Weather Question
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Weather Question
A question from a low hour student - something I can't quite figure out the theory -
Why does high pressure normally result in stable atmosphere but poor visibility?
Why does high pressure normally result in stable atmosphere but poor visibility?
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"High pressure" is a relative term. i.e. it is higher pressure than the surrounding areas. This makes the air stable, which means it is not going anywhere very fast, so it does not clear away the tiny particles, which eventually will be enough to reduce the visibility.
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When the upper airmass is descendig, it is pressing the lower
layers against the ground - the pressure on the surface is high.
There is adiabatic heating in the compressed air. Additionally,
the lowest layer of the air which is in contact with the ground
is cooled by the cold ground. Cold air is near the surface,
ower it lays a relatively warmer air. This layers does not mix
very well.
Cold air is heavier, so there is something like an ocean of cold air
and lighter air ower it. This is called inversion. There is only very
little vertical movement = stable atmosphere. Simply there is no force
which would make the heavy particle of cold air climb upwards into
the lighter air and vice versa.
Dust, smoke, different particles can not climb trough the
inversion layer - the visibility under the inversion is
negatively affected. Over the inversion layer the visibility
can be good.
The descending air is pushed to the sides. There is a flow OUT of the
centre of the high. This dampens the rotating movement of the air
around the centre of the high - coriolis force. The winds are low.
In contrary, in the atmospheric low there is a movement TO the
centre - coriolis force accelerates the rotation around the centre
of the low - stronger winds, the dust and smoke can dissipate better.
layers against the ground - the pressure on the surface is high.
There is adiabatic heating in the compressed air. Additionally,
the lowest layer of the air which is in contact with the ground
is cooled by the cold ground. Cold air is near the surface,
ower it lays a relatively warmer air. This layers does not mix
very well.
Cold air is heavier, so there is something like an ocean of cold air
and lighter air ower it. This is called inversion. There is only very
little vertical movement = stable atmosphere. Simply there is no force
which would make the heavy particle of cold air climb upwards into
the lighter air and vice versa.
Dust, smoke, different particles can not climb trough the
inversion layer - the visibility under the inversion is
negatively affected. Over the inversion layer the visibility
can be good.
The descending air is pushed to the sides. There is a flow OUT of the
centre of the high. This dampens the rotating movement of the air
around the centre of the high - coriolis force. The winds are low.
In contrary, in the atmospheric low there is a movement TO the
centre - coriolis force accelerates the rotation around the centre
of the low - stronger winds, the dust and smoke can dissipate better.
Pompey till I die
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Interesting
Whilst aware of the text book answer I'd also attributed it to the fact that the air is denser and can thus support more water \ particles in the atmosphere.
Every time I watch the outro to "Antiques Roadshow" I always think it's a text book example of anti-cyclonic haze.
Every time I watch the outro to "Antiques Roadshow" I always think it's a text book example of anti-cyclonic haze.
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Think of low air pressure as a Hoover, moving all the muck and dust away, even in low wind speeds. High pressure is a cushion, pushing down the dust, water vapour etc, and compressing it, close to the surface.
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airborne - I've been teaching (different subjects) for a very long time, and often use "think of it as a....." suggestions to help with understanding.
I do, however, then enforce that with some means of remembering which is which, and why.
Apart from simply remembering (in which case you may as well remember the high/low answer anyway), how does your suggestion not get *some* people to say "Think of a low as a cushion, and a high as a 'hoover'".
Hoovers, indeed any brand of upright vacuum cleaner - and to that effect, most cylinder brands, are "higher" than cushions (well, taller).
I completely agree with your idea, I just like to know the "Why" behind many statements.
I do, however, then enforce that with some means of remembering which is which, and why.
Apart from simply remembering (in which case you may as well remember the high/low answer anyway), how does your suggestion not get *some* people to say "Think of a low as a cushion, and a high as a 'hoover'".
Hoovers, indeed any brand of upright vacuum cleaner - and to that effect, most cylinder brands, are "higher" than cushions (well, taller).
I completely agree with your idea, I just like to know the "Why" behind many statements.
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I've got to admit, some of these answers are amongst the most bizarre I come across when describing why you get poor viz under a high!
