Stability and the Weather
By KD1LD

In general, fog or horizontal development and uniformed thickness of clouds indicates stability of the atmosphere. We can recognize these signs of stable weather on the adiabatic chart also know as the rayob. The radioson gives us the existing temperature aloft. When you plot them on the rayob, it
is called the existing lapse rate line. It also gives us the relative humidity readings to convert to the dew-point line. The rayob itself provides the dry adiabat. Which represents the constant rate of three degrees centigrade per thousand feet, at which dry air cools adiabaticly as it rises. And the moist
adiabat which represents the constant rate of two degrees centigrade per thousand feet, at which saturated air cools adiabaticly as it rises. By comparing the existing temperature line with the dew-point line we can determine the levels of dry and saturated air. Where the two lines coincide
through a layer, we know that we have saturated air. Thus we can recognize clouds on a rayob chart. We determine the stability or instability of each layer by comparing the existing temperature line with one or the other of the adiabat. Where the existing air is dry we compare the existing temperature line with the dry adiabat. And were the existing air is saturated we compare it with the moist adiabat. Let us first consider dry air. The existing laps rate can be less than the dry adiabat or greater than the dry adiabat. If it is less than the dry adiabat, any parcel of air lifted adiabaticly would be cooler than the surrounding air so it would return to it's original level. This air would be stable. If the existing laps rate is greater than the dry adiabat any parcel of air lifted adiabaticly would be warmer than the surrounding air so it would continue to rise. This air would be unstable. Now
we will consider moist air. If the existing laps rate is less than the moist adiabat any parcel of air lifted adiabaticly would be colder than the surrounding air, so it would return to it's original level. This air would be stable. If the existing laps rate is greater than the moist adiabat any parcel of air lifted adiabaticly would be warmer than the surrounding air, So it would continue to rise. This air would be unstable. Thus any existing laps rate greater than three degrees centigrade per thousand feet would indicate instability, weather the air is dry or moist. Any existing laps rate less than two degrees centigrade per thousand feet would indicate stability, weather the air is dry or moist. Any existing laps rate that is greater than two degrees but less than three degrees centigrade per thousand feet would indicate conditional stability. This means that if the air is dry, it would be stable if the air is moist, it would be unstable.

Lets make an example. Lets look at the adiabatic chart. There are now two lines on the chart. The laps rate line and the dew-point line. In the first layer notice the temperature and the dew-point are the same. This layer is saturated. The saturated layer at the surface would indicate fog. The temperature increases with altitude so we would also have an inversion. Since the lapse rate is less than two degrees centigrade per thousand feet we know that this layer would be stable. Fog and inversion layers are typical indicators of stability. In the second layer the air is dry and the temperature remains the same with an increase in altitude. This is called an iceothermel layer. It to would be stable. Iceothermel layers are typical indicators of stability. In the third layer, also dry, the temperature decreases one degree per thousand feet, this layer would be stable.

Lets make another example. In the first layer we notice the existing lapse rate is four degrees centigrade per thousand feet. This air would be unstable. In the second layer the air is saturated, and the existing lapse rate is three degrees per thousand feet, thus the air would be unstable. These conditions would indicate a cumuaformed layer of clouds. If the existing lapse rate is only one degree per thousand feet, this layer would be stable and the cloud formation would be stratiform.

Atmospheric stability is the key stone to all weather in the troposphere. it links together all concepts of temperature, moisture, pressure, and clouds.