Refer to figure.
Adiabatic temperature changes – occur when a gas is compressed or expanded with no external exchange of heat.
THE DRY ADIABATIC LAPSE RATE DALR – Lapse rate for rising dry (unsaturated) air.
Constant/fixed value of 1°C/100 m (about 3°C/1000 ft)
THE SATURATED LAPSE RATE SALR – Lapse rate for rising saturated air.
Constant/fixed value of 0.6°C/100 m (about 1.8°C/1000 ft)
=> Saturated air cools down at a slower rate than unsaturated/dry air => because when saturated air is forced to rise and cools, condensation will take place releasing latent heat.
THE ENVIRONMENT LAPSE RATE ELR – Actual temperature profile of troposphere. It varies with time and position. ISA ELR = 2°C per 1000 ft (0.65°C per 100 m).
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 1000 ft |
| ABSOLUTE STABILITY | ELR < SALR | stable conditions for both dry and saturated air. ELR is less than 1.8°C per 1000 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 1000 ft |
| NEUTRAL STABILITY | ELR = DALR Or ELR = SALR | Neutral stability for unsaturated/dry air Neutral stability for saturated air. |
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