Heat and Adiabatic Processes
Summary of Heat and Adiabatic Processes
This section discusses Heat and Adiabatic Processes as part of a module on Temperature and Water. The focus is on three main concepts: heat, latent and sensible heat, and adiabatic processes, which are crucial for understanding atmospheric dynamics.
qna
what does temp measure
Temperature measures the kinetic energy of the molecules, in other words the energy associated with how fast the molecules are moving.
what does lapse rate tell us about the atmosphere
The lapse rate describes how much the temperature decreases or increases with altitude
what does the adiabatic lapse rate change depending on whether the air is dry or moist?
The lapse rate is lower when water is present because the water has a higher heat capacity than the dry air.
Heat
Definition: Heat is the energy transferred to or from a system, typically through conduction, radiation, or friction, but not through work or matter transfer.
Distinction: Heat is not a property of a system but rather a transfer between systems. For example, when consuming hot food, heat is transferred to our body, not the food itself.
Latent Heat vs. Sensible Heat
Latent Heat:
Refers to the hidden energy associated with phase changes (e.g., melting ice or boiling water) that does not change temperature.
Example: Water absorbs latent heat when it boils at 100°C without a temperature increase.
Sensible Heat:
Associated with temperature changes that can be sensed or measured.
Example: Heating water from 20°C to 100°C involves sensible heat, as the temperature increases can be felt or measured.
Adiabatic Processes
Definition: Adiabatic processes occur when a system does not exchange heat with its surroundings. Changes in temperature, pressure, and volume happen without heat transfer.
Example: In mountainous regions, air forced upward cools adiabatically, leading to cloud formation when it reaches the dew point. Conversely, descending air compresses and warms, dissipating clouds.
Importance in Atmospheric Processes
The concepts of latent heat, sensible heat, and adiabatic processes are essential for understanding how energy is stored and transferred in the atmosphere, influencing weather patterns and climate.
In summary, these three concepts—heat, latent and sensible heat, and adiabatic processes—are fundamental in defining the properties that control atmospheric processes.
(light music) - This section is on Heat and Adiabatic Processes and this is part of our module two on Temperature and Water. We're gonna look at three different things. We're gonna go back to heat which we introduced in previous sections and then we're gonna define specific types of heat. One's called latent heat and the other is called sensible heat, and then we'll think about adiabatic processes and apply them to the atmosphere. And these three things help us to define the properties that control atmospheric processes. Heat, so last time we looked a lot at what temperature was and we decided it was really the energy of motion of the molecules in a fluid, or a liquid, or a solid. Heat is energy that is being transferred to or from a system, typically by conduction, right? One thing in contact with another. Can also be by radiation such as light from the sun. It could even be by friction, right? If you're driving along the road, there's gonna be heat transferred in that process. Mechanisms other than thermodynamic work or transfer of matter are those that qualify as heat. So it's the energy being transferred, but it's not being transferred by work and it's not being transferred by actually moving matter from one to another. Those can also transfer heat, right? If you add cold ice to a warm glass of water, then you're gonna bring some heat or lack of heat with the ice to the water, and that's by transferring matter. So but that's the matter that's bringing the different temperature, not heat. So heat involves transfer between a system and its surroundings or could be between two systems or objects. It's just not a property of one system alone. It contributes to changing the system's internal energy which is the property of the system alone. So heat is the thing that gives you more temperature, but it isn't something that you're going to contain in your system. So it's an important, it's kind of a picky distinction but it's important that we get this straight so we understand when we're distinguishing between energy that's in a system and heat that's transferred between systems. And so heat as we've defined it now is different from the ordinary language in everyday life and the usage we have for heat as a property of the system itself 'cause sometimes you'll say, "Oh, that chicken pop pie has a lot of heat in it," but what we really mean is its temperature is hot and only if we eat the chicken pot pie and our tongue gets hot because we're eating it, then we've had heat transferred from the chicken pot pie to us. So the quantity of energy transferred as heat in a process is the amount of transferred energy excluding any work that was done and any energy contained in the matter transferred. So yes, in my chicken pot pie example, I'd have to spit the chicken pot pie back out so that I didn't transfer the matter to me, that it just transferred the heat and not the matter. Differences between latent and sensible heat are important to keep track of as well. Latent heat, and we'll think about what that word means in a second, is the potential energy of particles which is hidden as it does not change the temperature. So by latent, we mean hidden, right? And in what way is it hidden? It's hidden because it doesn't reflect itself in what the temperature of the object is. So you can have a lot of latent heat or a little bit, but it doesn't change the temperature. One way to think about that is you can have water that has latent heat and when it starts to boil at 100 degrees Celsius, it's gonna keep taking up