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Introduction
When the air is compressed or expanded without external transfer of heat, adiabatic temperature changes will occur.
Air lifted = Expanding and will cool adiabatically
Air descending = Compressing and will warm adiabatically
- The dry adiabatic lapse rate (DALR)
Is the lapse rate for rising dry (not saturated) air
= 1°C/100m (3°C/1000ft)
- The saturated adiabatic lapse rate (SALR)
Is the lapse rate for rising saturated air (RH 100%)
= 0.6°C/100m (1.8°C/1000ft)
Why does it have a slower lapse rate?
When saturated air is lifted it will not only cool down but also condensation will take place which releases latent heat. This heat will slow down the lapse rate at which the air cools.
At higher altitudes, the SALR tends more and more towards the DALR because the air is now colder, containing not as much water vapour and therefore not as much latent heat will be released.
- The environmental lapse rate (ELR)
Is the actual temperature profile of the troposphere as measured by radiosondes, it varies with time and position
Stability
- When the air that is lifted by a force returns to its original position once that lifting force is removed (atmosphere = stable)
At every level the air is forced to rise to, it'll be always colder than the surrounding air and therefore more dense (heavy)
- The air is stable when the ELR is less than the SALR (lies in the area to the right of the SALR on the graph)
Instability
- When the air that is lifted by a force continues to rise once that lifting force is removed (atmosphere = unstable)
At every level that the air is forced to rise to, it'll be always warmer than the surrounding air and therefore less dense (lighter)
- The air is unstable when the ELR is greater than the DALR (lies in the area to the left of the DALR on the graph)
Conditional Instability
- When the air that is lifted by a force will continue to rise if saturated or return to its original position if unsaturated when the lifting force is removed (atmosphere = conditionally unstable)
- The stability of the atmosphere is now not only conditional upon the value of the ELR but also the moisture content
- The air is conditionally unstable when the ELR lies between the SALR and the DALR
Neutral Stability
- When the lifted air's lapse rate (DALR or SALR) is equal to the ELR
At every level the air is forced to rise to, it'll have the same temperature as the surrounding air and therefore the same density
Summary
1. ELR less than SALR = air is stable/absolute stability
Stable weather includes: bad visibility, light turbulence, stratiform clouds, intermittent to continuous precipitation
2. ELR greater than DALR = air is unstable/absolute instability
Unstable weather includes: good visibility, moderate turbulence, cumuliform clouds, showery precipitation
3. ELR between SALR and DALR = air is stable (if dry) and unstable (if saturated)
When the air is compressed or expanded without external transfer of heat, adiabatic temperature changes will occur.
Air lifted = Expanding and will cool adiabatically
Air descending = Compressing and will warm adiabatically
- The dry adiabatic lapse rate (DALR)
Is the lapse rate for rising dry (not saturated) air
= 1°C/100m (3°C/1000ft)
- The saturated adiabatic lapse rate (SALR)
Is the lapse rate for rising saturated air (RH 100%)
= 0.6°C/100m (1.8°C/1000ft)
Why does it have a slower lapse rate?
When saturated air is lifted it will not only cool down but also condensation will take place which releases latent heat. This heat will slow down the lapse rate at which the air cools.
At higher altitudes, the SALR tends more and more towards the DALR because the air is now colder, containing not as much water vapour and therefore not as much latent heat will be released.
- The environmental lapse rate (ELR)
Is the actual temperature profile of the troposphere as measured by radiosondes, it varies with time and position
Stability
- When the air that is lifted by a force returns to its original position once that lifting force is removed (atmosphere = stable)
At every level the air is forced to rise to, it'll be always colder than the surrounding air and therefore more dense (heavy)
- The air is stable when the ELR is less than the SALR (lies in the area to the right of the SALR on the graph)
Instability
- When the air that is lifted by a force continues to rise once that lifting force is removed (atmosphere = unstable)
At every level that the air is forced to rise to, it'll be always warmer than the surrounding air and therefore less dense (lighter)
- The air is unstable when the ELR is greater than the DALR (lies in the area to the left of the DALR on the graph)
Conditional Instability
- When the air that is lifted by a force will continue to rise if saturated or return to its original position if unsaturated when the lifting force is removed (atmosphere = conditionally unstable)
- The stability of the atmosphere is now not only conditional upon the value of the ELR but also the moisture content
- The air is conditionally unstable when the ELR lies between the SALR and the DALR
Neutral Stability
- When the lifted air's lapse rate (DALR or SALR) is equal to the ELR
At every level the air is forced to rise to, it'll have the same temperature as the surrounding air and therefore the same density
Summary
1. ELR less than SALR = air is stable/absolute stability
Stable weather includes: bad visibility, light turbulence, stratiform clouds, intermittent to continuous precipitation
2. ELR greater than DALR = air is unstable/absolute instability
Unstable weather includes: good visibility, moderate turbulence, cumuliform clouds, showery precipitation
3. ELR between SALR and DALR = air is stable (if dry) and unstable (if saturated)
