# Lecture 4:  Introduction to evaporation and latent heat

https://www.youtube.com/watch?v=0oQUQ4wPGqg

[00:07] Hey, I'm Mel Strong, and this lecture is on evaporation in latent heat.
[00:13] We're gonna start by looking at phase transitions of water molecules.
[00:15] So here I've got a simulation from Phet, and what we're looking at are a bunch of water molecules that are in a container that we can see through.
[00:25] And we talked about this before, but water molecules have hydrogen on one side of the oxygen, so they kind of have this Mickey Mouse head appearance to them.
[00:34] So if you notice that the water molecules right now are not perfectly still.
[00:41] They're jiggling around a little bit.
[00:43] They're vibrating; what we would say is that they have vibrational energy.
[00:50] Now, despite the fact that they're moving a little bit, if you watch one for a long time, you'll notice that even though it's vibrating, it's not really changing its position.
[00:59] It doesn't really move very far, and it does not change position with its neighbors.
[01:05] Okay, I want to add a little bit of heat, okay, so and I've warmed it.
[01:11] Up and so the water molecules now look nervous.
[01:15] They are vibrating more and they have more vibrational energy.
[01:19] So this is true with all atoms and molecules.
[01:22] At Absolute Zero, which is zero degrees Kelvin, which is the coldest possible temperature, all motion stops and all atoms and all molecules cease to vibrate.
[01:35] But at any other temperature, there's some vibration of all substances.
[01:38] Now the warmer I make it, the more vibrational energy they have.
[01:44] So despite the fact that these guys are now vibrating more, you'll also notice that they still don't really change the position with their neighbors.
[01:54] They stay bonded with their neighbors.
[01:56] Also, I want to point out these holes: this one, that one, that one, a little one down there.
[02:03] Okay, keep an eye on those holes here, and I'm going to warm it up some more.
[02:12] Okay, right there something happened.
[02:15] And what happened is what we call a phase transition.
[02:17] We went from solid to liquid.
[02:20] In other words, we went from ice to liquid water.
[02:23] And you'll notice what happened when I did that.
[02:25] So now the water molecules are free to rotate, so you'll see some of them spinning.
[02:30] Spinning around, they're free to move.
[02:33] They don't have to stay in one spot anymore.
[02:35] Those open spaces went away.
[02:40] So right now in the liquid state, these water molecules are free to bang into each other and very slowly will migrate around.
[02:49] Okay, one that I'm gonna refreeze it, so let's cool it down.
[02:57] And we don't have the big holes that we had before, but here's one down here; it's a little one right there.
[03:01] So water molecules like to form six-sided rings when they're in the solid state.
[03:08] And if you look at a snowflake, a snowflake is often six-sided.
[03:14] Some are twelve sided and that six sided symmetry comes from this.
[03:17] It comes from the fact that water molecules like to assemble themselves with six-sided symmetry even at the molecular level.
[03:28] Okay, so in a solid, water molecules they're not perfectly still, but they're in general are not moving very, very quickly.
[03:37] They're not changing places with their neighbors; they're mostly staying in the same spot.
[03:42] When I warm it up, they are able to move around.
[03:45] It's kind of like this giant mosh pit of water molecules; they're all bumping into each other and they are free to spin or move around.
[03:53] Okay, so I'm gonna heat it up more.
[03:57] So as I heat it up more, what we're gonna see is that they have more vibrational energy; they're spinning faster and occasionally one of them breaks apart from the herd.
[04:07] And when that happens, we're seeing another phase transition, the transition between liquid and vapor.
[04:16] So at this particular temperature I can see I've got a couple vapor molecules up there and most of them are still.
[04:26] I'm going to heat it up some more.
[04:30] And as I heated up some more I can see more and more of the water molecules going into the vapor state.
[04:36] Now let me cool it back down.
[04:40] So I cool it back down and as I cool it down one of the things that's going to happen is the vapor that I have is going to condense down here with the liquid and we turn to the liquid state.
