Stability – tendency of the atmosphere to encourage or resits vertical movement
Lifting Forces:
- Airmass Movements (fronts)
- Geography wind blowing air up a mountain
- Surface heating
The lifting just start the movement and the atmosphere stability tend to continue the movement once the initial force is gone.
When volume of air is lifted it expand due to reduced air pressure this expansion causes cooling at a rate of 5.4F decrease per 1000ft of altitude dry air
The rate is called the lapse rate.
Atmospheric Stability /Instability is determined by actual lapse rate
As the lapse rate increases (the air cools at a faster rate with increasing height) the air becomes more unstable
There is a difference between the temperature lifted parcel and the ambient temperature
at sea level 70F for both but at 1000ft 64.6F for the lifted parcel and 67F for the ambient
if the ambient was 63F the lifted parcel will continue to rise because it is warmer, less dense unstable than ambient temp
the cooler the ambient the surrounding air the higher the parcel will be lifted
Humidity also plays a role in stability and instability, we know that humid air rises invisibly until temp & dew point converge at that point condensation occurs and we see the cloud base form. The precess of condensation release heat into the parcel of air and this alters the lapse rate.
A saturated volume of air cool more slowly its lapse rate is only 3.3F decrease per 1000ft of altitude gain, this reduced lapse causes a greater temperature differential between the lifted parcel and its surrounding air, the result is that it climbs faster and will result even higher.
Stability – measure of atmosphere ability to resist vertical motion
Lifting force – required to begin the process
Cooler surrounding air aloft continues the upward movement (when air is warmed from below, it rises and causes instability)
High humidity increases the tendency of the parcel to ascend
The lapse rate is the decrease in temperature with increased altitude
Turbulence and clouds with extensive vertical development result when unstable air rises
Stability is determined by the actual lapse rate
Warming air from below tends to decrease stability