Atmospheric Stability

3 Answers

1. 
   

   
   

   SkyBoy98046 on Feb 11, 2011

   Yes, hazy day = stable atmosphere. but you need smog to have haze so you can have clear stable if you’re not in Los Angeles.

   Another way is simply to look at the pressure trend. A high barometric pressure is good weather, your in a high. If the trend shows an increase, your getting better weather. If it is low and getting lower, the bad stuff is coming.

   I’m sure the scientist would laugh at me but I’ve always theorized that the power or strength of a high pressure area holds down weather and forces stability. And a low is too weak to do that and the weather in a low does what it wants to. Kinda like two dudes wrestling, if you hold force, or pressure against the other guy, he can’t do much but if you apply low pressure he can overpower you and win the match. 😛 weird analogy huh? 😉

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2. 
   

   
   

   Steve Pomroy on Feb 11, 2011

   Hi Skyhawk.
    
   To answer your first question, you can indeed have stable conditions on a clear day.  You need some obstruction to visibility (such as the smog mentioned in Skyboy’s post).  Turbulence tends to scatter and dissipate these obstructions, but the absence of turbulence won’t create them — they have to be ther in the first place for the low visibility we associate with stable air to occur.  In the area where I currently fly, we routinely have inversions in the winter with silky smooth flying and crystal clear visibility.
    
   The key to determining stability is the environmental lapse rate, and only the environmental lapse rate (ELR) — as it compares to the Adiabatic Lapse Rate (ALR).  The reason the ELR is so important is because of it’s relationship to the dry and saturated adiabatic lapse rates.  Other factors, such as the presence (or absence) of turbulence, good (or poor) visibility, and cumulus (or stratus) clouds are simply the effects of stability/instability.  Some of them make good indicators when you are sitting on the ground trying to anticipate the smoothness of the ride you’re about to go on, but they are symptoms, not causes.
    
   When the ELR is low, the temperature of the air mass is dropping slowly as altitude is increased.  For example, if the lapse rate is 1C/1,000′, and the temperature is 15C at the surface, the temperature at 1,000′ AGL will be 14C.  Now consider what happens to a parcel of air as it rises due to some disturbance (wind over a hill, for example).  As the air rises, pressure drops, and the air expands.  As it expands, it exerts energy, and therefore it’s temperature drops.  If the air is dry, it will cool at about 3C/1,000′ as it rises.  So, starting at the surface at 15C, it will be 12C at 1,000′ AGL.  The prevailing temperature at 1,000′ AGL is 14C, but this rising parcel is cooler at 12C.  The lower temperature implies an increased density.  And so that parcel of air now will tend to sink back downward, as it is heavier than the surrounding air.  This is a stable situation as the displaced air parcel inhernetly tends to return to it’s original position.
    
   What about when the ELR is high?  Let’s use an example of ELR = 5C/1,000′ and surface temperature of 15C.  Now when we displace a parcel of air (wind over a hill again), it will once again reach a temperature of 12C at 1,000′ AGL.  But the surrounding air will be at 10C.  The warmer air is less dense, and will tend to continue rising.  At 2,000′ AGL, the rising air will be at 9C, but the surrounding air will have reached 5C.  As you can note, the difference in temperature is increasing as the rising air rises.  The rising air will continue to rise until it hits a stable ELR.  In extreme cases, this occurs at the tropopause (where the ELR goese to zero), and we get big thunderstorms.
    
   So, here are the rules:
   ELR > ALR = Unstable Air
   ELR < ALR = Stable Air
    
   Note here that the ALR for unsaturated air is different than the ALR for saturated air.  The Dry Adiabatic Lapse Rate (DALR) is about 3C/1,000′.  The Saturated Adiabatic Lapser Rate (SALR) varies quite a bit with temperature, but averages at around 1.5C/1,000′, and will always be lower than the DALR.  The difference is casued by the latent heat released by the water vapor as it condenses to water droplets.
    
   Because of this difference, the ELR boundary between stability and instability is 3C/1,000′ in unsaturated air, and about 1.5C/1,000′ in saturated air.  So for example, if you have an ELR of 2C/1,000′ (which is close to the average), dry unsaturated air would be stable, but saturated air would be unstable.  This is why, if you fly IFR, turbulence often intensifies when you enter cloud.
    
   You should also note that some types of turbulence can occur in stable air.  Mechanical turbulence in strong winds is worse in unstable air, but can still occur in stable air (but will only affect the very low altitudes).  Mountain wave phenomenon actually requires stable air in order to occur.
    
   Cheers,
   Steve
   http://www.flightwriter.com

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3. 
   

   
   

   LovemyGF on Feb 15, 2011

   Steve Pomroy has it correct.  In fact, he did a pretty good explanantion on a VERY complicated subject.  Trust me, I minored in meteorology, and one can get “hardcore” into weather theory with it.  Using the lapse rates, we can find out CAPE (convective available potentential energy) and how high the risk of CB will occur, and how severe the resultant TS will be.  We can figure out cloud heights, and many other things.  The worst torture was when one of my professors made us plot out a Skew T diagram by hand. 
   In truth, you cannot figure out the actual lapse rates, unless you have access to lots of money and scientific equipment (mainly weather ballons and radiosondes).  That is the only way this information si obtained, regular weather ballon launching. 
   Steve already did a good job explaining lapse rates.  To add to what he stated, in regards to high and low pressure systems and their associated weather, just think about what Steve said. 
   With high pressure systems, we have cold air decending.  Cold air is dense, and wants to keep sinking.  Without any sort of major lifting (cold air trying to sink), we cannot get any condensation of the moisture in this air, and thus, released energy into the atmosphere (this is called latent heating of condensation).  This released energy is the fuel for most weather. 
   With low pressure systems, we have warm air ascending.  This is perfect for the releation of energy into the atmosphere. 
   High and low pressure systems form when the jet stream fluctates.  Where it tends to spread apart, creates a “vaccum” which sucks air up into it…aka, a low pressure system.  Where it squeezes together, tends to “squeeze” extra air down…aka, a high pressure system.
    
    
    
    
   
    
   

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