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Mother Nature's lapse rate
limits 
There are three inherent properties of the atmosphere that all pilots
should be able to discuss without hesitation.
1. Here's the easy one. All pilots know that warm
air is always
less dense than cold air. If you are having trouble with
this one, take a ride in a hot air balloon.
2. Not as intuitive, but pilots should understand that moist
air is always
less dense than dry air at the same temperature. Golfers
understand this one…the more moisture in the air, the further you can
drive the ball.
3. Lastly, pilots should be aware that when air ascends, it
will always
expand and cool. Just take a look at any developing cumulus
cloud
and you’ll witness this process in
action.
There are very few topics in meteorology that
don’t have
their origins in these three basic properties of the
atmosphere.
Becoming comfortable with these basic properties will pave the way to
learn more advanced concepts of weather. Once you've
convinced
yourself that you have mastered all three, it’s time to have a serious
conversation about a little more advanced topic, namely, lapse
rates. The discussion to follow may seem very technical at
times,
but all pilots should strive to comprehend its content.
A lapse rate is simply the
change of atmospheric temperature over a given change of pressure or
altitude. It wouldn't be surprising if the term lapse rate
conjures up a thought about the standard
lapse rate. All pilots should be familiar with the standard
atmospheric lapse rate. That is, for every 1,000 feet
increase in altitude, the temperature decreases by 2
degrees Celsius on
average. Remember that? Now, forget
it.
That’s right…from a meteorological perspective the standard lapse rate
is a parameter that you should put on the shelf. In fact, on
any given day the actual environmental temperature rarely
matches the standard lapse rate. The Skew-T log (p) diagram
on the left shows a rare case where the lapse rate is nearly standard.
Click here to view a larger image.
Routine use of the standard lapse rate should be used only to
determine how far the environmental temperature has deviated from
standard when using all of those rarely used performance tables in the
pilot’s operating handbook (POH). Using the standard lapse
rate to calculate the freezing level, for example, will leave you
sorely disappointed most of the time. Moreover, the actual
environmental lapse rate may be
greater than the standard implying a lower freezing level than you
calculated!
How much greater? You may be surprised to learn that the
environmental lapse rate in unsaturated air can be as large as 3
degrees for every 1,000 feet gain in altitude – that’s one whole degree
Celsius greater than the standard! This is referred to as the
dry adiabatic lapse rate (DALR) and it is actually very common,
especially within the first several thousand feet above the surface
during the afternoon hours as shown to the right (click here to view the complete
diagram). The DALR is essentially nature’s unsaturated limit
in the atmosphere. Once the atmosphere reaches the DALR, it
stops right there and can be no greater.
There is one exception,
however. It is called a super-adiabatic lapse rate – a
lapse rate
greater than 3 degrees Celsius for every 1,000 feet gain in
altitude. It is most observed on clear,  dry
days when surface heating by incoming shortwave radiation is the most
intense. A super-adiabatic lapse rate is very shallow,
typically extending no greater than 500 feet above the surface as shown
in the red square on the left diagram (click here to view the complete
diagram). It is created from an imbalance between
the rate at which air adjacent to the ground is heated by conduction
and the rate at which dry convective eddies or thermals can transport
the heated parcels of air upward. These convective eddies are
not able to transport heat upward fast enough to maintain the
DALR. In other words, the surface heats up quicker than the
atmosphere can move away the heat. Such a super-adiabatic
lapse rate drops off very quickly with height above the
ground where it then quickly transitions to the DALR above about 500
feet.
When the air is saturated or
slightly super-saturated the rules of the game change
slightly. When
the air is saturated as it is in the diagram on the right, the
moist adiabatic lapse rate (MALR) is Mother Nature's limit. Click here to view the complete
diagram.
Unlike the DALR that is a constant,
the MALR
varies with temperature. For very warm temperatures, the MALR
is significantly less than the DALR. For very cold
temperatures, the MALR nearly equals the
DALR. So there’s no way to express a specific rate since the
MALR changes depending on the temperature.
Is it possible to have a
saturated environment where the temperature and dewpoint cool off at a
rate greater than the MALR? Yes, it is physically realistic,
but not a common occurrence in layers over 100 mb (3,000 feet)
deep. Such saturated lapse rates typically occur in
proximity to the zone where the inflow to thunderstorms (mesoscale
convective systems) is being
lifted by the thunderstorm complex’s moist downdraft outflow.
It is somewhat common to see these excessive (and unrealistic) lapse rates
on radiosonde observations (RAOBs). When a
radiosonde rises up and leaves a cloud, the wet or ice-covered sensor
in a radiosonde package can continue to report saturated
conditions. If the air above the cloud is excessively dry (as
it is for the RAOB sounding on the left), evaporative cooling can cause
the sensor to report a saturated lapse rate greater than the MALR (click here to view the complete
diagram). It is also very common to see numerical weather
prediction model analyses and forecasts depict a saturated lapse rate
greater than the MALR in shallow layers. These are also
likely unrealistic.
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