Earth Science: Lecture 18 - Atmospheric Moisture

[00:00:00] Speaker 1: Hello, and welcome to Earth Science, Lecture 18, Atmospheric Moisture. Water vapor is an odorless, colorless gas that mixes freely with the other gases of the atmosphere. Unlike oxygen and nitrogen, the two most abundant components of the atmosphere, water can change from one state of matter to another, that being solid, liquid, or gas, at the temperatures and pressures that are experienced at Earth's surface. Because of this unique property, water leaves the oceans as a gas, but returns to them as a liquid. As you observe day-to-day weather changes, you might ask yourself: Why is it generally more humid in the summer than in the winter? Why do clouds form on some occasions but not others? Why do some clouds look thin and harmless, whereas others form dark and ominous towers? Answers to these questions involve the role of water vapor in the atmosphere, and is the central theme of this lecture. Water is the only substance that naturally exists on Earth as a solid, such as ice, liquid, and a gas, water vapor. Ice is composed of water molecules that form a tight, orderly network. Thus, water molecules in ice are not free to move, but rather vibrate about fixed sites. When ice is heated, the molecules oscillate more rapidly. When vibrations increase sufficiently, the bonds begin to break, resulting in the process of melting. In the liquid state, water molecules are still tightly packed, but they are moving fast enough that they are able to slide past one another. As a result, liquid water is fluid and takes the shape of its container. As liquid water gains heat from its environment, some of the molecules acquire enough energy to break the remaining molecular attractions and escape from the surface, becoming water vapor. Water vapor molecules are widely spaced compared to liquid water, and exhibit very energetic random motions. Unlike a liquid, a gas will expand to occupy a container of any size, and it can also be compressed. For example, you can easily put more and more air into a tire and increase its volume only slightly. However, you can't put 10 gallons of gasoline into a 5-gallon can. So this slide here just serves as a reference for my students. So this shows the different states. We have solid in the bottom left, liquid in the middle, and gas in the upper right. And then I threw in some random animations just because I thought they were kind of cool. But what you also see here is the different processes that lead from one state of matter to the other, and also how much energy is absorbed or released. So this will be the focus of our discussion moving forward for at least a little while. So use this as a reference if you need it. We'll return to these ideas later. Whenever water changes state, heat is exchanged between water and its surroundings. Heat is absorbed, for example, when water evaporates. Meteorologists often refer to heat energy or measure heat energy in calories. One calorie is the amount of heat required to raise the temperature of one gram of liquid one degree Celsius, or 1.8 degrees Fahrenheit. Thus, when 10 calories of heat are added to one gram of water, the water molecules vibrate faster, and a 10 degrees Celsius temperature rise occurs. Under certain conditions, a substance can absorb heat without an accompanying rise in temperature. For example, when a glass of water with ice is warmed, the temperature of the ice water mixture remains constant at 32 degrees Fahrenheit, or 0 degrees Celsius, until all of the ice is melted. So, this is really interesting. If adding heat does not raise the temperature, where does the energy go? If we're adding energy to this, how can it not increase its temperature? Well, in this case, the added energy goes into breaking the molecular attractions between the water molecules and the ice cubes. Because the heat used to melt ice does not produce a temperature change, it is referred to as latent heat, where latent means hidden. This energy can be thought of as being stored in the liquid water, and is not released into its surroundings as heat until the liquid has returned to the solid state. Measuring one gram of ice requires 80 calories, I'm sorry, melting one gram of ice requires 80 calories, an amount referred to as the latent heat of melting. Freezing, which is the reverse process, releases the 80 calories per gram to its surroundings as the latent heat of freezing. So, what this is telling us is that temperatures do not change whenever you change phase. And we can see that with this chart. So, on the right we see an interesting chart. Temperature is on the vertical, and this is the heat input. So, from left to right, we're heating up. Ice warms to the melting point. So, we see that in the very beginning here. So, maybe we're starting at, say, -20 degrees Celsius. We're heating up our ice. Ice warms to the melting point, then absorbs heat during the phase change as the temperature remains constant. During a change in state, the heat energy is used to change the bonding between the molecules. In the case of melting, as we mentioned previously, that added energy is used to break the bonds between the molecules. When all of the ice has melted, the water will warm, then, to its