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about Atmospheric Stability
Perhaps one of the
most confusing aspects of meteorology for most beginning students is
the concept of atmospheric stability.
Atmospheric stability is certainly a complex
concept! Stability in the atmosphere is usually described in terms
of 'lapse rates'. An understanding of latent heat is
also imperative to conceptualizing atmospheric stability, therefore
this page will focus on both lapse rates and latent heat
as precursors to our understanding of atmospheric stability.
Even before we begin here, it is highly recommended
that you visit our "Water
& Weather" page. There you will learn some of the important
properties of water that will greatly enhance your understanding of
atmospheric stability here.
Lapse rate defines the way in which temperature varies with
altitude. Lapse rates come in two
general flavors: The Environmental Lapse Rate (ELR) and The
Adiabatic Lapse Rates. The latter
comes in two flavors of its own: The Dry Adiabatic Lapse Rate (DALR)
and The Saturated
Adiabatic Lapse Rate (SALR) sometimes called The Wet Adiabatic Lapse
Rate (WALR) and sometimes it is also called The Moist Adiabatic
Lapse Rate (MALR)! Ugggg! We'll just call it SALR.
The ELR is the actual variation of temperature with
height at a certain time and place. It varies from place to place
and from time to time so there really is no fixed rate.
Meteorologists measure temperatures through a vertical profile by
releasing weather balloons with mini-weather stations attached to
them called radiosondes. Sometimes, meteorologists drop these
mini-weather stations from a plane at high altitude with a parachute
attached. These latter type of measuring devices are called
The word 'adiabatic' refers to a thermodynamic
process in which heat neither enters nor leaves the system in
question. Yet temperatures drop with altitude in the troposphere.
Why is that? Considering the fact that no heat leaves the system,
how is it possible that air gets cooler with altitude?
The following illustration shows how pressure
decreases with increasing altitude. The reason pressure is lower
with altitude is due to the fact that there are fewer 'air
molecules' at altitude than there are closer to Earth. Air becomes
thinner (air molecules become increasingly scarce) as you go up.
As a result, air expands. The following illustration opens in
separate window. Be sure to close the window to return to this page!
This is a good point to list a couple important laws
of physics! These links will open in a new window, so just close the
windows when you are ready to return to this page!
The Law of Adiabatic Expansion/Compression
The Law of Hydrostatic Balance
Since pressure decreases with elevation, rising air
Law of Buoyancy) will expand. There will be less inward pressure
relative to outward pressure exerted on that parcel. The process of
expansion effectively takes up some internal energy of that parcel;
this energy is heat energy, therefore the result is removal of heat
translating in lower temperatures. The energy is still there (it is
neither created or destroyed). It is simply not detectable by
measuring instruments (thermometers)!
The Ideal Gas Law!
Temperature doesn't just suddenly drop. It drops at
a rate (-deg/elev) known generically as the Adiabatic Lapse Rate. So
what is this rate? It depends on whether the air parcel ascending
into the sky is saturated or not. If it is 'dry' (not saturated), it
will rise and expand experiencing temperature drop as heat energy is
absorbed during the expansion process. If, on the other hand, it is
saturated, then it will rise, expand and reach its dew point
temperature causing the water vapor in it to condense on
During condensation, water vapor is undergoing a
phase change from gas to liquid requiring that every gram of water
vapor release 539 calories of heat energy (read more about this in
our "Weather's Water" section). This heat energy is released thus
lowering the rate at which the air parcel cools as it ascends. For
example: if air cooled 10 units per 1000 feet (lost 10 units of
heat), then condensed and released 3 units of heat, then we would
subtract 3 units from the original 10 giving us a slower rate of 7
units per 1000 feet.
The dry adiabatic lapse rate is 9.8ºC
per kilometer (9.8ºC/km). This rate is constant until the air parcel
in ascent becomes saturated (reaches its dew point temperature). As
explained above, once dew point is reached (saturation), latent heat
is released as a result of condensation (gas to liquid change of
state of water) and the rate drops.
saturated adiabatic lapse rate is variable since it largely depends
on how much latent heat is made available within the air parcel as
its moisture condenses. Generally, the higher the parcel ascends,
the greater the amount of heat released by condensation since
expansion is more pronounced (pressure decreases with elevation).
Temperature will also affect this rate. Another thing to keep in
mind is that as the parcel condenses and continues its ascent, less
and less water vapor will remain to be condensed meaning that less
heat energy will be released. In the lower elevations the SALR
varies from about 3.9ºC/1km when ambient temperature is around 26ºC
to about 7.2ºC/1km when ambient temperature is around -10ºC.
Considering the fact that less water vapor is available for
condensation after prolonged ascent, the SALR will eventually
increase to the point where it is almost identical to the DALR
(9.8ºC/1km)! This is most likely to occur at very high elevations
where ambient temperature can drop below -40ºC! That sounds pretty
cold eh?! Consider that when you fly across country, the air
temperature outside your window is probably around -100ºF! Chilly!
should note here that in a beginning course, you will probably be
taught the average SALR, which is between or at 5-6ºC/1km, an
approximate average of the extremes.