[04:55] And notice that right now compared to where I just had the temperature I have very little water molecules up in the vapor state.
[05:01] Okay, so this relates to something we learned about two lectures ago and we had that chart looking at maximum possible mixing ratio with temperature.
[05:10] And what we said at the time is at the higher the temperature the higher the maximum.
[05:18] Possible mixing ratio could be, and this is an illustration of that.
[05:22] When I make it hotter, more water molecules can enter the vapor state.
[05:25] When I have a colder, less water molecules can enter the vapor state.
[05:31] So here I'm warming it up again, and we're starting to see more of those water molecules enter the state as it is a vapor.
[05:37] So those are the things that keep in mind when you're trying to visualize water molecules.
[05:42] In a solid, they are gonna stay in one spot; they still vibrate a little bit.
[05:51] In liquid, it's like the mosh pit; they're all elbowing each other.
[05:53] In the vapor, they're free to move around, and they typically move in a straight line until they hit something else or hit each other.
[06:06] So we're going to get into evaporation now.
[06:08] And one of the things to keep in mind with evaporation is that whenever you have a body of liquid water that's exposed to the atmosphere, there are.
[06:18] Always going to be water molecules leaving that liquid water and going into the atmosphere.
[06:23] And at the same time, there will be water molecules leaving the atmosphere and entering the liquid.
[06:30] This is going on all the time, so anytime you've got liquid water, no matter how big the body of water is, you've got water molecules leaving, you've got water molecules coming back.
[06:42] Now, temperature is what controls the rate at which they leave.
[06:45] See, if you've got really cold water, the water molecules in this glass have relatively low kinetic energy, meaning that they're not moving very fast.
[06:56] And because they're not moving very fast, it's more difficult for them to leave the liquid surface of the water.
[07:02] Whereas this glass, because it's hotter, the water molecules have more kinetic energy.
[07:09] They're moving faster and they escape the surface of the water or more easier, so there's a higher flux of water.
[07:19] Molecules leaving the hot water than there is the cold water.
[07:22] Now this relates to something that we've talked about a couple of lectures ago.
[07:28] If I have an atmosphere sitting over the ocean and we would call that big blob of air an air mass, so we've got a big blob of air sitting over the Pacific Ocean, this air mass will pick up water vapor out of the Pacific Ocean.
[07:45] But the Pacific Ocean is pretty cold, and so even though it's picking up water vapor out of the ocean, it doesn't get that much compared if that same air mass was sitting over, say, the Gulf of Mexico.
[08:00] Gulf of Mexico, because that ocean is warmer, has a higher flux of water vapor entering the air mass that's sitting above it.
[08:09] So when we talked about how dewpoint will change with wind direction, this is the reason why air mass sits over a warm ocean; it picks up way more.
[08:20] Water than the air mass sits over a cold ocean.
[08:22] If the wind comes from the west, it blows in this relatively dry air.
[08:25] Our dew points typically are low.
[08:28] If the wind comes from the east and blows in this air with relatively high dew points, our dew points go up.
[08:35] So I'm going to show a couple diagrams with arrows that represent the fluxes of the water molecules leaving versus the water of water molecules that are turning.
[08:50] So in this case, what I'm trying to show here is that I've got about twice as many of the water molecules leaving as I do of those returning.
[08:57] Okay, that's what that means.
[09:02] So imagine I've got three glasses of water, and each glass of water is the same temperature, but in each case they are in an environment of different relative humidities.
[09:12] So what you would notice is that in each case the amount of water molecules leaving the flux is.
[09:20] The same for each, remember that's what the red arrow stands for.
[09:26] And it's dependent on temperature, and the temperature is the same, so all three red arrows are the same.
[09:32] But where we have low relative humidities, we actually see very few water molecules returning.
[09:38] Whereas with moderate humidities, we see more water molecules returning.
[09:45] And when we get to a hundred percent relative humidity, we see equal rates of those leaving and those returning.
