Stable or Unstable Atmosphere, Water Vapour, Wind and Turbulence

Take off, Cruise and Climbing Performance

A property of air is that it can be compressed. Its motion develops as a result of spatial differences in atmospheric pressure. Most likely because of uneven absorption of solar radiation. 

A stable atmosphere resists any upward or downward displacement, while unstable allows for upward or downward disturbance to grow into vertical or convective current which can vary throughout the day.

Convective heating and cooling occurs when cooler, less dense, lower pressure air sinks and hotter, more dense, higher pressure, air rises. In regards to your flight, surface heating, cooling aloft, converging or upslope winds and fronts (where two different masses of air of differing pressure and temperatures converge) can affect your flight.

These masses of ascending and descending air create areas of low and high pressure. As air warms it ascends leaving low pressure at the surface. As air cools it descends causing more dense and higher pressure air at surface. Generally, low pressure brings unstable weather and high pressure leads to stable weather conditions. In northern hemisphere winds blow clockwise in high pressure and anticlockwise in low pressure due to earths rotation and coriolis effect.  

Surface Pressure Charts can help you identify High and Lows of where you will be flying. The lines drawn on surface charts connecting areas of equal pressure are called isobars.The closer together the isobars are the faster the wind speed will be. Turbulence can be experienced when flying in air that is being displaced either by convective currents, other aircraft or other obstacles that influence the air path.

Picture an invisible section of air called an air parcel. If you compare the temperature of this air section to the temperature of air surrounding it, you can tell if it is stable (likely to stay like that) or unstable (likely to move and weather will change).

There are three things to take note of when determining stable or unstable:

1) Saturation (unsaturated air is relative humidity less than 100%)  This will determine how quickly a rising parcel of air will cool and which lapse rate will apply, generally moister air will cool less rapidly than dry.

2) Dry adiabatic lapse rate is 3 degrees per 1000 feet up to 36,000 ft.

3) Saturated adiabatic lapse rate is 1.5 degrees per 1000 feet at low levels, reverts back to 3 at higher altitude.

Whether the air sinks or rises depends on plotting the existing environmental  temperature of surrounding air and the dry adiabatic lapse rate relative to increasing height.  

In lower density air your plane will fly FASTER. Clouds give a good indication of what the air is doing. Below is a chart for pollution but is a good visual of cloud shape.

Air Density has a significant effect on aircraft performance because as air becomes less dense it reduces power to engine, thrust as less air for propeller and lift as thinner air is less force on airfoils keeping you in fight.

Stable Air = Air that sinks 

Unstable Air = Rising Air