GEOG 1101 Lecture Notes - Lecture 5: Fluid Parcel, Lapse Rate

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Atmospheric Stability refers to the tendency of an air parcel wit its water vapor, to either remain in
place or to change vertical position by ascending (rising) or descending (falling).
* A stable parcel of air resists vertical displacement or, when
disturbed, tends to return to its starting place.
* An unstable parcel of air continues to rise until it reaches an
altitude where the surrounding air has a density & T° similar
to its own.
Rules of Stability
1) when an air parcel is warmer (less dense) than the surrounding air, the parcel will rise:
UNSTABLE air or condition
2) when an air parcel is cooler (denser) than the surrounding air, it will tend to stay at the same
level or sink : STABLE air
3) The Environmental Lapse Rate (ELR) is the Tº profile of the
atmosphere (surrounding air). The actual T° lapse rate in the
lower atmosphere at any particular time under local weather
conditions is the ELR.
Whether a particular parcel of air will rise (unstable) or not rise (stable)
is a function of the T° inside the parcel of air as compared to the T°
outside the parcel of air, and thus the ELR determines air stability
or instability.
Types of Stability
Stable condition exist when the ELR < DAR
Absolutely stable conditions or Absolute Stability
* This usually results in NO UPLIFT of air, and may bring about
subsidence or sinking of the air parcel. This usually causes
clear sky conditions.
* high pressure cells form stable atmospheric conditions
* The most severe example of stable conditions is a inversion
when the air Tº is increasing with increasing altitude in the
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