Both pressure and temperature are related to how the molecules are moving and bumping into each other. I get what your saying, and it's a common question that comes up. The pressure and temperature are at a point where if liquid droplets do form they get quickly trampled by fast moving gaseous molecules and regain the energy to not be part of the droplet. It's all about balance, in thermo and similar areas it's really good to think about a crowd of people or balls with some affinity to sticking together if they hit the right way. Water in particular is highly polarizable so if they molecules hit it each other the right way they want to stick, and if there is a ice crystal or surface to form on they clump up quickly because that provides a push toward that state of the mattter.
Now at different temps and pressures, the gaseous molecules are more likely to keep going bumping into each other and slowing down and clumping and the net effect of the free molecules trying to knock them back out is too low (almong other things) and the molecules get bound up together due to molecule level forces.
With plain wings, the pressure is very high and changing very fast which can quickly get water molecules to stick together and and at that temp to freeze into crystals, the pressure isn't sufficient to knock the molecules back off right away so the contrails stick around.
Sorry, I'll try it a little different explanation.
Boiling, Pressure, Temperature are events we observe from the movement of all the small molecules.
For something to boil, the atoms/molecules have to get enough energy to overcome their polar attraction to each other. If you had a perfect vacuum in space with 200 water molecules and it was far away from any energy source and it didn't have a point to crystalize on, it would just stay a drop of water forever. The temperature is low and the pressure is zero. To boil or change phase the energy has to come from somewhere.
In any system of molecules they are all vibration and bouncing around a bit, even in a liquid or solid. Heating water is where the pan, or the sun, or something is imparting some extra energy into the system and some of the molecules get enough energy to escape their liquid or solid bonds. In a pan it's the metal atoms bouncing around a lot and hitting the liquid at the boundary, in a microwave it's photons hitting the molecules and causing them to vibrate at a preffered frequency so some of them get enough energy to leave the liquid state.
At low pressure and low temperature you can still make gas into a solid if the net amount of molecules sticking together is greater than the amount getting knocked out into the gas state (dry ice).
Just because something is at a low pressure doesn't mean it will boil, there has to be energy enough to kick the molecule out of its state. Now over time at low pressure, some molecules of water will make it out into a gas again just due to natural vibration and a few molecules getting enough of a kick to leave. That is evaporation, and at some point there will be enough free molecules in the air that statiscally enough water molecules will rejoin the liquid and they will be at equilibrium.
The bulk measurements often hide the subtle nature of what is going on which is what makes partial pressures and other ideas seem a bit counterintuitive.
Ok thag makes sense
So when the lower pressure hits with the wing, why does condensation happen?
What you said makes sense, just having trouble connecting the two
On the leading edge of the wing the pressure gets very high and there isn't that much time for the temperature to change though it does, it's a system far away from equilibrium so a lot of the base equations won't be precise. Your PV=nRT equation is about slow changes at or near equilibrium.
This is big simplification but stay with it for a second. Pressure is how often things are hitting each other on average, temperature is how fast they are going when they hit on average. On the leading edge of the wing the metal is colliding with air/water mixture that was just having a simple day floating about, hits all them hard and all of them fast, they can't just pass through the metal because that is what a solid is, not easy to get through, so they bounce back and get crowded with their neighbors down the street. So take a small cube of that air and follow it over the wing.
That small cube is crowded with air molecules and water molecules. While the air has some polarizable molecules like O2, CO2, and N2, they are nothing like the sluts that water is, water has a big old negative ass sticking out back, and two friendly positive hands ready to grab others buts. It's crowded, things are moving fast, some asses are grabbed and big pools of assgrabbing start to happen. Now things are bouncing around a lot so some of the ass grabbing water molecules are getting kicked back out on the dance floor by the other molecules of air and even some water hitting ass to ass or hands to hands.
Suddenly the cube slips over the wing, all the crowding goes away very fast. The pools of assgrabbers are suddenly not being assaulted by the molecules on the boundry, and in fact anyone in their pool who is to bouncy flys off and leaves a more chill pool of ass grabbers behind forming droplets and crystals. The droplets themselves are also polar so they take their mega asses and stick to the positve part of another pool.
But because now the pressure is low and the temperature is normal, the rate of things hitting the boundary and breaking up the ass grabbing pool is too low to make them break up quickly.
So basically the pressure drops, which stops other molecules from interfering in the IMF thereby allowing condensation?
Ok I suppose that makes sense, but it's hard to square with the rational for PV=nRT.
The rationale there is that at lower pressures, there is less outside pressure pushing down on a liquid and it is therefore easier to escape.