boiling point. Where temperatures, again, will remain constant as heat is absorbed during the second phase, from liquid to gas. After all the liquid has changed to gas, the continued warming will increase the temperature of the water vapor. So, again, during any phase changes, we do not see a change in temperature, because that energy is being used, in this case, to break apart the bonds between the molecules. As you will see, latent heat plays an important role in many atmospheric processes. In particular, when water vapor condenses to form cloud droplets, latent heat of condensation is released, warming the surrounding air and giving it buoyancy. When the moisture content of air is high, this process can spur the growth of towering storm clouds. So, at this point, I'd like you to stop this video and watch the one that I've linked in the YouTube description. I will warn you, it is very technical and detailed, so you might have to watch it two, maybe even three times, which is what I would do in my lecture, in a face-to-face course. Moving on, we saw that heat is absorbed when ice is converted to liquid water. Heat is also absorbed during the process of evaporation, the process of converting a liquid to a gas. The energy absorbed by water molecules during evaporation is used to give them the motions needed to escape the surface of a liquid and become a gas. This energy is referred to as the latent heat of vaporization, because it's turning into a vapor. During the process of evaporation, the higher temperature, that is, faster moving molecules, escape from the surface. As a result, the average molecular motion, or the average temperature, of the remaining water is reduced. Hence the common expression that evaporation is a cooling process. You have undoubtedly experienced this cooling effect. From whenever you step out of the shower or a swimming pool when you're wet, in such a situation, the energy used to evaporate water comes from your skin and you feel cool. So you're not necessarily cold because you're stepping out into colder air. The air is probably the same temperature as when you first got in. You are just experiencing the process of evaporation, which is a cooling effect. Because the most energetic molecules are leaving. So here in this cup, we see 25s. These are the most energetic ones. They leave. And so the energy is being reduced. And so it has a cooling effect. The reverse process of condensation occurs when water vapor changes to the liquid state. During condensation, water vapor molecules release energy, that is the latent heat of condensation, in an amount equivalent to what was absorbed during evaporation. When condensation occurs in the atmosphere, it results in the formation of such phenomena as fog and clouds. Sublimation is the conversion of a solid directly into a gas, without passing through the liquid state. Examples you may have observed include the gradual shrinking of unused ice cubes in the freezer and the rapid conversion of dry ice to wispy clouds that quickly disappear. Deposition, on the other hand, refers to the reverse process. The conversion of a vapor directly into a solid. This change occurs, for example, when water vapor is deposited as ice on solid objects such as grass and windows. These deposits, called hoarfrost or simply frost, is called hoarfrost or just frost. A household example of this process is the frost that accumulates in the freezer. Deposition releases an amount of energy equal to the total amount released by condensation and freezing. So now let's discuss water vapor in the atmosphere. Water vapor constitutes only a small fraction of our atmosphere, varying from as little as 1/10 of 1% up to about 4% by volume. But the importance of water vapor in the air is far greater than these small percentages would indicate. Indeed, scientists agree that water vapor is the most important gas in the atmosphere when it comes to understanding atmospheric processes. Humidity is the amount of water vapor in the air. Meteorologists employ several methods to express water vapor content in the air. Before we consider these humidity measures further, it is important to understand the concept of saturation. Imagine a closed jar that contains water overlain by dry air, both at the same temperature. As the water begins to evaporate from the water surface, a small increase in pressure can be detected in the air above. This increase is the result of the motion of water vapor molecules that were added to the air through evaporation. In the open atmosphere, this pressure is called vapor pressure, and is defined as the part of the total atmospheric pressure that can be attributed to the water vapor content. So this is where it gets pretty interesting and very, very important. In that closed container, as more and more molecules escape from the water surface, the steadily increasing vapor pressure in the air above forces more and more of those molecules to remain or return to the liquid. Eventually, the number of vapor molecules returning to the water will balance the number leaving. At that point, the air is said to be saturated, which means it can hold no more water vapor in its region. However, if we add heat to the container, which would increase the temperature of the water