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

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GEOG LESSON FIVE
ATMOSPHERIC STABILITY
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
troposphere.
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