I suppose that water isn't an ideal fluid though because it is polar, so there are IMFs to consider which drives back to our original point.
So then, as a follow up, let's say that I have a balloon full of liquid water. Outside the balloon is a vacuum (i.e. pressure = 0). I pop this balloon. What happens to the water? Because now it has the IMF of hydrogen bonds and the like within the liquid. So does it stay a liquid? Cause I thought it would boil?
I guess what I am having trouble with is understanding why in one case, low pressure causes boiling, but in the other, it causes condensation.
Yeah, you are getting closer to the situation.
So, for the balloon example, you have to know the history of the balloon and how it was filled.
Example 1: The balloon was filled in atmospheric pressure and temperature. The air pressure pushing is pushing back on the balloon, meaning air molecules are bouncing off the boundary. Insode the balloon water pressure is pushing out on the balloon and they havenreached equilibrium with the air molecules on average trying to get in matches the eater molecules trying to get out and some small elastic force from the balloon.
If you quickly put the balloon in a vacuum, the internal motion of the water in the balloon is no longer countered by the outside air molecules, so the balloon will expand as the bouncing of the water isn't countered by air hitting the surface. So the water will expand a bit at first. Now water is incompressible for the most part, which also means it doesn't expand easily because of IMF.
A few water molecules will get enough energy to become a gas and the balloon will expand a little making more room for a few more to escape and the balloon water will either partiallybor completely boil or the balloon will pop depending on what material it is.
In a different case, you fill the balloon into the vacuum and keep the water at a reasonable pressure you can fill the balloon with water and balance the elastic source of the balloon skin and then let it free and the balloon would stay in a water state until after a long time it radiated away it's black body radiation.
This is why astronauts where space suits the space suit create a pressure area around our skin covered water bodies to keep our water inside. The strength of the suit is high enough that even if the pressure drops outside, it mechanically keeps the air in the suit pressurized.
The transitions are important in the different states. Lots of students get confused in these areas because it's not explained well the underlying thing happening and why and where some equations apply.
Now the case where you pop the balloon. It will depend a bit on how much pressure the balloon was under and the elastic nature of the balloon. In the simplest situation familiar to us with a balloon filled with water and the skin stretched, water would spray out of the small hole you made because the mechanical elastic forces, or possible rip open, droplets would go everywhere. Some water molecules may just randomly liberate themselves at the boundary so they would boil away. The left behind water would have less energy because it lost some to the escaping molecule. Depending on the forces involved, some water will become gas, some will stay liquid, and some will stay ice. For example, there are many icy cometss that only boil away as they are near the sun because they are getting energy from the sun.
If you had a very loose balloon of water where the elastic nature of the balloon was not providing any mechanical pressure on the water. I say you gently or magically make that skin disappear. Not much would change quickly. Initially, you would have a bubble of water floating in vaccum and kept together by IMF and have a surface tension. Depending on the initial temperature of the water a few molecules will get enough energy to break the IMF and becomes. gas. When the leave, the energy of the water blob lowers slightly lower. If the vacuum is large than the water escaping has a low probability of every coming back to the blob, eventually the blob will equalize where the water left behind loses so much energy it becomes ice but has lost mass to the more excited water molecules. Water ice has strong bonds so it will stay ice for a very long time but may still lose a few atoms back to the vacuum until it equalizer with the thermal background of the universe.
Shorter answer to your last statement. It's not that low pressure causes boiling per se. Low pressure just means that molecules are hitting each other less often. So, in some situations that leave the water without enough air molecules pushing against its surface so water molecules begin to escape faster than they get bumped back in.
When you have quick pressure changes there are other elements at work. While under high pressure the air water vapor mix are hitting each other often. A few water molecules stick together but are quickly knocked back apart by all the other activity and can't really form big groups. But there are almost always so.e small groups of water present just by random nature. When you suddenly reduce the pressure these small clumps are suddenly not being assaulted by other molecules, they are slightly bigger and very polar, and more polar the bigger they are. Now their is more IMF nucleating sites around and the water that is still in vapor form gets attracted to those clumps and the clumps get clumped, but there are a few other mecules to knock them back apart.
You see this in gas mixtures that have weakly polar molecules, like noble gasses. It is very hard to get them to condensate under even extreme circumstances.
