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CE-2036-ENVIRONMENTAL-IMPACT-ASSESSMENT-OF-CIVIL-ENGINEERING--PROJECTS--->View question

Write a note on Atmospheric stability and temperature inversions?

(JUNE/JULY 13, DEC 2013/JAN 2014, JUNE / JULY 14)


Asked On2017-05-17 16:45:07 by:manjarimattur

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ATMOSPHERIC STABILITY AND TEMPERATURE INVERSIONS
LAPSE RATE:
In well mixed air which is dry for every 100m MCI
case in altitude, the temperature
decreases by about 1o C. This vertical temperature
gradient is known as dry adiabatic lapse rate
(DALR). Ambient and adiabatic lapse rates are a mea
sure of atmospheric stability. Since the
stability of the air reflects the susceptibility of
rising air parcel to vertical motion, consideratio
n
of atmospheric stability or instability is essentia
l in establishing the dispersion rate of pollutants
.
The atmospheric is said to be unstable as long as
a rising parcel of air remains warmer
than the surrounding air.
Conversely, when a rising parcel of air arrives at
an altitude in a colder and denser state
than the surrounding air, the resultant downward bu
oyancy force pushes the parcel of air
downward (Earthward) and away from the direction of
displacement. Under such conditions
atmosphere is said to be stable.
Stability is a function of vertical distribution o
f atmospheric temperature and plotting the
ambient lapse rate can give an indication of the st
ability of the atmosphere. Though dry, moist or
wet adiabatic lapse rate may be used, in such a com
parison, the DALR issued as the measure
against which several possible ambient lapse rates
are plotted. Thus the boundary line between
the stability and instability is DALR
When ALR exceeds DALR, the ALR is said to be SUPER
ADIABTIC and the
atmosphere is highly unstable. When the two lapse r
ate are exactly equal, the atmosphere is said
o be neutral. When the ALR is less than the DALR,
the ALR is termed SUB-ADIABATIC and
the atmosphere is stable. If air temperature is con
stant throughout a layer of atmospheric, ALR
=O, the atmospheric layer is described as isotherma
l and the atmosphere is stable.
When temperature of the ambient air increases rath
er than decrease with attitude, the
lapse rate is negative or inverted from the normal
state. Negative lapse rate occurs under
conditions, commonly referred to as an INVERSION, a
state in which warmer air blankets colder
air.
NVERSIONS:
When the reverse or negative lapse rate occurs, a
dense cold stratum of air at ground level
gets covered by lighter warmer air at higher level,
This phenomena is known as INVERSION.
During inversion vertical air movement is stopped a
nd pollution will be concentrated beneath the
inversion layer i.e. in the denser air at ground le
vel. As a result, during temperature inversion, the
atmospheric is stable and very little turbulence o
r mixing takes place under such conditions
pollutants in the air do not disperse.
Inversion is a frequent occurrence in the autumn a
nd winter months and the accumulation
of smoke and other contaminants further aggravates
pollution by preventing the sunrays from
warming the ground and adjacent air. Fog is commonl
y associated with inversions, narrow
valleys are favourable to inversions. The horizonta
l air movement is restricted. At the time of
inversions, visibility is greatly reduced and conta
minants are at a maximum. Inversion occurs
when dT/dz is +ve, when n < 1 i.e. when the tempera
ture of atmosphere increase with elevation
instead of decreasing


Answerd on:2015-01-17 Answerd By:Nikhil-bharadwaj

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