and air, more water will evaporate before a balance is reached. Consequently, at higher temperatures, more moisture is required to reach saturation. So this chart on the right is going to be almost annoying because we are going to look at this a lot. In fact, we'll be flying back to this slide multiple times throughout the rest of this lecture. What this is showing you is that as temperatures increase, so here you see temperature in Celsius on the left, Fahrenheit on the right, as our temperatures increase, the water vapor content at saturation is also increasing. In other words, for example, at 14 degrees Fahrenheit, we only need 2 grams of water vapor to reach saturation. But at 104 degrees, we need 47 grams of water vapor content in our atmosphere to reach the same level of saturation. So this is very, very important. Not all air is saturated, of course. Thus, we need ways to express how humid a parcel of air is. One method is to specify the amount of water vapor contained in a unit of air. The mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of dry air. So the equation is something along the lines of the mixing ratio is equal to the mass of the water vapor in grams divided by the mass of the dry air in kilograms. Because the mixing ratio is expressed in units of mass, it is not affected by changes in pressure or temperature. The quantity of water vapor in the air remains the same. However, measuring the mixing ratio by direct sampling is time consuming. Thus, meteorologists commonly use methods different from this to express the moisture content of air. These include relative humidity and dew point temperature, things we will look at in just a moment. So here we are with that chart once again. The most familiar and, unfortunately, the most misunderstood term used to describe the moisture content of the air is relative humidity. Relative humidity is the ratio of the air's actual water vapor content to the amount of water vapor that is required for saturation at that temperature and pressure. So this compares what's there and what is needed to be saturated. Thus, unlike the mixing ratio, relative humidity indicates how near the air is to saturation rather than the actual quantity of water vapor in the air. For example, at 25 degrees Celsius or 77 degrees Fahrenheit, air is saturated when it contains 20 grams of water vapor per kilogram of dry air. Thus, if the air contains only 10 grams of water vapor at that temperature, the relative humidity is 10 over 20. In other words, 50% relative humidity. Right, so we need 20 grams for it to be fully saturated or 100% humidity. But we only, say in this example, have 10 grams. So we only have half of what is needed for saturation, and so we're at 50% relative humidity. If we were to increase to 20 grams, then we would be at 100% humidity or saturation. Because relative humidity depends both on the air's water vapor content and on the amount of moisture required for saturation, it can be changed in either of two ways. First, relative humidity can be changed by adding or removing water vapor. Fairly straightforward. You add more water, you're going to change how humid it is. And second, because the amount of moisture required for saturation depends on the air temperature, that is, again, the warmer the air, the more water that is required to saturate it, relative humidity also varies with temperature. So we're going to look at an example of both. First, let's take a look at how humidity varies with moisture. And I understand that this figure has a lot going on, but we'll take a good look at it and try to step through it without any confusion, I hope. In nature, moisture is added to the air mainly via evaporation from the oceans. However, plants, soil, and smaller bodies of water also make substantial contributions. When water vapor is added to a parcel of air, the relative humidity of that parcel increases until saturation occurs, that is, until we reach 100% relative humidity. What if even more moisture is added to this parcel of saturated air? In other words, what if we're at 100% humidity or saturation and we're still adding more? Does the relative humidity exceed 100%? Normally, this situation does not occur. Instead, as in the closed container we described previously, the excess water vapor condenses to form liquid water. You may have experienced such a situation while taking a hot shower. The water is composed of very energetic hot molecules, which means that the rate of evaporation is high, because they're moving so fast they can escape the surface. As long as you run the shower, the process of evaporation continually adds water vapor to the unsaturated air in the bathroom. If you stay in a hot shower long enough, the air eventually becomes saturated, and the excess water vapor begins to condense on the mirror, the windows, the tiles, and whatever other cool surfaces are in the room. So this figure here gives you an example. From the table that we saw previously, at 25 degrees Celsius, we need 20 grams of water vapor to saturate the air. So here's an example. We're at 25 degrees, and so number one here is saying we need 20 grams of water vapor content for us to be saturated. But we only have 5, but we only have 5 in this example. So if we have 5, but 20 