Met
Thread Starter
Joined: May 2007
Posts: 3
Likes: 0
From: Sheffield
Met
Hi,
I'm new to this forum, and new to flying.
I've only resently passed Airlaw and have started studying for Met.
Why is it so difficult to understand even though it's all around us.
I'm having problems understanding air density... could someone help with this??
thank you
I'm new to this forum, and new to flying.
I've only resently passed Airlaw and have started studying for Met.
Why is it so difficult to understand even though it's all around us.
I'm having problems understanding air density... could someone help with this??
thank you
Blah Blah Blah
Joined: Jan 2006
Posts: 927
Likes: 0
From: Malmesbury VRP
Here is one way to understand air density.
Take a pint glass and fill it with Air. Would you then agree that in that pint glass you have maximum air density? You should agree as you cannot get more than one pint of air in a pint glass.
Pour half a pint water in to your pint glass of air. Water is heavier than air so it is going to push air out of your pint glass. So now you only have half the air in the pint glass. So would you now agree that the air in the glass is now less dense? You should because you have a 1 pint container with only half pint of air in it, which means you do not have maximum air density in the glass.
There are other properties that affect air density such as:
Temperature
Altitude
Maybe someone else will explain how those properties affect air density.
Take a pint glass and fill it with Air. Would you then agree that in that pint glass you have maximum air density? You should agree as you cannot get more than one pint of air in a pint glass.
Pour half a pint water in to your pint glass of air. Water is heavier than air so it is going to push air out of your pint glass. So now you only have half the air in the pint glass. So would you now agree that the air in the glass is now less dense? You should because you have a 1 pint container with only half pint of air in it, which means you do not have maximum air density in the glass.
There are other properties that affect air density such as:
Temperature
Altitude
Maybe someone else will explain how those properties affect air density.
Joined: Feb 2007
Posts: 4,598
Likes: 0
From: Amsterdam
Air density is simply the amount of "air molecules" (actually a mixture of oxygen, nitrogen and a bunch of others) in a certain volume.
All these air molecules bounce around, bump into each other and bump into whatever happens to be in the air as well. This bumping into objects exerts a force on these objects which we call air pressure.
By their very nature, air molecules want to be as far from each other as possible, but there are a few factors that force them together. In the free atmosphere, gravity is the most important one: it sucks the air molecules to the surface of the earth. The air molecules resist this to some degree, depending on a number of factors. And it's these factors that eventually determine air density and air pressure:
- Temperature: the higher the temperature, the more "bouncier" the air molecules become. So they are able to resist the compression effect from gravity better, making sure that there are less air molecules in a given space. So when temperature goes up, air density goes down.
- Altitude: the higher up you go, the less air molecules are above you, sucked down by gravity and squeezing the air molecules below together. So the higher you go, the less dense the air becomes.
If it wasn't for the sun, the air density and air pressure on the planet would be even throughout the surface, and uniformly declining as you go up. But the sun heats the earths surface unevenly, causing spots where the air is less dense (due to a higher temperature) than elsewhere. Less air molecules means less gravitational pull so lower pressure. Nature doesn't like this pressure differential so air starts flowing from the lower temperature/higher pressure regions to the higher temperature/lower pressure region. Due to the coriolis force this air movement will not take a direct path but will "swing" to the right (northern hemisphere), creating the cyclonic and anti-cyclonic winds around high- and low pressure regions. And between the cold and warm air masses are the cold and warm fronts.
Now there are a few other effects, but on a broad level this is what causes "weather".
All these air molecules bounce around, bump into each other and bump into whatever happens to be in the air as well. This bumping into objects exerts a force on these objects which we call air pressure.
By their very nature, air molecules want to be as far from each other as possible, but there are a few factors that force them together. In the free atmosphere, gravity is the most important one: it sucks the air molecules to the surface of the earth. The air molecules resist this to some degree, depending on a number of factors. And it's these factors that eventually determine air density and air pressure:
- Temperature: the higher the temperature, the more "bouncier" the air molecules become. So they are able to resist the compression effect from gravity better, making sure that there are less air molecules in a given space. So when temperature goes up, air density goes down.
- Altitude: the higher up you go, the less air molecules are above you, sucked down by gravity and squeezing the air molecules below together. So the higher you go, the less dense the air becomes.
If it wasn't for the sun, the air density and air pressure on the planet would be even throughout the surface, and uniformly declining as you go up. But the sun heats the earths surface unevenly, causing spots where the air is less dense (due to a higher temperature) than elsewhere. Less air molecules means less gravitational pull so lower pressure. Nature doesn't like this pressure differential so air starts flowing from the lower temperature/higher pressure regions to the higher temperature/lower pressure region. Due to the coriolis force this air movement will not take a direct path but will "swing" to the right (northern hemisphere), creating the cyclonic and anti-cyclonic winds around high- and low pressure regions. And between the cold and warm air masses are the cold and warm fronts.