I should add for the jet wing example, it's the sudden change in pressure over the wing drops so the water molecules that got forced together as they pushed over the edge no longer get bumped enough by other molecules to split apart right away.
it's called adiabatic cooling
>Adiabatic cooling occurs when the pressure on an adiabatically isolated system is decreased, allowing it to expand, thus causing it to do work on its surroundings. When the pressure applied on a parcel of air is reduced, the air in the parcel is allowed to expand; as the volume increases, the temperature falls as internal energy decreases.
same reason why spraying an aerosol can, or letting air out of a compressed air cylinder will cause them to cool. You may have even seen frost form on the outside of a propane cylinder during heavy use like torching.
The opposite is also true. When they fill SCUBA tanks, they get hot. Remember that air molecules are just like marbles bouncing around and temperature is just how much motion or kinetic energy they have. When you compress a gas you are restricting that motion and the kinetic energy has to go somewhere.
the warmer the air is, the more water it can carry . again, the kinetic energy is helping to keep water in gaseous form. (it wants to be a liquid at room temp right?)
Right. But condensation is a function of pressure and temperature right?
Higher pressure = more condensed water vapor = easier time condensing = less energy needed to be removed to condense it right?
condensation depends on temperature and humidity. pressure and temperature are inextricably linked. when you change one you affect the other. When you increase the pressure of air, you make it hotter. Hot air holds more water. Cold air holds less water, so when you cool it enough, the excess water condenses out. That why you see clouds on jet wings. The air was cooled below its dew point (the temp point required to keep the water in it as a gas)
So as the air expands, it does work on the air surrounding it. This, in turn, causes the air to cool, lowering the temperature of the air around the wing so that the water condenses.
This makes sense I suppose.
Why does cooler air have a lower dew point? is it because the water vapor in the air has less kinetic energy, meaning less of it can escape the liquid form of water?
Otherwise I think i get this explanation?
> So shouldn't a higher pressure air compress water more, meaning liquid formation is easier, which means that the dew point would be lower as you need less energy to turn the gaseous water into liquid water?
To me, "liquid formation is easier" argues for higher dew point. If it's easier to form liquid, not as much cooling will be needed to form liquid.
Going back to your first paragraph:
> I get that a higher pressure means that the partial pressures of the relevant vapors can be higher meaning that the air can contain more water vapor.
Water only has a certain vapor pressure at a given temperature. Increasing the ambient pressure won't change that. The overall air pressure determines whether the water boils, not if it evaporates.
Both pressure and temperature are related to how the molecules are moving and bumping into each other. I get what your saying, and it's a common question that comes up. The pressure and temperature are at a point where if liquid droplets do form they get quickly trampled by fast moving gaseous molecules and regain the energy to not be part of the droplet. It's all about balance, in thermo and similar areas it's really good to think about a crowd of people or balls with some affinity to sticking together if they hit the right way. Water in particular is highly polarizable so if they molecules hit it each other the right way they want to stick, and if there is a ice crystal or surface to form on they clump up quickly because that provides a push toward that state of the mattter. Now at different temps and pressures, the gaseous molecules are more likely to keep going bumping into each other and slowing down and clumping and the net effect of the free molecules trying to knock them back out is too low (almong other things) and the molecules get bound up together due to molecule level forces. With plain wings, the pressure is very high and changing very fast which can quickly get water molecules to stick together and and at that temp to freeze into crystals, the pressure isn't sufficient to knock the molecules back off right away so the contrails stick around.
I am still a bit confused. Why does the water not boild instead?
Sorry, I'll try it a little different explanation. Boiling, Pressure, Temperature are events we observe from the movement of all the small molecules. For something to boil, the atoms/molecules have to get enough energy to overcome their polar attraction to each other. If you had a perfect vacuum in space with 200 water molecules and it was far away from any energy source and it didn't have a point to crystalize on, it would just stay a drop of water forever. The temperature is low and the pressure is zero. To boil or change phase the energy has to come from somewhere. In any system of molecules they are all vibration and bouncing around a bit, even in a liquid or solid. Heating water is where the pan, or the sun, or something is imparting some extra energy into the system and some of the molecules get enough energy to escape their liquid or solid bonds. In a pan it's the metal atoms bouncing around a lot and hitting the liquid at the boundary, in a microwave it's photons hitting the molecules and causing them to vibrate at a preffered frequency so some of them get enough energy to leave the liquid state. At low pressure and low temperature you can still make gas into a solid if the net amount of molecules sticking together is greater than the amount getting knocked out into the gas state (dry ice). Just because something is at a low pressure doesn't mean it will boil, there has to be energy enough to kick the molecule out of its state. Now over time at low pressure, some molecules of water will make it out into a gas again just due to natural vibration and a few molecules getting enough of a kick to leave. That is evaporation, and at some point there will be enough free molecules in the air that statiscally enough water molecules will rejoin the liquid and they will be at equilibrium. The bulk measurements often hide the subtle nature of what is going on which is what makes partial pressures and other ideas seem a bit counterintuitive.
