Earlier in this course we learned that stable atmospheric
conditions can lead to a buildup of air pollutants. Thunderstorms
on the other hand form when the atmosphere is unstable. We are
going to look at
this topic in a little more detail. Meteorologists look for
situations that might make the atmosphere unstable because this could
warn of severe weather.
We'll start with a relatively easy
to understand conceptual test
that we can perform to determine whether the atmosphere is stable or
You simply take some air and
it in a parcel. At this point the air inside the parcel is
exactly the same as the air outside (same temperature, density, and
pressure). We'll assume that once the air is sealed in the parcel
it can't mix
or exchange heat with the surrounding air.
Next you imagine lifting the
air. The air parcel will expand and the air in the parcel will
cool somewhat. After being lifted, the air inside the
parcel may have a different temperature than the surrounding air
outside the parcel. This is because the inside the parcel is
"insulated" from the surrounding air
In some ways this is like getting inside an automobile and driving
up to the top of a mountain. You might have the heater or maybe
the air conditioner running in the car. In either event, once you
arrive at the mountain summit the air inside the car would probably
have a different temperature than the air outside.
Once we have lifted the air we let go of the parcel and watch to
see what happens next. If the air comes back to where it started,
atmosphere is stable. If the air continues to rise the atmosphere
In the figure above the air in the
parcel has ended up colder and denser than the surrounding air.
In this case the parcel would sink back to the ground.
In the analogy shown above at right you can imagine giving the
the picture a shove then watch to see what happens. In the
situation shown above the rock would roll part way up the slope but
stop, turn around, and come back down to where it started.
Now the lifted air parcel has found
itself warmer and less dense than the surrounding air. It will
continue to float upward on its own. This indicates an unstable
We need a little more information to be able to perform the test
described above. First we need to know how quickly a rising
parcel of air will cool.
Unsaturated air (relative humidity less than 100%) always cools at
rate of 10o C/km. This is known as the dry adiabatic lapse
rate. The term lapse rate just means rate of decrease with
increasing altitude, adiabatic means that heat is not being exchanged
between the air inside and outside the parcel.
Saturated air cools a
little more slowly, we will use an average rate of 6o C/km (the moist
adiabatic lapse rate). As
saturated air rises, expands, and cools, condensation releases latent
heat inside the parcel. The latent heat energy offsets and
reduces the cooling due to expansion. There isn't enough latent
heat energy to cause the rising parcel to warm.
We also need to know the temperature of the atmosphere at different
altitudes above the ground.
The atmosphere can do just about anything. The middle figure
shows temperature decreasing at a rate of 8o C/km (the environmental
lapse rate). The left and right examples show the air cooling
more slowly and more rapidly, respectively, with increasing altitude.
Usually atmospheric temperature doesn't decrease at a uniform rate
as shown above. A more realistic example is shown below.
A plot of temperature versus
altitude is called a sounding.
We now have all the tools we need.
In this first example we assume the
environmental lapse rate is 4o C/km. This is shown in the
column of figures in the figure above. The next two columns show
the temperature inside rising parcels of unsaturated (green) and
saturated (orange) air (they cool at 10o C/km and 6o C/km). The
environmental temperatures and the parcel temperatures are also plotted
on a graph on the right side of the figure.
The parcel curves (green and red) lie to the left of the purple,
environment, curve. Rising parcels of unsaturated or saturated
air will both end up colder and denser than the surrounding
environmental air. If they are lifted and released, they will
sink back to the ground. The atmosphere is absolutely stable in
We'll change the environmental lapse rate to 11o C/km in Example #2.
Now, because the atmosphere is cooling so quickly with increasing
altitude, lifted parcels of both unsaturated and saturated air
end up warmer and less dense than the surrounding air. Both the
orange and green curve lie above and to the right of the purple curve
on the graph. When released these parcels will continue to rise
on their own. The atmosphere is absolutely unstable in this case.
We'll pick an intermediate value, 8o C/km, for
the environmental lapse
rate in the next example.
The atmosphere is conditionally
unstable. A rising parcel of unsaturated air ends up cooler and
denser than the surroundings. A parcel of saturated air, which
cools at a slower rate, ends up warmer than the air around it.
The condition for instability is that the air must be saturated.
It was a little harder coming up with a rock/hill analogy in this
case. The condition for instability in that case is the direction
of the initial push that you give to the rock.
We'll leave the environmental lapse rate the same for the last and most
In this case the parcel of air
starts out unsaturated. It becomes saturated when lifted to 1.5
km altitude, once it has cooled to a temperature of 0o C. This is
called the Lifted Condensation Level (LCL) and you would see cloud
begin to form at this point. From that point on
upward the rising parcel will cool at the moist adiabatic rate.
Initially the rising parcel is colder and denser than the surrounding
air. If the parcel is lifted to 3 km it has the same temperature
as the air around it. If lifted above 3 km the parcel air finds
itself warmer and less than the air outside. If lifted just a
little bit beyond 3 km altitude the parcel will be able to continue to
rise on its own. 3 km in this case is the Level of Free
The atmosphere is conditionally unstable in this case. A
rising parcel must first of all become saturated. Then it must be
lifted to and just above the level of free convection.
The value of the environmental lapse rate is one of the main
that determines whether the atmosphere will be stable or unstable.
(eg. 4o C/km)
DALR & the MALR
(eg. 8o C/km)
(eg. 11o C/km)