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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.

Tapas Bhattacharya
Web-project : Phys-645, Fall-2007, UAF
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