more latent heat to get the energy for those vapor molecules to leave the liquid phase. And so it's kind of hidden in there and it is shown in the phase change, but it's hidden from the temperature. So that's all we're thinking about is the difference between the heat that goes into a phase change versus the heat that goes into a temperature change. And so the temperature change one is gonna be the sensible one, so we'll get to that next. But for latent heat, the energy possessed is because of the distancing of particles where attraction was over a greater distance, IE in the form of potential energy. And so the latent heat is because of the distance between molecules in the fluid, and the latent heat is the heat released or absorbed by a system during a change of state that occurs without a change in temperature. And what is that usually? That's usually a phase change. So examples are phase changes such as the melting of ice to form water or the boiling of water to form steam. So that's what latent heat to us. It's sort of the heat associated with a phase change. So what's sensible and why do we call it sensible? Well, it's something that's able to be sensed. So what can we sense? We can sense temperature. We may not know how much of the water, we can't tell how much of the water is liquid and how much is ice, but we can measure and sense what its temperature is. So the heat that's associated with changing the temperature is what we call sensible heat and since temperature was associated with the energy and motion of the molecules or its kinetic energy, the sensible heat is really a measure of how much kinetic energy is being transferred. So sensible heat is the heat associated with the energy involving the motion of particles, namely kinetic energy and examples are adding heat that results in an increase in temperature. And so for example, if you put a pot of water on the stove, it's gonna start out near room temperature. Maybe it's 20 degrees Celsius. You turn the heat up on the stove and it's gonna be adding heat to that water, and that water is gonna go from 20 degrees Celsius, and 30, and then 40, and then 80, and then maybe 100, and you can measure that with a thermometer and you could probably also touch it and tell that it's getting hotter. And so I wouldn't do this after about 50 degrees, but you can sense where that energy is going, what that heat is doing. But once you hit 100 degrees, it's gonna feel the same to a thermometer, not your finger at 100 degrees. And what's gonna happen is more and more of the liquid is gonna change to steam and it's just gonna sit there at 100 degrees even though your stove is still putting lots of heat in, why is that? It's because all that heat is now going into latent heat, right? Instead of sensible heat. So that's what our important difference is between sensible heat and latent heat. And here's another way to think about the same set of changes that I just described with slightly more quantified aspects here. And so this graph shows temperature on the left and the amount of heat added on the bottom. So in my example, sort of as time passes, I was adding more and more heat. Here we're just looking at, we don't have time, we just have heat increasing and we're going from minus 100 degrees Celsius up to 200 degrees Celsius. And so let's start at the bottom left. We've got ice at minus 100 degrees Celsius. If you add heat to it, you can increase the temperature from minus 100, to minus 80, to minus 50, to minus 20, to zero. And if you read the bottom axis, that'll take approximately 200 of our kilojoules per kilogram and that's because something called the specific heat capacity of the ice is that it takes 2.1 kilojoules per kilogram per kelvin. So for 100 kelvin change, each kilojoule per kilogram is gonna need 2.1 times 100 or 210 kilojoules per kilogram. So that gets us up to zero, right? So we're now where the blue starts going sideways. So what does that mean that it's going sideways? Well, it means it's staying at the same temperature, but look at the bottom axis. We're still adding over 300 kilojoules per kilogram while it's staying at zero degrees Celsius. So as you melt the ice, you're adding in this latent heat and that's what's changing from solid with very little motion to liquid with a little more motion, but they're all at zero degrees celsius. So the average of their motion is the same until you raise the temperature again. So that's our latent heat, 334 kilojoules per kilogram to go from solid at zero C to liquid at zero C. So we get to liquid now and now we're on the green line, and we can keep going up. So here we can go from zero to 100 degrees and here for liquid water, our specific heat capacity is about 4.2 kilos per kilogram per kelvin. So we actually need twice as much heat for every degree Celsius to get up this next bit. And so it takes, for that 100 we're now gonna need 420 kilojoules per kilogram to get up to 100 degrees Celsius. So that's one difference between our phases is we actually need more heat to heat water up per gram than we do to heat ice up per gram per degree. So that's really interesting as well and now we get to 100. So we're at 100 and again, we go sideways. And now it's this huge trip sideways and that's because once you hit 100 degrees Celsius, you're gonna change from liquid to steam and this has a huge amount of latent heat required. It actually needs 2,265 kilojoules per kilogram to go from water to steam. So that's a huge amount of latent heat, so this is our hidden heat where we're not changing the temperature at all. We can't sense the difference. The temperature and the kinetic energy of the molecules is staying the same. We're just changing phase. But that's exactly why both water is so unique because it's latent heat is so high and it's latent heat is so high because of the polarity of the water molecules and how they hydrogen bond to each other. So breaking them from