[09:49] So if you think about evaporation as a net loss of water, which glass is going to dry up first?
[09:58] Well, this one is losing way more than it's gaining, so this one is going to dry up first.
[10:04] And in fact, relative humidity is correlated to the evaporation rate.
[10:10] So when you have low relative humidities, you have rapid evaporation.
[10:16] Whereas when you get to moderate relative humidities, the evaporation rate is slower, and when.
[10:22] You get to 100% evaporate relative humidity, you actually get no evaporation at all.
[10:27] So for us in New Mexico in June, we typically have the highest temperatures with the lowest dew points.
[10:35] And what that ends up with is very, very low relative humidities, so we'll get relative humidities sometimes 1% or less.
[10:44] Now, if you've ever mopped your floor in June, one of the things you may notice is that your floor will dry just about as fast as you can mop.
[10:53] There's a very, very high evaporation rate.
[10:56] When do we get into the monsoon?
[10:59] And we'll talk about the monsoon in the future lecture, but the relative humidity does go up.
[11:05] We get more water in the atmosphere, the evaporation rate slows down a little bit.
[11:10] In your, if you mop your floor, it takes a little longer for it to dry.
[11:14] Now here's an example question.
[11:18] We have three sponges and each one have the same amount of water in them and.
[11:23] Each are the same temperature.
[11:25] The only difference is that each of these are in a different environment of relative humidity.
[11:29] Which sponge dries up first?
[11:34] This one has the lowest relative humidity, so this sponge is going to dry it first because its evaporation rate will be higher.
[11:48] Relative humidity of a hundred percent is kind of a rare thing, especially here in New Mexico, but I want to talk about it a little bit because it's kind of a weird to think about.
[11:56] So if you lived in an environment where the relative humidity was a hundred percent, every time one water molecule leaves the glass, another one would immediately have to return and take its place.
[12:09] So what that means is that it doesn't matter how long you left this glass of water out, it would never ever ever evaporate.
[12:18] However, let's imagine you had some way to go in there and identify all the water molecules.
[12:23] Let's say you could put little.
[12:24] Tiny nametags and all the water molecules and you, and then you let it sit there for days and weeks and months.
[12:31] And you could go in and you could check and check who's in there.
[12:34] What you would notice as time went on as you'd see less and less of the original water molecules that were in your glass.
[12:43] And they've all been replaced by these strangers that came in from the atmosphere.
[12:48] So eventually, if you did, if you let this stay here long enough, all of the water molecules in your glass of water would be gone.
[12:56] And they'd all be replaced with these new ones that came in from the atmosphere.
[13:00] Okay, for this next part you need to use your imagination a little bit and we're working up towards something called latent heat.
[13:09] But to get there I have a goofy story that I have to explain.
[13:16] Just stick with me a little bit; it'll make sense in the end, I hope.
[13:19] So the first thing I want you to imagine is that the world is covered in these little tiny microscopic boxes you can't.
[13:26] See them, but if you could, they have a label on them.
[13:28] Each one says thermal energy, and these boxes are all over the place.
[13:33] But if you took a bunch of these boxes and you started adding them to something, the thing you add them to would get warmer.
[13:40] So we start throwing these boxes in this little pond; the pond would warm up.
[13:45] However, we could also reach down into the pond and grab those same boxes and pull them out, and if you took them away, the pond would become cooler.
[13:56] So anything you add them to becomes warmer; anything you take them away from becomes cooler.
[14:01] Okay, so now imagine that the water molecules, they live in this kingdom that they have, and there are these rules.
[14:12] And the rules are regarding these little boxes.
[14:16] And the first rule that they have is that if you are ever in the vapor state, you have to carry one of these boxes, a thermal energy, with you everywhere you go.
[14:23] Okay.
[14:26] Doesn't matter how long you're in the atmosphere.
[14:29] It doesn't matter where you go if you're in the vapor state.
