January 15, 2010  

Greetings!

Mother Nature's lapse rate limits Cumulus clouds

     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. 

Standard lapse rate sounding     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
Adiabatic lapse rate diagram 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, Super-adiabatic lapse ratedry 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.
   Moist adiabatic lapse rate
     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.
Moist adiabatic diagram.
     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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