Basically, a high pressure system (anticyclone) is dominated by slow descent throughout the depth of the troposphere. As air descends it warms due to adiabatic compression, thus the atmospheric column is warmer than normal. Superimposed on this near the Earth's surface lowest 1km or so is the planetary boundary layer that is driven by solar heating that is characterized by buoyancy driven plumes. Under normal (i.e. non anticyclonic conditions) a certain percentage of these plumes are buoyant enough to escape the boundary layer and this results in the 'ventilation' of particulates into the free atmosphere. Also these plumes tend to form clouds which help to 'wash out' particles in the cloud droplets and/or precip.
Anywho, the warmer than normal atmosphere under the anticyclone reduces the buoyancy of boundary layer plumes and reduces the amount of 'ventilation', thus the particulates build up and drive down the viz. There's also some chemical effects from these nasties hanging around that further drives down viz, but that's not really my area.
Hope this helps!
Gareth.
Basically, a high pressure system (anticyclone) is dominated by slow descent throughout the depth of the troposphere. As air descends it warms due to adiabatic compression, thus the atmospheric column is warmer than normal. Superimposed on this near the Earth's surface lowest 1km or so is the planetary boundary layer that is driven by solar heating that is characterized by buoyancy driven plumes. Under normal (i.e. non anticyclonic conditions) a certain percentage of these plumes are buoyant enough to escape the boundary layer and this results in the 'ventilation' of particulates into the free atmosphere. Also these plumes tend to form clouds which help to 'wash out' particles in the cloud droplets and/or precip.
Anywho, the warmer than normal atmosphere under the anticyclone reduces the buoyancy of boundary layer plumes and reduces the amount of 'ventilation', thus the particulates build up and drive down the viz. There's also some chemical effects from these nasties hanging around that further drives down viz, but that's not really my area.
Hope this helps!
Gareth.
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gfunc - that sounds a fairly heavy handed response in your opening line. Bit harsh?
From what I can see, your cut/paste answer from a textbook is very similar to miroc's cut/paste; Farmer1 says the same but much more simplisticly, airborne only gave a "look at it this way", but the answer is the same. Pompey's? Hmmm.
What's your point about bizarre answers (plural)?
From what I can see, your cut/paste answer from a textbook is very similar to miroc's cut/paste; Farmer1 says the same but much more simplisticly, airborne only gave a "look at it this way", but the answer is the same. Pompey's? Hmmm.
What's your point about bizarre answers (plural)?
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I wonder
has anybody else noticed that when you talk about vis, pressure & weather to non-pilots they look very bored very quickly ? It's almost like they are completely disinterested in one of the most interesting things in the world...
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Fair play Keygrip, just the few hundred students giving all sorts of weird and wonderful answers over the past few years is enough to make anyone cranky, especially if said students are majoring in Met. Sorry if rubbed anyone the wrong way!
Anywho, it's not a cut n paste job - it's all my own work! My standard of grammar is far too poor to get into a textbook.
I said these answers are bizarre as there seems to be a lot of confusion about fairly basic physical principles that I always thought were reasonably well covered in the PPL texts. Simplifications are absolutley fine, but there becomes a point when you start hammering a square peg into a round hole with a sledge hammer, so to speak. You start getting more problems building on these if the simplifications propagate to other areas where they don't fit and people may start second guessing other areas that they are already clear on. A lot of the reasons people have come up with on this thread mention the key principles but are couched incorrectly that leads to a haze (pun intended) of confusion.
For example the hoover and cushion analogy is nice as it does conceptually fit with the vertical motion in lows and highs, except when you think about it, the time scales of the dominant physical processes are so different it begins to fall apart. The mean vertical motion in a high or low is of the order a few m per hour, whereas a convective updraft is a few m per second - the observed build up of haze isn't really going to result from a 1/3600 difference in the vertical motion.
Similarly there's a bit of confusion about the temperature in miroc's post. I can understand what they are going but if you don't know the answer in the first place it can be very confusing. Here miroc is talking about the concept of potential temperature, which is the temperature air would have if it was transported adiabatically to 1000hPa, as we all know from experience that the coldest air is not present at the surface. A rising plume approximately retains its original potential temperature and the buoyancy that I talked about in my first post is defined by the potential temperature difference between the environment and the rising plume.
