EAS 308, Meeting 4
Stability in the atmosphere:
To this point, we have been studying the large scale motions in the atmosphere: the
polar-front jet-stream in Chapter 1, low and high pressure regions in Chapter 2 and air
masses and fronts in Chapter 3. These large-scale motions flow primarily horizontally.
Now we study the vertical motions in the atmosphere. Briefly, a stable atmosphere
suppresses vertical motions and an unstable atmosphere enhances vertical motions.
Strataform clouds are a visible manifestation of a stable atmosphere while cumulform
clouds are a visible manifestation of an unstable atmosphere.
To determine atmospheric stability you must know the vertical variation of temperature
and moisture. The measurements are routinely made around the world using a radartracked balloon-borne instrument called the radiosonde. The measurements can be
understood in terms of lapse rates: the way temperature varies with altitude.
The environmental lapse-rate: The actual variation of temperature with altitude.
The dry (unsaturated)y-adiabatic lapse rate: The variation of temperature within a rising
or sinking air parcel that is unsaturated (no condensation). The lapse rate is about
10C/km.
The saturated-adiabatic lapse rate: The variation of temperature within a rising or
sinking air parcel that is saturated (contains condensate, eg. cloud droplets). The lapse
rate, near the earth’s surface, is approximately 6C/km. The rate is less than the dry-rate
because of the latent heat released by the condensing water vapor.
Graphical representation of lapse rates: See Figure 46. The height units are in
thousands of feet and the temperature units are in Fahrenheit.
A definition of stability: An air parcel is stable if it returns to its original level after an
initial vertical displacement. To determine the stability of the atmosphere, we must study
and understand the adiabatic diagram. The one used most in the USA is the Skew-T Logp diagram (the author describes the British Tephigram which is similar). You can obtain
a Skew-T diagram at en.wikipedia.org/wiki/Image:Skew-T.gif.
Using this diagram sketched on the board I will define the following stability states:
1. Absolute instability
2. Neutral stability
3. Conditional stability
4. Absolute stability
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Also, I will show you how to determine the base and top of a cumulus cloud:
This discussion will lead to Assignment 4
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