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An air mass is unstable when:
  • A
    pressure shows a marked variation over a given horizontal area.
  • B
    parcels of air may rise or descend spontaneously.
  • C
    temperature increases with height.
  • D
    temperature and humidity are not constant.

Refer to figure.

Atmospheric stability is determined by comparing the ELR with the DALR and the SALR.

ABSOLUTE INSTABILITY ELR > DALR when the environmental lapse rate is greater than DALR – air is unstable for both dry and saturated air.
ELR is greater than 3°C per 1 000 ft

After a parcel is forced to rise, when lifting force is removed => the parcel of air continues to rise. This parcel of air will always be warmer than the surroundings and, therefore, have the tendency to continue rising.
Continues to move further away from its original position.
ABSOLUTE STABILITY ELR < SALR stable conditions for both dry and saturated air.
ELR is less than 1.8°C per 1 000 ft
CONDITIONAL INSTABILITY DALR > ELR > SALR stability of air is dependent on whether the air is saturated or unsaturated. The atmosphere is said to be stable for unsaturated/dry air and unstable for saturated air.
ELR is between 1.8°C and 3°C per 1 000 ft
NEUTRAL STABILITY ELR = DALR
Or
ELR = SALR		 Neutral stability for unsaturated/dry air

Neutral stability for saturated air.

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