[14:33] You have to carry a box with you at all times, no exceptions.
[14:36] Okay, they're very strict about this, all right.
[14:40] So rule number two: if you go from the vapor state and land back into liquid somewhere, you have to release your box into the liquid.
[14:49] Doesn't matter if you landed in the ocean, if you land in the river, if you land in someone's glass of water, you've got to release your box as soon as you land.
[14:59] That's the rule, okay.
[15:02] So what that means is if you could look really closely at a body of liquid water, you would see there are boxes of thermal energy floating around from all the water molecules that have landed.
[15:14] And then you'd see the water molecules bouncing into each other like the animation that we saw, okay.
[15:20] So these boxes are just going to stay there until somebody leaves, because rule number three is if you ever leave.
[15:29] Liquid state, you have to take a box of thermal energy with you, okay?
[15:34] That's the rule, because to be in vapor state you have to have a box.
[15:38] So what they do is, these boxes are floating around.
[15:41] It doesn't matter whose is who; they don't keep track.
[15:43] It doesn't matter, they're all the same.
[15:44] You just have to grab one before you go, okay?
[15:46] So what this means is, if we think about fluxes of water molecules leaving and returning, if we've got more guys leaving than we have coming back, these guys are pulling out more boxes of thermal energy than these guys are returning.
[16:06] We have a net loss of thermal energy.
[16:11] We also have a net loss of water molecules.
[16:13] But if we do that, because we're removing more thermal energy than we are replacing, the liquid down here is going to become cooler because it's losing thermal energy as a net loss.
[16:29] Thermal energy, on the other hand, if we had a situation where we had more of these guys arriving, dropping their boxes off into the liquid, then we had leaving, then we would have a net addition of thermal energy and the liquid would warm on account of this.
[16:45] Okay, so depending on the fluxes of how many you have leaving and how many have coming back, the body of water could get cooler or could get warmer.
[16:55] So let's go over a couple examples of how this works in real life.
[17:01] So as humans, when we get hot, we sweat, and the sweat is mostly water.
[17:07] And the water comes out, you know, it's exactly the same temperature as your body, so the sweat by itself does not cool you.
[17:15] But your body is relying on the evaporation, and if the conditions are right, then when water molecules evaporate off of your skin, they are removing those little units of thermal energy.
[17:28] And the...
[17:31] Remaining water, the remaining sweat, or your skin becomes cooler, which means that if you're human and you're sweating, what relative humidity conditions do you prefer?
[17:47] So in order for sweat to work, you need rapid evaporation.
[17:49] Rapid evaporation requires low relative humidity.
[17:52] So if you're sweating, it works best if you have low relative humidity.
[17:58] You've probably been in a situation where you've been in high relative humidity and hot temperatures.
[18:04] That's pretty miserable, and one of the reasons it's miserable is because your sweat simply is not evaporating very much.
[18:10] So there's very little latent heat being removed from your body.
[18:13] There's another example kind of in the opposite direction.
[18:19] So we've got a teapot here; there's water in there that's boiling, and what's coming out is water vapor, except you actually can't see water.
[18:31] Water vapor is invisible and so we commonly refer to this as steam.
[18:36] Steam has several definitions, but one definition is that it's a mixture of water vapor that you cannot see and little tiny droplets of liquid that you can see.
[18:48] So the part we can see is actually a little tiny microscopic droplets, but there's water vapor in here.
[18:53] For this example, let's imagine that this is all just water vapor, okay?
[18:57] And so the water vapor is coming out, and if you are unfortunate enough to put your hand in the stream of water vapor that's coming out of the teapot, the individual water molecules that come out are the temperature of boiling water because that's where they just came from.
[19:14] And they land on your hand and they transfer that heat to your hand and that burns you, right?
[19:21] That hurts, but it's actually worse than that because not only are they transferring the heat that they themselves are carrying, but they're also showing up with their little boxes of.
