Behind the Forecast: How inversions affect everything from fog to smog

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Science Behind the Forecast: Inversion (2/21)

LOUISVILLE, Ky. (WAVE) - Inversions are not well-known weather phenomena, but they can play a big part in the type of weather seen across a region.

As you rise from the ground up into the atmosphere through the troposphere (the part of the atmosphere closest to the surface), the temperature typically decreases; colder air sits above warm air. With a temperature inversion temperatures rise as we climb; warm air lays over colder air.

Inversions can dictate how clouds form, types of precipitation, and visibility. An inversion is a Tupperware lid for the atmosphere; it limits the upward movement of air from lower parts of the atmosphere. Air pollutants, smoke, and dust aren't able to be dispersed throughout the atmosphere. This accumulation of particulates can cause reduced visibility and even health issues for those with breathing ailments. Convection (the movement of heat throughout the atmosphere) is also limited. Limited convection keeps clouds from growing high enough to produce precipitation. Since air is colder near the bottom of an inversion, fog usually forms there.

There are four types of inversions: ground, frontal, turbulence, and subsidence.

A ground inversion forms when air is cooled by contact with a colder surface until it's temperature drops below that of the atmosphere above. Ground inversions often happen on clear nights when ground temperatures drop quickly due to radiational cooling. If the temperature drops below or meets the dewpoint, then fog may form.

Inversions are not well-known weather phenomena, but they can play a big part in the type of weather seen across a region.
Inversions are not well-known weather phenomena, but they can play a big part in the type of weather seen across a region. (Source: Pexels)

A frontal inversion happens when a cold air mass slides under a warm air mass and lifts it.

Turbulence inversions form when calm air sits over turbulent air. In turbulent air, consistent movement pushes heat towards the ground, thus cooling the upper parts of that layer. Eventually, air below becomes colder than the calm, warmer air above.

Subsidence inversion happens when a large slab of air sinks through the atmosphere. This layer is heated and compressed by the resulting increase in atmospheric pressure; this reduces the rate of change in temperature. If the air sinks low enough, eventually air at higher altitudes becomes warmer than the air beneath it, creating an inversion.

Temperature inversions typically happen during the late afternoon and early evening and last through sunrise, or shortly after, the next day.

Here are a few signs of an inversion:

  1. Clear Skies (no clouds)
  2. Calm (wind < 3 mph)
  3. Closer to sunrise or sunset
  4. Dew present
  5. Horizontal smoke patterns
  6. Ground fog in low-lying areas.

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