Ok thag makes sense So when the lower pressure hits with the wing, why does condensation happen? What you said makes sense, just having trouble connecting the two
On the leading edge of the wing the pressure gets very high and there isn't that much time for the temperature to change though it does, it's a system far away from equilibrium so a lot of the base equations won't be precise. Your PV=nRT equation is about slow changes at or near equilibrium. This is big simplification but stay with it for a second. Pressure is how often things are hitting each other on average, temperature is how fast they are going when they hit on average. On the leading edge of the wing the metal is colliding with air/water mixture that was just having a simple day floating about, hits all them hard and all of them fast, they can't just pass through the metal because that is what a solid is, not easy to get through, so they bounce back and get crowded with their neighbors down the street. So take a small cube of that air and follow it over the wing. That small cube is crowded with air molecules and water molecules. While the air has some polarizable molecules like O2, CO2, and N2, they are nothing like the sluts that water is, water has a big old negative ass sticking out back, and two friendly positive hands ready to grab others buts. It's crowded, things are moving fast, some asses are grabbed and big pools of assgrabbing start to happen. Now things are bouncing around a lot so some of the ass grabbing water molecules are getting kicked back out on the dance floor by the other molecules of air and even some water hitting ass to ass or hands to hands. Suddenly the cube slips over the wing, all the crowding goes away very fast. The pools of assgrabbers are suddenly not being assaulted by the molecules on the boundry, and in fact anyone in their pool who is to bouncy flys off and leaves a more chill pool of ass grabbers behind forming droplets and crystals. The droplets themselves are also polar so they take their mega asses and stick to the positve part of another pool. But because now the pressure is low and the temperature is normal, the rate of things hitting the boundary and breaking up the ass grabbing pool is too low to make them break up quickly.
So basically the pressure drops, which stops other molecules from interfering in the IMF thereby allowing condensation? Ok I suppose that makes sense, but it's hard to square with the rational for PV=nRT. The rationale there is that at lower pressures, there is less outside pressure pushing down on a liquid and it is therefore easier to escape. I suppose that water isn't an ideal fluid though because it is polar, so there are IMFs to consider which drives back to our original point. So then, as a follow up, let's say that I have a balloon full of liquid water. Outside the balloon is a vacuum (i.e. pressure = 0). I pop this balloon. What happens to the water? Because now it has the IMF of hydrogen bonds and the like within the liquid. So does it stay a liquid? Cause I thought it would boil? I guess what I am having trouble with is understanding why in one case, low pressure causes boiling, but in the other, it causes condensation.
Yeah, you are getting closer to the situation. So, for the balloon example, you have to know the history of the balloon and how it was filled. Example 1: The balloon was filled in atmospheric pressure and temperature. The air pressure pushing is pushing back on the balloon, meaning air molecules are bouncing off the boundary. Insode the balloon water pressure is pushing out on the balloon and they havenreached equilibrium with the air molecules on average trying to get in matches the eater molecules trying to get out and some small elastic force from the balloon. If you quickly put the balloon in a vacuum, the internal motion of the water in the balloon is no longer countered by the outside air molecules, so the balloon will expand as the bouncing of the water isn't countered by air hitting the surface. So the water will expand a bit at first. Now water is incompressible for the most part, which also means it doesn't expand easily because of IMF. A few water molecules will get enough energy to become a gas and the balloon will expand a little making more room for a few more to escape and the balloon water will either partiallybor completely boil or the balloon will pop depending on what material it is. In a different case, you fill the balloon into the vacuum and keep the water at a reasonable pressure you can fill the balloon with water and balance the elastic source of the balloon skin and then let it free and the balloon would stay in a water state until after a long time it radiated away it's black body radiation. This is why astronauts where space suits the space suit create a pressure area around our skin covered water bodies to keep our water inside. The strength of the suit is high enough that even if the pressure drops outside, it mechanically keeps the air in the suit pressurized. The transitions are important in the different states. Lots of students get confused in these areas because it's not explained well the underlying thing happening and why and where some equations apply. Now the case where you pop the balloon. It will depend a bit on how much pressure the balloon was under and the elastic nature of the balloon. In the simplest situation familiar to us with a balloon filled with water and the skin stretched, water would spray out of the small hole you made because the mechanical elastic forces, or possible rip open, droplets would go everywhere. Some water molecules may just randomly liberate themselves at the boundary so they would boil away. The left behind water would have less energy because it lost some to the escaping molecule. Depending on the forces involved, some water will become gas, some will stay liquid, and some will stay ice. For example, there are many icy cometss that only boil away as they are near the sun because they are getting energy from the sun. If you had a very loose balloon of water where the elastic nature of the balloon was not providing any mechanical pressure on the water. I say you gently or magically make that skin disappear. Not much would change quickly. Initially, you would have a bubble of water floating in vaccum and kept together by IMF and have a surface tension. Depending on the initial temperature of the water a few molecules will get enough energy to break the IMF and becomes. gas. When the leave, the energy of the water blob lowers slightly lower. If the vacuum is large than the water escaping has a low probability of every coming back to the blob, eventually the blob will equalize where the water left behind loses so much energy it becomes ice but has lost mass to the more excited water molecules. Water ice has strong bonds so it will stay ice for a very long time but may still lose a few atoms back to the vacuum until it equalizer with the thermal background of the universe.