Air is considered to be stable if it
descends to its original level after an initial vertical
displacement. Rising air parcels are considered to be unstable, and
can sometimes result in impressive cumulonimbus clouds! You may have
heard the term 'thermal' used before. A thermal is simply a term
given to a rising parcel of air. From this point on we will refer to
rising air parcels simply as thermals.
We know thermals won't ascend into space! So how far
do they get? It depends on numerous factors. We will start with the
very basic concept before we get into other factors. Let's begin
with a hypothetical example:
If a thermal's temperature near Earth's surface is
and ambient air temperature has been measured at 26ºC, and we have
determined by reading the data from the radiosonde we released into
the air that the environmental lapse rate (the actual temperature
through a vertical column of air) is 5.8ºC/1km, then we should have
no problem determining the point at which the thermal's ascent will
stop. For simplicity, let's say the thermal rises only at the DALR
thermal will stop its ascent as soon as its temperature equals the
ambient air temperature around it! After the first kilometer, the
thermal will have cooled due to expansion by 9.8ºC (the DALR).
Meanwhile, air around it will have cooled 6.8ºC (the ELR). This
means the thermal will be 19.2ºC and ambient air temperature will
also be 19.2ºC. At equal temperatures, the thermal no longer rises,
and stabilizes at 1km of elevation! If you have the free Flash
www.macromedia.com, you can see an example of what we just went
over by clicking>>
here. If the window
is blank, you need to download the free Flash plug-in!
Though a bubble was used to represent the thermal, the more likely
'shape' of a thermal would be like this >>>
THERMAL. For simplicity, a bubble is used to illustrate
Now let's throw in
another variable: momentum. The momentum of any object is just its
mass multiplied by its velocity, or mv. Momentum expresses the
persistence of movement, or inertia, of a body in motion. The
pressure exerted by a gas (in this case air of a thermal) is related
to the molecular motion of the gas molecules. When a molecule
collides with a surface (another molecule) it produces a force on
that surface. That force multiplied by the by the time duration of
the collision is called the impulse. Impulse is related to momentum
(both are properties of mass).
It is likely that
the thermal will have momentum on its side as it ascends. So even if
it reaches a temperature equal to that of surrounding air, it may
still ascend another 300 meters or so just from momentum! In this
scenario, the thermal will become slightly cooler than surrounding
air, but this makes it denser than surrounding air and it will soon
descend. Though, it is likely to have momentum on its side as it
descends now! In which case it will simply descend below the level
at which its temperature equals the temperature of surrounding air
and heat adiabatically causing it to become slightly warmer than
surrounding air. Again, it will begin to rise and may pass the point
again, each time having less and less momentum. See it here>>
Okay, now let's
throw another factor into the mix: moisture! We discussed the SALR
earlier, and learned that it is lower than the DALR due to release
of latent heat as water vapor condenses.
We'll focus on what its predicted
temperature should be at a given altitude. Let's say the thermal has
a temperature of 25ºC
at Earth's surface, and we have determined that SALR is 5.5ºC/1km
(average rate taught in most courses). What would the thermal's
temperature be at 2km above Earth's surface if it rises at the DALR
for the first kilometer, and at the SALR for the second kilometer?
Check your answer here
We have covered DALR, ELR, and SALR, now
let's add one more: Dew Point Lapse Rate. Again, the exact rate
varies depending on atmospheric conditions, but the range is
relatively small nevertheless. NOAA meteorologist, Thomas Schlatter,
performed numerous calculations of dew point lapse rate under
various conditions and found that values range from 1.6ºC/1km
to nearly 1.9ºC/1km.
Let's refer to dew point lapse rate simply as DPLR. Okay, quick
recap before we move on:
DALR = dry adiabatic lapse rate = 9.8ºC/1km
SALR = saturated adiabatic lapse rate = 3.9ºC/1km - 7.2ºC/1km
ELR = varies from place to place and from time to time...
DPLR = 1.6ºC/1km - 1.9ºC/1km
is important to know dew point changes slightly with altitude since
it affects the level at which condensation occurs. This level is
officially termed the condensation level by meteorologists or
lifting condensation level (LCL). What is condensation level?
is the level at which moisture in rising air (thermals) starts to
condense and form droplets of cloud. It is the point at which dew
point temperature and air temperature become equal. You should
already be aware that dew point temperature is the temperature to
which air must be cooled to become saturated with water vapor.
Further cooling leads to the formation of cloud droplets or fog in
the atmosphere or the deposit of dew on surfaces. If air temperature
equals dew point temperature, relative humidity equals 100%.
is very nearly the base of any cumulus cloud. You can indirectly see
it best when there are large cumulonimbus or towering cumulus clouds
in the sky. Their flat bases are very nearly indicative of
condensation level! You'll notice there aren't any clouds below this
animation will illustrate an air parcel's orographic ascent up a
mountain slope, reaching condensation level, then proceeding up the
slope and descending the other side. This example is commonly used
in the classroom because it not only illustrates DALR and SALR, but
it also shows condensation level and DPLR as well! It also
effectively illustrates the cause of rain shadow desert
See it here >>click
Having watched the animation at
the link above, you are now aware of the lifting condensation level,
and how it can be considered to be the boundary delimiting areas
where a thermal will rise at DALR and where it will rise at SALR.
