Stability
- II

**Gravitational
Stability & Gravitational
Instability:**

**As dry air goes up, it’s temperature
decreases
approximately at a rate of 1˚C/100m. This happens as the gas gets a
scope to
expand at a higher altitude. This temperature gradient is called ‘dry
adiabatic
lapse rate’. we can estimate the stability of an air mass by comparing
this
‘lapse arte’ with the vertical temperature gradient in the atmosphere.
**

** **

**In the above diagram the red line
represents the
atmospheric temperature gradient and the blue line shows the ‘dry
adiabatic
lapse rate’. Both are negative, but the air-mass is much colder at the
higher
altitude (above the point of intersection, in the plot). So, as the
mass goes with acceleration, from a position below the point of
intersection to
a point above it, the air-mass experiences a net force in the downward
direction and falls back.
Practically
it bounces back and forth. This is an example of ‘gravitational****
stability’.**

In
this diagram, the green line represents the
atmospheric
temperature gradient and the blue line shows the ‘dry adiabatic lapse
rate’.
Though both are negative, at any particular altitude the air mass
remains
warmer. So, the mass continues to go up with acceleration, if it
remains unmixed
with the surroundings. This is an
example of ‘gravitational instability’

**Dynamics for
moist air:**

**
**

**Still now we
discussed about the dry air.If the air-parcel is humid it contains some
water
vapor. So, as this air-parcel rises up
and becomes colder, it is more and more towards the saturation. After
being
saturated, further cooling in the atmosphere causes condensation of the
water
vapor. The latent heat thus released is absorbed by the rising air.
This is the
reason the rate of cooling of a saturated air-mass is less compared to
it’s
unsaturated counterpart.In this situation the atmosphere is stable if
it is unsaturated but it is unstable if saturated.**

**This
conditioanally unstable atmosphere, which is cold at the upper level
and warm and moist at the lower level, supports the formation of
convective cloud.**