During a thermal inversion, normal convection currents are stalled.
Understanding Thermal Inversions and Convection
Under typical atmospheric conditions, the air temperature decreases with altitude. This allows warmer, less dense air near the surface to rise, while cooler, denser air aloft sinks. This vertical movement of air is known as convection, and it helps mix the atmosphere.
The provided reference highlights that strong convection in the atmosphere can create vertical currents, making the air unstable. This natural instability is key to processes like cloud formation and the dispersal of pollutants.
The Impact on Convection During an Inversion
A thermal inversion (also known as a temperature inversion) occurs when the normal temperature profile of the atmosphere is reversed. Instead of getting colder with height, a layer of warm air sits above a layer of cooler air near the ground.
The reference explicitly states: "...when a temperature inversion occurs and cool air is trapped under a layer of warm air, normal convection is stalled."
Why does this happen? Because the air near the surface is cooler and denser than the air above it, it lacks the buoyancy needed to rise. The warm layer acts like a lid, trapping the cooler air below and preventing the typical upward movement associated with convection.
Consequences of Stalled Convection
When convection is stalled by a thermal inversion, several things happen:
- Increased Stability: The air trapped beneath the warm layer becomes much more stable. There is very little vertical mixing.
- Altered Cloud Formation: As noted in the reference, cloud formation changes. The stable conditions can inhibit the development of cumuliform clouds (like cumulus or cumulonimbus) which rely on strong updrafts from convection. Fog or low-lying stratus clouds might persist instead.
- Pollutant Trapping: Without vertical mixing, pollutants emitted near the surface can get trapped in the cool layer, leading to increased concentrations and poor air quality.
Here's a simple comparison:
| Feature | Normal Conditions | Thermal Inversion Conditions |
|---|---|---|
| Temperature vs. Height | Decreases with height | Warm layer above cool layer |
| Convection | Normal, upward movement (unstable) | Stalled (stable) |
| Air Mixing | Good vertical mixing | Limited vertical mixing |
| Pollutants | Dispersed vertically | Trapped near surface |
In summary, during a thermal inversion, the fundamental atmospheric process of convection is interrupted, leading to a stable, poorly mixed lower atmosphere with significant environmental consequences.