is what we need for saturation, we have 5 divided by 20, which is 25% humidity. Then, as we increase the amount of water vapor, now we're up to 10 grams. Well, we still only need 20 to reach saturation. We have 10 over 20 here, which is 50% humidity. And then we continue adding water vapor to this. We're up to 20 grams. We need 20 to reach saturation. So at this point, we have reached saturation, and so we have 100% humidity. So this is pretty interesting. So this shows you how relative humidity actually works. A decrease in temperature, so now we're looking at how temperature affects this. A decrease in temperature is very important. So the second condition that affects relative humidity is the temperature of the air. So examine this figure now carefully. It's different than the one previously. Note in that condition A, so that's on the left, that when air at 25 degrees Celsius contains 10 grams of water vapor per kilogram of air, it has a relative humidity of 50%. Again, because we need 20 to be saturated. This can be verified, again, by looking at the table that we've been seeing. Here we can see that at 25 degrees Celsius, air is saturated when it contains 20 grams of water vapor. Because the air in A contains 10 grams, its humidity is again 50%. But when the air in the flask is cooled from 25 to 15 degrees, as shown in B, the relative humidity increases from 50 to 100%. Because at 15 degrees Celsius, if we look at our table, we only need 10 grams of water vapor to saturate. So, we're now down to 10 grams, and we have 10 grams in the flask, so we're at 100% humidity. We can conclude that when the water vapor content remains constant, a decrease in temperature results in an increase in relative humidity. And then if we wanted to go further, we can decrease the temperature even more. We're already at 100% humidity. So, where that goes is to the liquid form. It condenses out of the air, and now we're picking up moisture, actual liquid, in the jar. Oh, that's the point here, so sorry, I forgot I had a second slide of this. So, let's just go through it again. But what if there is no reason to assume that cooling would cease the moment the air reaches saturation? What happens when the air is cooled below the temperature at which saturation occurs, as in Part C? Notice, again from that table, that when the flask is cooled to 5 degrees, let's take a look, only 5 grams of water vapor is needed to saturate the air. But we have 10 grams in our jar. So, what we find is that 5 grams of water vapor is going to condense as a liquid and form droplets at the bottom of the flask. Similarly, when rising air reaches an elevation where it is cooled below its dew point temperature, some of the water vapor condenses to form clouds. Because clouds are made out of tiny liquid droplets or ice crystals, the moisture is no longer part of the water vapor content of the air. So, we can summarize the effects of temperature on relative humidity as follows. When the water vapor content of air remains at a constant level, a decrease in temperature will result in an increase in humidity. And an increase in temperature causes a decrease in relative humidity. So, this figure illustrates the variations in temperature and humidity during a typical day and their relationship described. So, here's just a random day at a random location. Around 6 a.m. we experience our coldest temperature. And because it's cooler, we're seeing the highest relative humidity. But that changes during the daytime. As it warms up, the relative humidity drops. So, make sure to study the previous couple of slides and make sure you can really understand this point. I understand that it's a lot of detail to consider when studying this material. So, relative humidity is just one of the two main ways that we represent the moisture content of air. The dew point temperature, or simply the dew point, of a given parcel of air is the temperature at which water vapor begins to condense. In other words, it's the temperature at which we reach saturation. The term dew point stems from the fact that at night, objects near the ground often cool below the dew point temperature and become coated with dew. You have undoubtedly seen dew form on an ice-cold drink or on a humid summer day such as that in the figure. In nature, cooling air below its dew point temperature typically generates dew, fog, or clouds when the dew point is above freezing and frost when it's below freezing. The dew point temperature can also be defined as the temperature at which a parcel of air reaches saturation. And hence, it is directly related to the actual moisture content of that parcel of air. Recall that the saturation vapor pressure is temperature dependent. In fact, for every 10 degrees Celsius, or 18 degrees Fahrenheit, increase in temperature, the amount of water vapor needed to reach saturation roughly doubles. Therefore, saturated air at 0 degrees Celsius, or 32 Fahrenheit, contains about half the water vapor of the saturated air at 10 degrees, or 50 degrees Fahrenheit, and roughly one-fourth that of air at 20 Celsius, or 68 Fahrenheit. Because the dew point is the temperature at which saturation occurs, we can conclude that high dew point temperatures indicate moist air, and conversely, low dew