[19:32] Thermal energy that they took out of the water in the teapot and when they land on your hand not only are they transferring their high temperature to your hand but they're also depositing their little boxes of thermal energy.
[19:46] So you kind of have to imagine thermal energy just be piling up on your hand as these water molecules land on your hand.
[19:53] So you have the heat of the hot water molecules plus you've got the accumulated heat of all of their thermal energy units being piled on.
[20:04] So this produces a steam burn.
[20:07] In a steam burn, if you've ever had one, it's especially nasty because it's a burn that's hotter than what you would get in boiling water.
[20:15] It is the temperature of boiling water plus extra thermal energy on top of that, so it's a really painful type of burn.
[20:24] In nature, we talked about how dew will form if we cool the air below the dew point.
[20:30] And so what we talked about earlier a.
[20:33] couple of lectures ago is that as we
[20:35] cool the temperatures off at night if we
[20:38] get below the dew point we will start
[20:40] growing do usually on a very thin
[20:42] surface like grass well the other thing
[20:45] that actually happens during this
[20:47] process is that as the water molecules
[20:49] come down to join this growing dew drop
[20:52] they're adding their little boxes of
[20:55] thermal energy to this drop in so the
[20:58] formation to do actually adds heat to
[21:02] the the grass so this is a little
[21:05] confusing because we said you got you
[21:08] got to cool the temperatures down to get
[21:10] to the dew point and that's still true
[21:11] but what we're saying now is as you make
[21:15] do that's actually adding some heat back
[21:17] into the environment it doesn't warm the
[21:20] environment up but it prevents it from
[21:22] cooling down as much as it normally
[21:23] would so here's back to our sponge
[21:28] question except I've I've changed it a
[21:30] little bit so the beginning is the same
[21:32] as we saw before so we have three
[21:34] sponges each is wet each is the same
[21:37] temperature each has the same amount of
[21:39] water in it and the only difference is
[21:41] the relative humidity next to each
[21:44] sponge right so we have these three
[21:47] sponges and we said before that this one
[21:50] is the one that's going to evaporate
[21:51] first so now the question is instead you
[21:55] let them sit for 15 minutes and you
[21:57] return to fine all three still damp
[21:59] which one is the coldest so the relative
[22:04] humidity here is the lowest this one
[22:07] dries the first but not only is
[22:10] evaporation the most rapid on this one
[22:12] but we're also losing thermal energy
[22:15] more rapidly on this one so when we come
[22:18] back in 15 minutes this one will be the
[22:21] driest and it will be the coldest
[22:23] because it has lost the most thermal
[22:25] energy so this concept that we've been
[22:29] talking about where I invented this with
[22:31] a world of boxes and X and all that that
[22:34] is actually known as science is latent
[22:36] heat in latent means hidden so latent
[22:41] heat are these little packets of energy
[22:43] that you cannot see but they're
[22:44] transferred between water molecules when
[22:48] they change phases so as they go from
[22:50] from liquid to vapor and back from vapor
[22:52] to liquid they transfer this energy back
[22:54] and forth latent heat is a giant source
[22:58] of energy in weather and climate so when
[23:02] we when we look at say thunderstorms or
[23:04] hurricanes part of the energy a big
[23:07] chunk of the energy that's keeping those
[23:09] storms going is actually latent heat
[23:12] it's the is these little tiny packets of
[23:15] energy being transferred from water
[23:17] molecules as they go from one phase to
[23:19] the other so it's a pretty big deal in
[23:21] weather and climate a couple more
[23:25] examples so humans we sweat and latent
[23:31] heat is removed from our body dogs
[23:33] cannot sweat so they pant and so what
[23:37] dogs are relying on is the removal of
[23:40] latent heat from their tongue so they
[23:42] have a big wet sloppy tongue and the
[23:46] evaporation occurs over the tongue and
[23:47] water molecules remove latent heat from
