSTs from the Galapagos Cold Pool that chill the boundary layer air
• Large-scale subsidence associated with the southeastern Pacific subtropical high
• The EUC-driven upwelling that maintains the cool SSTs

Below the inversion, moisture is trapped in a shallow mixed layer where it condenses into the stratocumulus and fog that characterize the Garua. Above the inversion, the air is warm and dry.

The inversion height determines the elevation at which fog deposition is most effective on the islands. Highland observation sites in the DARWIN project were chosen to capture this transition from fog-dominated to above-inversion conditions without exposing precise siting details in public notes.

During El Nino, warmer SSTs erode the inversion, allowing deep convection to develop and replacing the stratocumulus regime with convective Precipitation.

Role in urban climate

Inversions also play a key role in urban climate. Radiation inversions during calm winter nights trap pollutants and heat emissions in the urban canopy layer, exacerbating air quality problems and modifying the urban heat island effect. The micro-scale processes that govern urban atmospheric exchanges are strongly influenced by the presence or absence of low-level inversions.

See also: Garua, Galapagos Cold Pool, Sea surface temperature, Marine boundary layer, urban climate