Stability AND Instability PDF

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Stability AND Instability by SA sir...

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STABILITY AND INSTABILITY CONCEPT OF EQUILIBRIUM: 1. STABLE EQUILIBRIUMWhen air parcel is in a new position, & subjected to forces which tend to restore it to its original position, the atmosphere is considered to be in stable equilibrium. 2. UNSTABLE EQUILIBRIUMTends to continue in direction of displacement. 3. NEUTRAL EQUILIBRIUMIf in its new position, it is subjected to no forces restoring either to its original level or displace it still further than atmosphere is in north east.

ADIABATIC PROCESS OF TEMPERATURE CHANGE

Air mass heated at surface of earth—expands, less dense & light— surrounding cold air forces it to rise—rising air enters into a region pressure from high pressure and thus expands more due to leads pressure—GAS LAWS—temperature drops even though no heat has been removed – ADIABATIC COOLING--- air cools down & begin to fall --- as it sinks, it moves from low pressure to high pressure--- temperature rises (no heat added)— ADIABATIC WARMING.

GAS LAWS: a. Increase in volume of ascending air parcel b. When an object expands it needs energy c. If energy is not available outside, object—COOLING uses its own internal energy.

In adiabatic process, temperature changes but heat content is same, no exchange of heat between an ascending air & its environment.

DESCENDING AIR PARCEL Goes to high pressure—compressed & heated—gets warmed at the adiabatic lapse rate of 1-degree Celsius for every 100 m of descent. Adiabatic temperature changes are brought solely due to changes in air pressure.

ADIABATIC LAPSE RATES: The rate of change of temperatures in an ascending & descending air mass through adiabatic process is called the adiabatic lapse rate. Lapse rate of saturated & unsaturated air parcels are not the same.

DRY ADIABATIC LAPSE RATE: When ascending/ descending air parcel is dry or unsaturated, its temperature changes at a constant rate. It cools at 9.8 c from each km. this cooling rate is called dry adiabatic lapse rate. WET ADIABATIC RATE: Reduced rate of temperature change caused by addition of latent heat of condensation is called wet/moist adiabatic lapse rate (latent heat of condensation)— warming of rising air—rate of cooling reduced. This lapse rate depends upon temperature of saturated parcel —increasing temperature—more water vapour in saturated parcel—more condenation – less net cooling—smaller lapse rate.

It cools at 6 degrees Celsius per km.

INSTABILITY: ATMOSPHERIC INSTABILITY: When normal lapse rate> dry adiabatic lapse rate of ascending parcel of air, the rising air continues to rise upwards and expand & thus become unstable. But if normal lapse rate< parcel’s lapse rate—atmosphere is stable. Conditions associated with instability: 1. CONDITIONAL INSTABILITY: Wet adiabatic lapse rate< normal adiabatic lapse rate (6 degrees Celsius) < dry adiabatic rate (9.8 degrees Celsius). Initially air forced to move upwards at Dry adiabatic lapse rate (for ex- 25 degrees Celsius)

After reaching a height gets becomes saturated (at 25 degrees Celsius) Latent heat of condition returns back to rising air —result into warming of rising air—reduces rate of cooling—hence it cools at wet adiabatic lapse rate (6 degrees Celsius). 2. ABSOLUTE INSTABILITY: Wet ALR< dry ALR< normal ALR Wet and dry air as they go up, will receive much heat & they can rise at very great heights, instability prevails. There, doesn’t matter whether air is saturated or not a parcel disturbed upward from equilibrium position cools more slowly with altitude that does its surrounding or not. It therefore finds itself warmer than the surrounding air & continues to rise away from its equilibrium position.

3. MECHANICAL STABILITY: Abnormal conditions when normal lapse rate is exceptionally very high (15- 35 degrees Celsius per 1000 m). in such conditions, upper layers of atmosphere becomes exceptionally cold & dense than the underlying layers, thus resulting cold and denser upper layers to descend automatically. This is called mechanical instability & helps in formation of TORNADOES.

STABILITY: Atmospheric stability is defined “as that condition in the atmosphere in which vertical motions are absent or definitely restricted”. DALR of an ascending dry air> NLR & if it not saturated & does not attain dew point it becomes colder than surrounding air at certain height (becomes heavier & descends). This process causes stability of atmospheric circulation due to which vertical circulation of air is resisted. a. ABSOLUTELY STABLE: Normal ALR< wet ALR< dry ALR As dry and wet ALR cool more faster than normal ALR thus air will not be able to rise—no clouds—no rainfall. Thus stability condition. 4 ways to change to atmospheric stability: a. Heating from below (unstable atmosphere) b. Cooling from below (stable atmosphere) c. Cooling aloft (unstable atmosphere) d. Warming aloft (stable atmosphere) e. Movement of air & daily temperature changes f. Smaller lapse rate promotes stability

g. Steep lapse rate renders air more unstable. h. Temperature inversion produces most stable conditions in the atmosphere. SIGNIFICANCE OF ATMOSPHERIC STABILITY & INSTABILITY: Different forms of precipitation (dew, fog, rainfall, frost, snowfall, hailstorm etc). it also depends on stability & instability of atmosphere.

RELATIONSHIP

BETWEEN

STABILITY,

LAPSE

RATES AND

ALTITUDE RELATIONSHIPS: Y= prevailing lapse rate Yd= dry adiabatic lapse rate Ym= saturation- adiabatic lapse rate. 1. Y< yd Column of air is said to be stable when, its prevailing lapse rate is less than the dry- adiabatic lapse rate 2. Y>yd Column of air said to be unstable when prevailing lapse rate> dry ALR 3. Y=yd Said to be neutral equilibrium when prevailing LR= DALR 4. Yd>y> ym Conditional instability when prevailing LR lies between the value for DALR and SALR/ WALR....