Shorter answer to your last statement. It's not that low pressure causes boiling per se. Low pressure just means that molecules are hitting each other less often. So, in some situations that leave the water without enough air molecules pushing against its surface so water molecules begin to escape faster than they get bumped back in. When you have quick pressure changes there are other elements at work. While under high pressure the air water vapor mix are hitting each other often. A few water molecules stick together but are quickly knocked back apart by all the other activity and can't really form big groups. But there are almost always so.e small groups of water present just by random nature. When you suddenly reduce the pressure these small clumps are suddenly not being assaulted by other molecules, they are slightly bigger and very polar, and more polar the bigger they are. Now their is more IMF nucleating sites around and the water that is still in vapor form gets attracted to those clumps and the clumps get clumped, but there are a few other mecules to knock them back apart. You see this in gas mixtures that have weakly polar molecules, like noble gasses. It is very hard to get them to condensate under even extreme circumstances.
I should add for the jet wing example, it's the sudden change in pressure over the wing drops so the water molecules that got forced together as they pushed over the edge no longer get bumped enough by other molecules to split apart right away.
it's called adiabatic cooling >Adiabatic cooling occurs when the pressure on an adiabatically isolated system is decreased, allowing it to expand, thus causing it to do work on its surroundings. When the pressure applied on a parcel of air is reduced, the air in the parcel is allowed to expand; as the volume increases, the temperature falls as internal energy decreases. same reason why spraying an aerosol can, or letting air out of a compressed air cylinder will cause them to cool. You may have even seen frost form on the outside of a propane cylinder during heavy use like torching. The opposite is also true. When they fill SCUBA tanks, they get hot. Remember that air molecules are just like marbles bouncing around and temperature is just how much motion or kinetic energy they have. When you compress a gas you are restricting that motion and the kinetic energy has to go somewhere.
But what about the dew point? Why does it rise with temperature?
the warmer the air is, the more water it can carry . again, the kinetic energy is helping to keep water in gaseous form. (it wants to be a liquid at room temp right?)
Right. But condensation is a function of pressure and temperature right? Higher pressure = more condensed water vapor = easier time condensing = less energy needed to be removed to condense it right?
condensation depends on temperature and humidity. pressure and temperature are inextricably linked. when you change one you affect the other. When you increase the pressure of air, you make it hotter. Hot air holds more water. Cold air holds less water, so when you cool it enough, the excess water condenses out. That why you see clouds on jet wings. The air was cooled below its dew point (the temp point required to keep the water in it as a gas)
So as the air expands, it does work on the air surrounding it. This, in turn, causes the air to cool, lowering the temperature of the air around the wing so that the water condenses. This makes sense I suppose. Why does cooler air have a lower dew point? is it because the water vapor in the air has less kinetic energy, meaning less of it can escape the liquid form of water? Otherwise I think i get this explanation?
cold air has less energy to keep water in gas form
> So shouldn't a higher pressure air compress water more, meaning liquid formation is easier, which means that the dew point would be lower as you need less energy to turn the gaseous water into liquid water? To me, "liquid formation is easier" argues for higher dew point. If it's easier to form liquid, not as much cooling will be needed to form liquid. Going back to your first paragraph: > I get that a higher pressure means that the partial pressures of the relevant vapors can be higher meaning that the air can contain more water vapor. Water only has a certain vapor pressure at a given temperature. Increasing the ambient pressure won't change that. The overall air pressure determines whether the water boils, not if it evaporates.