Skip to main content

Temperature inversions: types and effects

Short answer

A temperature inversion is a layer where temperature increases with height, suppressing vertical mixing. Radiation inversion forms on clear calm nights from ground cooling and is strongest before sunrise; subsidence inversion results from sinking, warming air in high-pressure areas and persists aloft. Both trap pollutants, leading to smog episodes and high ground-level concentrations.

What is a temperature inversion? Explain radiation inversion and subsidence inversion and discuss how inversions affect air pollution.

A temperature inversion is a layer of the atmosphere in which temperature increases with height (negative lapse rate), so cold dense air lies below warmer lighter air. The layer is extremely stable and prevents vertical mixing.

Radiation (nocturnal) inversion

  • On clear, calm nights the ground loses heat by long-wave radiation and cools quickly, cooling the air next to it.
  • The surface layer becomes colder than the air above, forming a ground-based inversion.
  • It is deepest before sunrise and is destroyed after sunrise when the ground heats up and the "inversion lid" lifts.
  • Common in valleys such as the Kathmandu valley in winter, where cold air also drains in from the hills.

Subsidence inversion

  • Occurs in high-pressure (anticyclone) regions where a large air mass slowly sinks.
  • The sinking air is compressed and warms adiabatically, so it becomes warmer than the air below.
  • The inversion forms at some height above the ground (several hundred metres to 1-2 km) and can persist for days, e.g. Los Angeles.
 height
  |        warm air
  |  ======================  <- inversion base (lid)
  |        cooler air
  |  (pollutants trapped below)
  +------------------------> T

Effects on air pollution

  • Vertical mixing is suppressed, so pollutants accumulate in the layer below the inversion and concentrations rise (smog episodes).
  • Plumes released inside the inversion layer "fan"; plumes released above are kept away from the ground, while plumes released below are "trapped".
  • Photochemical smog and fog-smog (London type) episodes are linked to inversions.
  • Low-level sources such as vehicles and domestic fires cause the highest exposure.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Environment and Pollution Control chapter 2 questions →