Page 272 - PPL-engelsk 2025

Meteorology

A stable atmosphere will counteract upward and downward air movements.

An unstable atmosphere will allow upward and downward air currents to grow.

A conditionally unstable atmosphere can, if it is affected by an external

influence, become unstable.

The stability of the air is determined by the vertical distribution of temperature

and humidity.

Stability is triggered when warmer air comes in over colder air, e.g. warm
fronts.

Stability increases when:

- there is cooling in the lower layers and/or warming in the higher layers

(nocturnal cooling provides stability).
- there is advection of cold air in the lower layers and/or warm air in the

upper layers (the consequence of advection of cold air can then later be

that convection is created and thus instability).

- the content of moisture decreases.

- there is subsidence (sinking)

Instability will increase when:

- there is warming in the lower layers and/or cooling in the higher layers.

- there is an advection of warm air in the lower layers and/or cold air in the

higher layers.

- the humidity content increases.

- the air is forced upwards, whereby conditional instability can be triggered
(can happen by lift over mountains or over fronts)

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Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 272