What I'm trying to do in my replies is to help others by pointing out where the concepts are applied correctly and incorrectly. In the case of the haze question there are multiple time and space scales involved where other concepts and simplifications don't mesh properly. Maybe most aren't really that interested in the long drawn out answer and its probably not that important in the grand scheme of things, but I'm going to give it anyways!
Cheers,
Gareth. (now 50% less cranky thanks to coffee injection).
Anywho, it's not a cut n paste job - it's all my own work! My standard of grammar is far too poor to get into a textbook.
I said these answers are bizarre as there seems to be a lot of confusion about fairly basic physical principles that I always thought were reasonably well covered in the PPL texts. Simplifications are absolutley fine, but there becomes a point when you start hammering a square peg into a round hole with a sledge hammer, so to speak. You start getting more problems building on these if the simplifications propagate to other areas where they don't fit and people may start second guessing other areas that they are already clear on. A lot of the reasons people have come up with on this thread mention the key principles but are couched incorrectly that leads to a haze (pun intended) of confusion.
For example the hoover and cushion analogy is nice as it does conceptually fit with the vertical motion in lows and highs, except when you think about it, the time scales of the dominant physical processes are so different it begins to fall apart. The mean vertical motion in a high or low is of the order a few m per hour, whereas a convective updraft is a few m per second - the observed build up of haze isn't really going to result from a 1/3600 difference in the vertical motion.
Similarly there's a bit of confusion about the temperature in miroc's post. I can understand what they are going but if you don't know the answer in the first place it can be very confusing. Here miroc is talking about the concept of potential temperature, which is the temperature air would have if it was transported adiabatically to 1000hPa, as we all know from experience that the coldest air is not present at the surface. A rising plume approximately retains its original potential temperature and the buoyancy that I talked about in my first post is defined by the potential temperature difference between the environment and the rising plume.
What I'm trying to do in my replies is to help others by pointing out where the concepts are applied correctly and incorrectly. In the case of the haze question there are multiple time and space scales involved where other concepts and simplifications don't mesh properly. Maybe most aren't really that interested in the long drawn out answer and its probably not that important in the grand scheme of things, but I'm going to give it anyways!
Cheers,
Gareth. (now 50% less cranky thanks to coffee injection).
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Well having skimmed through this I dont think anyone got the correct answer (though a few got closer than the rest).
The point is that a high pressure area is created when air descends from above. Air that is above is very dry (because all the moisture fell out when it ascended at some time in the past). Therefore (and this is the important bit) it warms at the Dry Adiabatic Lapse Rate (DALR), so becomes very warm (in relative terms) as it approaches the surface. [The same effect is true of warming winds like the fohn and chinook].
This makes the atmosphere very stable, as it is warm enought to stop most convection from the surface.
During cold nights, surface radation cools the air that is in contact with the ground, leading to the creation of an inversion layer. There may be some vertical movement below the inversion layer but in the main air cannot penetrate through it. The result is that smoke and dust from the surface is trapped in the lowest levels of the atmosphere, with clear conditions above.
The point is that a high pressure area is created when air descends from above. Air that is above is very dry (because all the moisture fell out when it ascended at some time in the past). Therefore (and this is the important bit) it warms at the Dry Adiabatic Lapse Rate (DALR), so becomes very warm (in relative terms) as it approaches the surface. [The same effect is true of warming winds like the fohn and chinook].
This makes the atmosphere very stable, as it is warm enought to stop most convection from the surface.
During cold nights, surface radation cools the air that is in contact with the ground, leading to the creation of an inversion layer. There may be some vertical movement below the inversion layer but in the main air cannot penetrate through it. The result is that smoke and dust from the surface is trapped in the lowest levels of the atmosphere, with clear conditions above.
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rsuggit,
Your answer is pretty much the same as mine I gave earlier. Don't get distracted by the moisture content, it does matter if the air is very dry or very moist, as long as it's not saturated, idealized air parcels will always warm or cool at the DALR (approx 9.8C/km).
