METEOROLOGY FOR DGCA CPL/ATPL EXAMINATION : LESSON 5 #ATMOSPHERIC STABILITY.
1. “Stability” is the property of an object to return back to its original position after being displaced.
2. If a parcel of air comes down when force is removed, then it is stable Atmosphere. If it continues to move further upwards, it is unstable atmosphere. And if it remains at the same location where it was last left, then it is neutral stability in atmosphere. Atmospheric stability governs the weather conditions in that geographical area.
3. In atmosphere when a parcel of air is forced to move upwards, Atmospheric pressure exerted on this parcel of air keeps reducing (since pressure reduces with incr in height), so there is expansion of this parcel of air which results in adiabatic cooling. Thus a parcel of air which is forced to move upwards will always cool down due to adiabatic expansion cooling. (Remember this cooling effect is purely due to adiabatic expansion and not due to environmental lapse rate).
4. DALR: Rate of cooling of parcel of dry air (which does not contain water vapor) moving vertically upwards is called Dry Adiabatic lapse rate. The value of DALR is 1deg/100m or 3 deg/1000 feet or 10 deg/Km.
5. SALR: The rate of adiabatic cooling of saturated parcel of air (parcel of air which contains water vapor) is different as compared to DALR. This happens since saturated air when cooled would not be able to hold all the water vapor content. Excess water vapor will get condensed releasing latent heat. This release of latent heat slows down the rate of expansion cooling of saturated parcel of air. SALR is 0.6 deg/100 m or 1.8 deg/1000 feet or 6 deg/Km.
SALR is not always constant or fixed at 0.6 deg/100 mtr, it is also dependent on Temperature. At higher temp, saturated air would hold large amount of water vapor. And when cooled would release more latent heat and slow down the cooling effect. Hence higher the temp of saturated air; lesser would be the value of SALR. SALR is lowest at low height/low lattitude
Notice that in all cases DALR is always more than SALR.
6. ELR: Environmental lapse rate. We have studied in lesson 1 of Met that as per International Standard Atmosphere, the sea level temp is + 15 deg and temp reduces with height at a rate of 6.5 deg/Km till tropopause. But this ideal conditions may not and will not exist everywhere and every time. In real world, at different geographical location on earth, at different altitudes, the temperature lapse rate would not be the same as ISA conditions. This is what is called Environmental Lapse rate or ELR. ELR would govern that when a parcel of air is forced to move upwards, the environment around this parcel is cooler/hotter/or at the same same temp; which would further decide the future path of the parcel of air. And would decide the Atmospheric stability/instability. DALR and SALR values are fixed. It is the ELR which varies with place, time and altitude. Now Let us discuss, one by one the conditions for Atmospheric stability.
7. Absolute Stability: Atmosphere is said to be absolutely stable if a parcel of dry/saturated air which is forced to move upwards falls back to original position. This will happen only if the parcel of air is cooler/heavier than the surrounding atmosphere. This is possible when the parcel of air is cooling at a faster rate than environment. Or you can say DALR is more than SALR, SALR more than ELR.
8. Absolute Instability: Atmosphere is said to be absolutely unstable if a parcel of dry/saturated air which is forced to move upwards, continues to move upwards when left. This will happen only if the parcel of air is hotter/lighter than the surrounding atmosphere. This is possible when the parcel of air is cooling at a lesser rate than environment. Or you can say SALR is less than DALR, DALR less than ELR.
9. Conditional Instability: Atmosphere is said to be conditionally unstable if it is stable for dry parcel of air and unstable for saturated parcel of air. And by now you can understand that it will happen if ELR is in between DALR and SALR.
DALR more than ELR, ELR more than SALR.
10. Neutral Stability. Atmosphere is said to be neutrally stable if a parcel of dry/saturated air which is forced to move upwards, remain at the same position where it is left. This will happen only if the parcel of air has the same temp than the surrounding atmosphere. This is possible when the parcel of air is cooling at the same rate as environment. Or you can say ELR=SALR (for neutral stability of saturated air). ELR=DALR (for neutral stability of dry air).
20 сен 2024
