Confused

FlyBoyJae

New Member
hello~
so, i did my solo flights...i have 4.7 PIC time now

i am still a little annoyed with those pilots not making the calls on the freq out at the practice area, but oh well~

i am now confused about this weather stuff~

I read in the book that as the temperature rises, the pressure would rise and the air would be less dense.

if the temperature goes down, then the pressure would decrease and the air would be more dense.

it that is the case, then why is the air less dense high up in the altitude? i can see that there would be less pressure up there, but the temperature is really cold there, so woudln't that make the air more dense?

also, I thought that the high pressure system that we see on the weather forecasts meant hot temperature...isn't it?
does the pressure system that is shown on the weather forecast have nothing to do with the temperature?

i dont' even know what i am confused about now.
 

jonnyb

Well-Known Member
Well dude, let me just tell you this.............

I'm not a freakin' meteorologist or anything
, but what I remember from teaching is: When it's hot = less pressure (hence a lower altimeter setting), and when there's less pressure = air is less dense. However, for an overall increase in altitude, there is an overall decrease in pressure. So, although temperature does decrease (overall) with an increase in altitude, so does pressure. Any current CFI's out there, please correct me if I'm wrong. It's been a while.
 

MidlifeFlyer

Well-Known Member
[ QUOTE ]

I read in the book that as the temperature rises, the pressure would rise and the air would be less dense.

if the temperature goes down, then the pressure would decrease and the air would be more dense.

it that is the case, then why is the air less dense high up in the altitude? i can see that there would be less pressure up there, but the temperature is really cold there, so wouldn't that make the air more dense?


[/ QUOTE ]Your confusion might be because you are making two assumptions that are not true:

1. You may be thinking that the changes in pressure that come from altitude and the changes in pressure that come from temperature act equally. They don't. The decrease in air density from altitude comes because there are less air molecules there. The increase in air density from temperature comes from slowing down and compacting whatever molecules are there to begin with.

2. You may be thinking that pressure changes happen =only= because of temperature changes. They don't. Well, they pretty much do, but the low pressure system that's generated by a temperature change off the coast of Oregon, may end up moving and being the weather factor in Texas.
 

flyguy

Well-Known Member
Although temperature and pressure are proportunal, the reason that pressure decreases with altitude is not because of temperature. The lower you are in the atmosphere the higher the pressure is due to gravity. The air at the surface has the weight of all of the air above it pushing down on it. As you go higher in elevation, you get closer to the top of the atmosphere and there is less air above you and less weight pushing down on the air around you. This is the same reason pressure in the ocean increases as you go deeper, except it is much more pronounced because water weighs much more than air.

As far as high and low pressure systems, a high pressure area like you see on a weather map actually indicates a cold airmass. That might clear up your confusion just knowing that fact.
 

flyguy

Well-Known Member
I just re-read your post and it seems that you already understand why pressure decreases with altitude so let me take a stab at your actual question. Think of air molecules as people. When its cold outside a group of people are going to want to huddle up to stay warm. If the people are huddled up you can fit more of them into a room. When its warm out they'll tend to spread out so they don't invade each other's personal space. If they are spread out you won't be able to fit as many into that same room. As you go up in altitude there are fewer people to start out with. The few people that are there will be huddled up because its cold, but they are huddled up in an empty room, and there's lots of empty space surrounding them, and the overall density is low.
 

MidlifeFlyer

Well-Known Member
[ QUOTE ]
I just re-read your post and it seems that you already understand why pressure decreases with altitude so let me take a stab at your actual question. Think of air molecules as people. When its cold outside a group of people are going to want to huddle up to stay warm. If the people are huddled up you can fit more of them into a room. When its warm out they'll tend to spread out so they don't invade each other's personal space. If they are spread out you won't be able to fit as many into that same room. As you go up in altitude there are fewer people to start out with. The few people that are there will be huddled up because its cold, but they are huddled up in an empty room, and there's lots of empty space surrounding them, and the overall density is low.

[/ QUOTE ]That is a truly great explanatory visual!
 

sorrygottarunway

Well-Known Member
Gentlemen, Boyle would be proud of us.

One thing thats important to remember is that the air is not in a fixed-volume container- it is free to move about in space. Therefore, if the temperature increases causing molecule average speed to increase, the molecules by consequence are free to AND WILL move apart from each other. In an enclosed container, an increase in the temp would actually cause a rise in pressure.

PV = kT (k = gas constant)

Up in the air, an increase in temp causes expansion- density decreases, and the air rises. As it rises the relative pressure of this sector of air is higher- because its not enclosed, it expands some more and cools.
 

sorrygottarunway

Well-Known Member
here's a nicer example:

lets say we take a trip into space in a nice super-reinforced BE10. Who cares about differential.

At around 250 miles altitude, we pop open the door and get sucked out.

We're now out in space and we would explode, because there is virtually no air pressure to speak of, to hold us together. There's an "infinite" amount of space to expand to. No molecules would be bumping into each other, it would be very, VERY cold.

Arms and legs flying everywhere!
 

FL410

New Member
Just think cold=High because of it being indicated blue on a weather chart, and a red low=Heat.

Keeping it simple.
 

mastermags

Well-Known Member *giggity*
[ QUOTE ]
here's a nicer example:

lets say we take a trip into space in a nice super-reinforced BE10. Who cares about differential.

At around 250 miles altitude, we pop open the door and get sucked out.

We're now out in space and we would explode, because there is virtually no air pressure to speak of, to hold us together. There's an "infinite" amount of space to expand to. No molecules would be bumping into each other, it would be very, VERY cold.

Arms and legs flying everywhere!

[/ QUOTE ]

Thats an awesome image
 

kellwolf

Piece of Trash
But I thought chemical and electrical bonds between molecules held us together instead of air pressure?
 

sorrygottarunway

Well-Known Member
[ QUOTE ]
But I thought chemical and electrical bonds between molecules held us together instead of air pressure?


[/ QUOTE ]

HA! Tell that to a cytoplasm as it experiences a vacuum!

remember, water is held as a liquid under certain vapor pressure conditions. Take away that pressure, and well... it turns into a gas! Thats why you get water droplets well below freezing at altitudes.
 
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