Remember that idealized parcels will retain their moisture content and dewpoint temperature until the air parcel temperature reduces to the dewpoint. As the temperature decreases condensation occurs and latent heat is released due to water changing phase (vapour to liquid) which modifies the lapse rate.
Also I think there might be confusion regarding the difference between the inversion resulting from the presence of a synoptic high pressure and the one associated with radiative cooling. Both have the same physical effect, in that they stop buoyant plumes and particulates leaving lowest layers, but it is not true that surface cooling is required for an inversion to form - note that they can persist during the daytime in summer. Similarly you have nocturnal inversion form under other synoptic conditions, all you need is a clear sky and light winds overnight.
However, the fact of the matter is that you do tend to get both the anticylonic inversion and the nocturnal inversion occuring together since anticyclonic conditions do favour clear skies and light winds, which doesn't help the understanding of things! If you have access to tephigrams or skew-t diagrams it is sometimes possible to see both synoptic (anticyclonic induced) and nocturnal inversions separately as the nocturnal one tends to exist near 900hPa and the synoptic one is around 850-700hPa depending on the conditions. If you have a time evolution you'll also see that the nocturnal one tends to be eaten up during the day.
Also as one of my collegues in the Atmos. Chem group pointed out, there is also an increase in photochemical smog during the day that drives down the viz over the daylight hours. Some talk of OH and ozone sulphates and aqueous phase blah blah. Chemistry isn't really my thing!
Anywho, I hope all this 'dry' meteorology is helping.....
Cheers,
Gareth.
Your answer is pretty much the same as mine I gave earlier. Don't get distracted by the moisture content, it does matter if the air is very dry or very moist, as long as it's not saturated, idealized air parcels will always warm or cool at the DALR (approx 9.8C/km).
Remember that idealized parcels will retain their moisture content and dewpoint temperature until the air parcel temperature reduces to the dewpoint. As the temperature decreases condensation occurs and latent heat is released due to water changing phase (vapour to liquid) which modifies the lapse rate.
Also I think there might be confusion regarding the difference between the inversion resulting from the presence of a synoptic high pressure and the one associated with radiative cooling. Both have the same physical effect, in that they stop buoyant plumes and particulates leaving lowest layers, but it is not true that surface cooling is required for an inversion to form - note that they can persist during the daytime in summer. Similarly you have nocturnal inversion form under other synoptic conditions, all you need is a clear sky and light winds overnight.
However, the fact of the matter is that you do tend to get both the anticylonic inversion and the nocturnal inversion occuring together since anticyclonic conditions do favour clear skies and light winds, which doesn't help the understanding of things! If you have access to tephigrams or skew-t diagrams it is sometimes possible to see both synoptic (anticyclonic induced) and nocturnal inversions separately as the nocturnal one tends to exist near 900hPa and the synoptic one is around 850-700hPa depending on the conditions. If you have a time evolution you'll also see that the nocturnal one tends to be eaten up during the day.
Also as one of my collegues in the Atmos. Chem group pointed out, there is also an increase in photochemical smog during the day that drives down the viz over the daylight hours. Some talk of OH and ozone sulphates and aqueous phase blah blah. Chemistry isn't really my thing!
Anywho, I hope all this 'dry' meteorology is helping.....
Cheers,
Gareth.
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SARM , would the visibility affect the ability of the average speed of an unladen african swallow if it were to fly gripping a coconut by the husk?????
Sorry for the thread creep but havent Monty pythonly sparred with SARM for a while
Sorry for the thread creep but havent Monty pythonly sparred with SARM for a while
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Now I remember why I stopped looking at my ATPL books
I have yet to find an example of weather theory which is actually useful to flying. The really useful stuff is knowing which websites to hit and how good or crap the data is likely to be, knowing about practical structural icing stuff, and practical stuff like that. All the stuff about Hadley cells is just a load of crap. About as useful as a radar altimeter when at FL150 over Dover.
Internet? That is an unapproved data source. Can't use that!!
Tabs please !
The permanent inversion traps all the pollution, dust and airborne crap in (typically) the bottom 3000 feet of the atmosphere. Ask a friendly balloon pilot to take you up one evening when there is high pressure and you will be surprised at how well defined the boundary layer is between the trapped air and the clear air above. The earth appears to have a second horizon !
