STABILITY Flashcards
Define Stable Air
Air displaced vertically is colder than surrounding (and therefore heavier), sinks back to its original level once lifting force is removed
Define unstable air
Air displaced vertically will become warmer than the surrounding environment (and therefore lighter), and will continue to rise once lifting force is removed
Define conditionally unstable air
Air displaced vertically that has a temp the same as the surrounding environment & will remain at the newly attained level once lifting force is removed.
The caveat is that if certain condxns are met, the parcel may become unstable & continue to rise.
Define environmental lapse rate (ELR)
General RoT? Exceptions?
The ACTUAL vertical temp profile that exists in the atmosphere.
As we ascend through the lower section of the atmosphere, temp generally decreases at a rate of 1.98deg/1000ft (ISA) as a general RoT.
However this can vary with inversions & isothermal layers.
How is ELR & dew point lapse rates obtained?
Weather balloons/aircraft/satellites
Describe the adiabatic process
Parcel of air is forced to rise > it is subject to less pressure > it expands > resulting in cooling.
And vice versa to cause heating
What does the term adiabatic actually mean
the temp changes within a parcel of air are purely as a result of work done INSIDE the parcel. (no heat exchange externally)
Value of DALR at low levels in mid-latitudes?
‘dry’ accounts for?
3deg/1000ft
Accounts for any parcel of air that has an internal RH of 0-99% (nil cloud)
Value of SALR at low levels in mid-latitudes?
‘saturated’ accounts for?
1.5deg/1000ft
RH of 100% and cloud has formed.
Why is the value of SALR less than DALR?
The rate of cooling during continued lifting is offset by latent heat being released into the atmosphere during the condensation process. (Additional heat doesn’t stop to cooling process, just slows it down).
State the process by which conditionally unstable air is forced to become unstable;
After: parcel forced to rise > Subject to less pressure > expansion > cooling
Continued cooling = condensation >
Cloud forms >
Cloud formation = release of latent heat >
Enough latent heat released = destabilisation >
Destabilise the air enough & rising parcel of air becomes warmer than surrounding air & will continue to rise after lifting mechanism is gone.
Define: Inversion
A shallow layer in the atmosphere where the inverse of ELR occurs. (i.e. increase of temp with height).
Define: Isothermal layer
A layer through which the temperature remains constant with increasing height.
How does an inversion affect a rising parcel of air?
Isothermal layer?
Parcel of air is lifted >
Must cool at either DALR/SALR (or both) >
Passing through inversion, parcel temp is cooler than INCREASING temp of inversion with height >
If lifting force removed whilst in inversion, parcel will sink.
Isothermal: same effect, except difference between the parcel & environment isn’t a marked
General RoTs (x3) wrt DALR/SALR/ELR & stability?
ELR < SALR: Air is STABLE
SALR < ELR < DALR: Air is CONDITIONALLY UNSTABLE
DALR < ELR: air is UNSTABLE.