Now there are a few other effects, but on a broad level this is what causes "weather".
Joined: May 2003
Posts: 664
Likes: 0
From: Yorkshire
gcolyer :
Here is one way to understand air density.
Take a pint glass and fill it with Air. Would you then agree that in that pint glass you have maximum air density? You should agree as you cannot get more than one pint of air in a pint glass.
Pour half a pint water in to your pint glass of air. Water is heavier than air so it is going to push air out of your pint glass. So now you only have half the air in the pint glass. So would you now agree that the air in the glass is now less dense? You should because you have a 1 pint container with only half pint of air in it, which means you do not have maximum air density in the glass.
Here is one way to understand air density.
Take a pint glass and fill it with Air. Would you then agree that in that pint glass you have maximum air density? You should agree as you cannot get more than one pint of air in a pint glass.
Pour half a pint water in to your pint glass of air. Water is heavier than air so it is going to push air out of your pint glass. So now you only have half the air in the pint glass. So would you now agree that the air in the glass is now less dense? You should because you have a 1 pint container with only half pint of air in it, which means you do not have maximum air density in the glass.
Flywanabee747, go check out the AOPA (US) website, or Google as I said. There are some excellent articles - including aviation-specific ones - which explain this properly.
FF
Joined: Jan 2005
Posts: 167
Likes: 0
From: UK
Welcome to Pprune, welcome to flying & congratulations on passing air law.
Air is made up of molecules, the amount of which determines its density. Molecules are affected by gravity like everything else. Closer to the Earths surface, lots of them (i.e, dense). Higher up in the atmosphere, fewer of them (i.e, less dense). Density is good for lift, thus more lift closer to the Earths surface.
The Wombat
Air is made up of molecules, the amount of which determines its density. Molecules are affected by gravity like everything else. Closer to the Earths surface, lots of them (i.e, dense). Higher up in the atmosphere, fewer of them (i.e, less dense). Density is good for lift, thus more lift closer to the Earths surface.
The Wombat
Joined: Mar 2006
Posts: 779
Likes: 1
From: Kent UK
OMG! Talk about confuse the poor chap! 
wombat13 wrote:
Air is made up of molecules, the amount of which determines its density.
It's the amount of molecules per unit volume which determines density. Two million air molecules filling Gcolyer's pint glass is more dense than 1 million molecules filling the glass.
wombat13 wrote:
Air is made up of molecules, the amount of which determines its density.
It's the amount of molecules per unit volume which determines density. Two million air molecules filling Gcolyer's pint glass is more dense than 1 million molecules filling the glass.
Joined: Oct 2004
Posts: 762
Likes: 0
From: UK
Fly wanabe 747
I would suggest going to the met office website and going to the learning section and then try the teachers section.
It has some brilliant explanations, diagrams, interactive etc. .
You can also work through questions etc.
I ask all my students to have a look at it.
Met is my specialist subject (apart from global climatololgy and hadley cells are boring) and i still don't get some of it. Very practical, very theoretical and very unpredictable.
It has some brilliant explanations, diagrams, interactive etc. .
You can also work through questions etc.
I ask all my students to have a look at it.
Met is my specialist subject (apart from global climatololgy and hadley cells are boring) and i still don't get some of it. Very practical, very theoretical and very unpredictable.
Carbonfibre-based lifeform
Joined: Mar 2002
Posts: 747
Likes: 0
From: London
gcolyer,
You were right to doubt your initial example!
If your glass contained two million molecules when it was full of air then it would simply end up with one million once you half filled it with water. Unless you changed something else (such as the pressure), all that you've done is halve the enclosing volume and halve the amout of air which leaves the density exactly the same.
Your statement
is a bit misleading. The term 'one pint of air' is meaningless on its own - you could easily get more air molecules into the glass by raising the air pressure. Lowering the glass from the table to the floor or taking it down a flight of stairs would do just that!
You were right to doubt your initial example!
If your glass contained two million molecules when it was full of air then it would simply end up with one million once you half filled it with water. Unless you changed something else (such as the pressure), all that you've done is halve the enclosing volume and halve the amout of air which leaves the density exactly the same.
Your statement
Would you then agree that in that pint glass you have maximum air density? You should agree as you cannot get more than one pint of air in a pint glass.
Thread Starter
Joined: May 2007
Posts: 3
Likes: 0
From: Sheffield
Thank you so much for all the replies.
I think i now understand what density is now
. I've read the book's over and over and i've been testing myself on pplquiz i'm taking my exam today 
so wish me good luck.
FWB
P.S i can't wait to get onto Nav.
I think i now understand what density is now
. I've read the book's over and over and i've been testing myself on pplquiz i'm taking my exam today so wish me good luck.FWB
P.S i can't wait to get onto Nav.