[23:50] their tongue and that cools their tongue
[23:53] and there's blood vessels in their
[23:55] tongue that then take that you know the
[23:57] blood cools on their tongue and that
[23:59] gets circulated throughout their body
[24:00] and that's their cooling mechanism
[24:02] there's a little bit of cooling as they
[24:04] move air back and forth over the tongue
[24:06] from the outside but the main purpose of
[24:08] panting is to increase evaporation over
[24:11] the tongue so if you have bullet hot
[24:14] soup and you blow on the soup that's to
[24:16] increase the evaporation rate that route
[24:18] that removes extra latent heat so in a
[24:22] in a
[24:23] panting dog as the air moves back and
[24:26] forth over his tongue it's increasing
[24:28] the evaporation rate losing latent heat
[24:30] to cool the dog or cat so what would a
[24:35] dog rather have high relative humidity
[24:37] or low relative humidity and so just
[24:41] like us in order for their panting to be
[24:43] effective like our sweating to be
[24:45] effective they prefer to be in the low
[24:48] relative humidity environments so if you
[24:50] have a dog and you live in a place with
[24:51] high relative humidity and he's panting
[24:54] his panting probably isn't doing much
[24:56] good as a little dog you bringing
[24:58] doesn't know this but he's panting and
[25:00] panting and panting like he thinks it's
[25:01] gonna help it's just not helping because
[25:03] wait and he is not being removed from
[25:06] his tongue like he's expecting so a dog
[25:09] in a environment that's hot with a
[25:12] hundred percent relative humidity has no
[25:14] way of cooling himself and for us in
[25:16] terms of if we were sweating in an
[25:18] environment of 100% relative humidity
[25:20] we can't cool ourselves either so if you
[25:26] live in New Mexico or almost anywhere in
[25:28] the in the southwest you probably have
[25:30] something called a swamp cooler on your
[25:32] roof also known as an evaporative cooler
[25:35] and inside this cooler on around each of
[25:39] the four sometimes they have three sighs
[25:41] some things they have four sides there
[25:43] are these what look like giant sponges
[25:46] that we call pads and inside the pads
[25:49] are kept wet so there's a pool of water
[25:51] underneath so this is what you cut one
[25:55] open this is what it looks like there so
[25:57] this is filled with water down here and
[25:59] there's a whole fountain and it dribbles
[26:01] the water down there's the pads right
[26:04] there and it keeps them wet then this
[26:06] thing is a big motor big fan that sucks
[26:09] air and it sucks air in and it boils it
[26:12] down into the house so that's what this
[26:15] is okay so the air comes in through the
[26:18] pads and as it comes in through the pads
[26:20] the the wetness of this of the pad
[26:24] causes evaporation so water is
[26:27] evaporating from these pads and as it
[26:29] does so the evaporation removes thermal
[26:32] energy removes latent heat and the
[26:35] result the residual water
[26:37] the left is cooler and as the air runs
[26:40] through that cooler pad it cools the air
[26:43] so in my animation here I have water
[26:47] molecules leaving and going up in the
[26:49] atmosphere some do that but a lot of
[26:51] them actually go the evaporate into the
[26:54] air that is then pushed into your house
[26:56] so part of the result of this is that it
[27:01] actually adds water vapor to the air in
[27:04] your house so typically you know this
[27:06] might be the setup it might be United
[27:08] five degrees outside only 10 percent
[27:10] relative humidity the air gets drawn in
[27:14] goes to the wet pads evaporation occurs
[27:16] cooling occurs remove of latent heat
[27:19] occurs and then that the air now is
[27:22] maybe 15 degrees colder than it was but
[27:25] now it's the relative humidity is higher
[27:28] and for two reasons
[27:29] number one as we learned in the previous
[27:31] lecture if I just cool the temperature
[27:33] down I will increase relative humidity
[27:35] and number two a lot of the evaporation
[27:38] that takes place actually adds water to
[27:41] the air that's going through the swamp
[27:43] cooler itself so the air that's coming
[27:45] into your house has more water vapor in