liquid to gas takes a lot of energy and this is hugely important for the atmosphere because it gives the atmosphere this way to store energy while effectively modulating the temperature 'cause you can't increase past that point until everything has turned to steam. That's on the very right-hand side, and then we could keep increasing temperature once we have it all as steam and it turns out that the heat capacity for steam is actually more similar to ice than water. So again, liquid water is super special 'cause it slows down the change in temperature associated with any heat change. This is why this latent heat of water at 2,265 kilojoules per kilogram is super important and it's why it's so closely tied, the concepts of latent heat, specific heat capacity, and temperature of the air. And this means that convection and other kinds of vertical transport are linked to the changes in the phases of water 'cause convection is gonna be this mixing up and down of air. But as you make ups and down, you're gonna have a lapse rate change in your temperature. And so those changes in temperature get modulated when you hit one of these phase changes and by that I mean it's gonna kind of dial down the impact, so it doesn't get as hot as you think it should. So next we wanna dive into this concept of adiabatic heating and what we mean by this is how heating happens when you can't transfer heat between the surroundings in the system. So here I've got a cartoon of a piston, sort of like the one that maybe used to be in your car. Now they're super fancy, but this is a really simple one. And so what it has is a fluid in the green area that as you pull the piston down can expand and then as you push the piston up it compresses. And so this allows your temperature and pressure and volume to change and what we specify when we select adiabatic is that those temperature pressure and volume changes of your fluid can exchange any heat with the surroundings. And so that's why the green box is drawn as thick because it's insulated, and it's keeping heat out and heat in and so they're not exchanging at all. And so heat was transferred and so if we don't have any heat going in or out, we call that adiabatic. And this is our simple example of a piston compressing a gas contained within a cylinder. The piston system is defined to be adiabatic here. if the heat condition from the system through the insulated walls is slow compared to your compressible time. So it's probably not perfectly adiabatic. It just means it's slow compared to the other changes that you're looking at. And that definition of adiabatic is important for this video, is thinking about how quickly heat is being transferred from the system to the surroundings and back and how that actually helps to form a particular type of clouds in mountainous regions. And so take a look at this video. - We've all seen clouds build up on one side of the mountain and then on the other side just dissipate into blue sky. Maybe you wanna know why that happens. Like most natural events, this one has an impressive scientific term attached to it. It's called adiabatic cooling and heating and occurs because of changes in air pressure. Here's some time lapse video that shows what happens. Basically as a parcel air encounters a mountain, it is forced upward. As air pressure decreases with altitude, the air parcel expands. Expansion causes the air to cool. When the air cools to its so-called dew point, the water vapor in the air condenses and becomes visible as a cloud. If there's enough moisture and the adiabatic cooling is strong enough, it rains or snows. Essentially the opposite occurs on the other side of the mountain. The cool air sinks and compresses. Compression results in increased temperature. When temperature rises above the dew point, the cloud dissipates into invisible water vapor. In Wyoming, especially in winter, most of the moisture-laden air masses come from the Pacific, approaching our mountains from the West. So as adiabatic cooling occurs, more rain and snow is dumped on West-facing slopes. As warmer drier air descends on the Eastern slopes, it accounts for another famous phenomenon of the plains, the so-called Chinook wins. So we've looked at clouds from both sides now. Knowing why they form and disappeared does not diminish their beauty. If it weren't for our mountains and the dynamic processes that occur, Wyoming would be a much drier place and frankly much less interesting. I'm Tom Hill from the University of Wyoming Cooperative Extension Service, exploring the nature of Wyoming. - Adiabatic cooling as illustrated in the video occurs when the pressure on an 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 gas is reduced, the gas in the parcel is allowed to expand. So when we put gas in a container and you can think about various examples, for instance, a carbonated beverage has some gas in it. When you contain it, it's under a certain pressure. But if you open the top of the carbonated beverage, the gas can expand and bubble out. And so when it's allowed to expand, the volume is going to increase and the temperature falls as its internal energy decreases. Adiabatic cooling occurs with what we call orographic lifting and lee waves and here orographic is just a fancy word to say that you have a lot of terrain that's probably mountainous as opposed to a flat surface. And so orographic lifting is when air comes up to a mountain and it gets pushed up and over the mountain, and that actually gives you a change in the updraft that can often form a cloud. And so clouds forming from orographic lifting can be really important and they're good examples of approximately adiabatic cooling. So in thinking about heat and adiabatic processes, we've covered three topics. Heat, the differences between latent and sensible heat, and adiabatic processes, and these three concepts allow us to define the properties that control atmospheric processes.