You have also learned that lifting condensation level is very nearly
located at the base of any CB (cumulonimbus) or TCU (towering
cumulus) cloud. If you have the free Flash plug-in, then check out
the following interactive animation depicting linear relationships
between DPLR, SALR, DALR and LCL! It opens in a new window,
As weather observers, there are times when we want
to know how high the base of these clouds are, especially for
purposes of aviation.
One way of course is to use special instrumentation,
such as a ceilometer used at Pierce College and at the Van Nuys
airport. However, there are times when no such instrument is
available, or there is a patch of blue sky over the ceilometer so
that the ceilometer thinks it's clear weather.
The way to determine cloud base without such
instrumentation available is to use a simple psychrometer. We can
calculate cloud base for cumulus clouds by basing our calculations
on Normand's Theorem. Simply multiply the difference between ambient
air temperature and dew point temperature by 400. Your answer will
be cloud base in feet above ground level. For example, if you take
the following measurements:
Air Temperature: 13.9ºC
Dew Point: 6.1ºC
13.9 - 6.1 = 7.8
7.8 * 400 = 3120
cloud base is approximately 3,120 feet above ground level.
should be noted here that this is restricted to morning to noon when
sunshine is warming the ground and the air in contact with it. It is
also restricted to cumulus and low stratus clouds. Beyond noon, and
with any other cloud, this equation only tells you the minimum level
a cloud base will be, but gives no indication of how high bases
Here in the San
Fernando Valley, I generally restrict this equation to times between
sunrise and about 10 to 11am when reporting cloud bases to aircraft.
We have covered DALR, SALR, DPLR, LCL and ELR. Now let's look at
inversions or as they are sometimes referred to as inversion layers.
An inversion is where air temperature increases with
altitude rather than the normal decrease in temperature in the
troposphere. Normally, the environmental lapse rate (ELR) shows us a
decrease in temperature with height. Earlier we learned that ELR is
variable from place to place and from time to time depending on
conditions, but that it is often defined as cooling with elevation
as a result of adiabatic cooling processes.
However, an inversion is where this cooling stops,
and a warming trend with height occurs. The following illustration
is adapted from the animation on the previous page you looked at
showing lapse rates' relationships with lifting condensation level.
The following illustration shows SALR, DALR, DPLR, and LCL, but it
also shows a new element: the ELR. It illustrates on one of
seemingly infinite amounts of possibilities available in nature!
The inversion in the above
illustration begins at 2,000 meters above the ground. From that
point upward, the actual air temperature increases with altitude.
This increase in temperature with elevation makes the atmosphere
above 2,000 meters very stable. This layer, or inversion layer as it
is called, acts as a 'lid' restricting the height to which a thermal
can rise. The thermal's temperature is relatively cooler than
ambient temperatures in the inversion layer, therefore the thermal
is technically denser than air in the inversion. As such, the
thermal will no longer be displaced by denser air around it and will
subsequently cease its ascent.
Anvil tops on cumulonimbus clouds most often are a
result of the cloud reaching an inversion layer. The inversion layer
for the really huge cumulonimbus clouds is the stratosphere where
ozone's absorption of solar radiation creates an inversion layer
many kilometers thick! Not all tops of clouds are necessarily the
base of inversions. Cut-off cloud tops may also be from wind shear
Okay, now the part you've all been waiting for...
Atmospheric stability comes in several "flavors". They include
Absolute Instability, Neutral Stability, Conditional Instability,
Absolute Stability, and Potential Instability.
Absolute instability occurs when ELR is greater than DALR. We
have learned that DALR is 9.8ºC/1km,
therefore we can conclude that absolute instability exists when ELR
is 9.9ºC/1km or greater. Meteorologists call this a "super-adiabatic
lapse rate" since heat loss with elevation is so rapid.
Neutral stability occurs when ELR and DALR are equal. That is,
when ELR is 9.8ºC/1km. It is called 'neutral' because the thermal
keeps its initial momentum and does not accelerate or slow down.
Conditional Instability occurs when ELR is less than DALR but
more than SALR. In other words, it is when ELR is between SALR
(which varies between 3.9ºC/1km to 7.2ºC/1km) and DALR (which is
9.8ºC/1km). The 'condition' for instability is only when the thermal
becomes saturated, not before.
Absolute stability occurs when ELR is less than SALR (and
therefore less than DALR). This means that ELR must be lower than
SALR (which varies between 3.9ºC/1km and 7.2ºC/1km) which will never
be more than 7.1ºC/1km (if SALR is at its maximum).
Potential Instability occurs when air at low levels (near the
ground) is moist, but is dryer higher up. The wet bulb temperature
(on a psychrometer) must decrease faster than the SALR. The
potential for instability is only realized when the thermal ascends,
most often up a slope of a mountain (orographic uplift), up a an air
mass front (frontal lifting), or by convergence at Earth's surface,
and reaches saturation. If you didn't see the animation on rain
shadow effect earlier, you can see an example of orographic uplift
by clicking >>here.
-by Steve W. Woodruff