point temperatures indicate dry air. More precisely, based on what we have learned about water vapor pressure and saturation, we can state that for every 10 degrees Celsius, or 18 degrees Fahrenheit, increase in the dew point temperature, again, air contains about twice as much water vapor. Therefore, we know that when air over Fort Myers, Florida, for example, has a dew point temperature of 25 degrees Celsius, or 77 Fahrenheit, it contains about twice the water vapor in the air as St. Louis, Missouri, which had a dew point of, say, 15 degrees Celsius, or 59 Fahrenheit, and four times as much that as the air over Tucson, Arizona, with a dew point of 5 degrees Celsius, or 41 Fahrenheit. Because the dew point temperature is a good measure of the amount of water vapor in the air, it commonly appears on weather maps. When the dew point exceeds 65 degrees Fahrenheit, or 18 degrees Celsius, most people will consider the air to fuel humid. Air with a dew point of 75 degrees or higher is considered oppressive. So, this is more prevalent to our face-to-face class, but you'll see this even in my online class as a demo that I do. But instruments called hydrometers, where hydro means moisture and metron means measuring instrument, these are used to measure the moisture content of air. One of the simplest of these devices is a psychrometer, also known as a sling psychrometer when connected to a handle and spun, like you see in the image on the right. These consist of two identical thermometers mounted side by side. One of the thermometers, called the dry bulb, measures the air temperature. And the other, called the wet bulb, has a thin cloth wick tied to the bottom. This cloth is saturated with water, and continuous currents of air is passed over the wick, either by swinging it side to side, or using a fan to move air past it. As a result, water is evaporating from that wick, absorbing heat energy from the wet bulb thermometer, which causes its temperature to drop. The amount of cooling that takes place is directly related to the dryness of the air. The more dry the air, the greater the evaporation, and the greater the amount of cooling. Therefore, the larger the difference between the wet and dry bulb temperatures, the more the relative humidity. By contrast, if the air is saturated, little to no evaporation will occur, and the two thermometers will have identical readings. By using a psychrometer, and the tables provided in Appendix B of our textbook, if you have it, you can easily determine the relative humidity and the dew point temperature. So, in my face-to-face class, we'll actually use these devices, both inside the classroom and outside, to measure how humid the air is. And, again, in my online classes, you'll see this as a video demonstration. So, either way, we get some experience with actually measuring these quantities. So, let's go ahead and do some questions to finish out this first lecture on moisture in the atmosphere. Question number one: More molecules are returning to the liquid state than are leaving the liquid state. The process is called what? Okay? So, in this case, if they're leaving the liquid state, we're seeing this as condensation. I'm sorry, if more are returning to the liquid state, we're seeing that as condensation. [00:28:39] Speaker ?: Question two: [00:28:41] Speaker 1: As a solid undergoes a phase change to a liquid, it's what? Okay, so remember, when we change phases, the temperature does not change, so it is constant, so that narrows us down to A or B. We just have to remember whether or not we absorb or release that heat. In the case of going from a solid to a liquid, we need to absorb energy in order to break those molecules apart. So, the answer here is B. Question number three: Which of the following processes absorbs or releases the most amount of heat? So, which one takes the most? Okay, so what we're looking for here is something that's going to step us from one extreme to the other. Either from a solid to a gas, or from a gas back to a solid. The only one that fits the bill here is sublimation. If you remember back all the way to our reference slide with the arrows all over it, we saw that sublimation will absorb 680 calories. Question four: You might need to reference the table previously used. At 25 degrees Celsius, it takes 20 grams of water vapor to saturate a kilogram of dry air. If there are five grams of water vapor in that same situation, what is the relative humidity? Okay, so in this case we have five grams, we need 20 for saturation. Five is one-fourth of 20, or 25%. You take the ratio of the actual amount of water vapor compared to what's needed for saturation. Last but not least, question five: The temperature at which a dry parcel of air reaches saturation is known as what? Okay, and this point, we call it the dew point temperature. The dew point of a given parcel of air is the temperature at which water vapor begins to condense. All right, so that's it for our first lecture on atmospheric moisture. From here, we're going to start talking about cloud formation, different types of clouds, and then in our third of these three lectures, we're going to start looking at things like severe weather as well. So, as always, thanks for watching and have a great day.