[27:48] the air itself and so you have a higher
[27:50] relative humidity and a higher mixing
[27:52] ratio and a higher dew point inside your
[27:54] house as a result so we have these all
[27:58] over the Southwest and this is a map of
[28:01] relative humidity of the entire country
[28:04] and this is a national average average
[28:07] over a whole whole year so what you'll
[28:11] see here now if we can read this but it
[28:13] says relative humidity percent and this
[28:14] is says 20 to 35 and then the 30 is 40s
[28:17] 50s 70s and this is average again over
[28:20] the whole year so the winter and summer
[28:22] the whole thing but what you notice is
[28:24] that right here this area that kind of
[28:27] brownish area this is the area that has
[28:29] the lowest relative humidity in the
[28:31] country and if you have a swamp cooler
[28:34] an evaporative cooler this is the part
[28:37] of the world the part of the country
[28:38] sorry that it's actually going to work
[28:41] best because if I go back if the
[28:43] relative humidity is too high I don't
[28:46] have much evaporation if I don't have
[28:48] much evaporation I don't have
[28:50] much cooling so the only way this thing
[28:52] works is if I have very low relative
[28:55] humidities once you get out into this
[28:58] part of the country
[28:59] nobody has swamp coolers people down
[29:02] here don't even know the swamp core it
[29:04] is they've never seen one because if you
[29:05] built one you put one on your house it
[29:07] would do nothing because you've got like
[29:09] like that green zone right there that's
[29:11] relative humidities that are routinely
[29:13] over 80% if you had a swamp cooler and
[29:17] you turn it on there'd be so little
[29:18] evaporation you notice no cooling effect
[29:21] at all so really realistically when we
[29:25] get really kind of east of the Sandia
[29:27] Mountains kind of right through here
[29:28] nobody over here has swamp cores because
[29:31] they just are very ineffective this area
[29:34] the desert southwest of swamp coolers
[29:37] worked especially during those months
[29:39] with a little relative humidity so why
[29:42] do we have that instead of normal
[29:44] refrigerated air because everybody over
[29:46] here has what we call air conditioning
[29:48] normal refrigerated air the advantage of
[29:52] swamp coolers is they are cheaper to
[29:53] make and they're cheaper to run so they
[29:55] don't require nearly as much electricity
[29:57] as refrigerated air so you could put
[30:00] refrigerated air on your house and it
[30:03] would be colder but it also cost a lot
[30:05] more so that's why you see a lot of
[30:07] these swamp coolers in this part of the
[30:08] country they are cheap to build and
[30:11] they're cheap to run and they work
[30:12] because we have low relative humidity
[30:16] okay so take-home points from this whole
[30:20] lecture water molecules are always
[30:23] leaving and entering the surface of a
[30:25] liquid all times and depending on the
[30:28] temperature the flux of water molecules
[30:31] you know there's height there's a higher
[30:32] flux of water leaving warmer water than
[30:35] there there is cooler water the rate of
[30:38] the molecules returning is controlled by
[30:40] relative humidity right so the higher
[30:42] the relative humidity the more water
[30:44] molecules are coming back the lower the
[30:48] relative humidity the higher the
[30:50] evaporation rate so we talked about that
[30:52] and then when water exchanges phases it
[30:56] exchanges these small little invisible
[30:58] packets of energy that we call thermal
[31:01] energy and so I made up the whole story
[31:02] with the little boxes so they
[31:04] you can visualize that just to make sure
[31:06] we're Claire there are no little tiny
[31:08] boxes that say thermal energy lying
[31:10] around okay that's a whole visual so you
[31:13] can kind of understand the concept but
[31:15] when water makes transitions between
[31:17] phases it transfers these little packets
[31:20] of energy along we call that latent heat
[31:22] evaporation removes latent heat
[31:25] condensation adds latent heat alright so
[31:29] next time we will talk about how all of
[31:31] this works to make clouds thanks for
[31:34] watching
[31:43] you