February 25, 2008

Phase Changes of Water, Latent heat (Review)

n Water in the gas phase (water vapor) contains more internal energy per gram of water than water in the liquid or solid phase. Water in the liquid phase (often just called water) contains more internal energy per gram of water than water in the solid phase (ice).

o Draw a diagram to show this and point out:

§ To go from a lower energy phase to a higher energy phase, energy needs to be added to the water

§ To go from a higher energy phase to a lower energy phase, energy must be removed from the water

n Latent Heat is the energy added to or removed from water as it changes phases … there is no measureable change in the temperature of the water. Latent literally means “hidden” in that even though energy was added to or removed from water, there is no way to measure (or sense) this using a thermometer.

n Sensible Heat is the energy added to or removed from water that results in a temperature change of the water, but no phase change. Sensible literally refers to a change that can be easily sensed using a thermometer.

n The calorie is a unit for measuring energy. We will use it in this discussion.

o One calorie is equal to the amount of energy required to raise the temperature of one gram of water by 1° C.

o In dietary science 1 Calorie = 1000 calories. In relation to the food, Calories give the maximum amount of energy that your body can extract by breaking down the food, so it is a measure of the energy content of food.

Phase Changes of Water, Latent heat (Continued)

n Draw a diagram showing the amount of energy involved in moving from one gram of ice at a temperature of -100° C to one gram of water vapor at a temperature of 100° C and back again. We will come to the following important conclusions:

o There is a lot of energy involved in phase changes of water, especially phase changes between liquid and gas.

o For water to evaporate (liquid à gas), energy must be added to the water. For climate and weather processes, this energy is supplied by the surrounding environment (where the water is evaporating), thus the surrounding environment gives up some of its energy and cools (temperature of surrounding environment goes down).

§ Example: evaporation of sweat cools the body

o For water to condense (gas à liquid), energy must be removed from the water. For climate and weather processes, this energy must be taken up by the surrounding environment (where water is condensing), thus the surrounding environment gains energy and warms (temperature of the surrounding environment goes up)

§ Example: the formation of clouds heats (or warms) the air

Energy Transfer via Conduction

n Conduction is the transfer of energy by direct collisions of molecules (touching). Energy can be transferred from one object to another or within a single object that contains temperature variations. The direction of the energy transfer is ALWAYS from warm to cold.

o Examine figure 2.5 from textbook, pointing out conductive heat transfer

n The rate at which energy is transferred within a material is referred to as its heat conductivity. For example, take a rod of steel. Heat the rod at one end and measure how quickly heat is conducted toward the other end. In general, solids and liquids are better heat conductors than gases because the molecules that make up solids and liquids are more tightly packed than in gases. Thus, water and metals are good heat conductors, while air is a poor heat conductor (or a good heat insulator).

o Go over table 2.1 from textbook. Use the table to compare how well heat is transferred by conduction through various materials. The higher the heat conductivity, the faster heat flows through the material by conduction.

n The heat conductivity of various substances can be used to explain the following:

o Still air is a very poor heat conductor. Therefore, it is commonly used as a heat insulator to slow the rate of conductive heat transfer.

§ Double-paned windows, insulation material, animal fur

o Metals, especially, and water are good heat conductors

§ Metals at 60° F can “feel” very cold to the touch because they are such good heat conductors that they conduct heat away from your body rapidly.

§ Water at 60° F also “feels” somewhat cold … partly because it is a good heat conductor and partly because it has a high heat capacity. Think of jumping in a pool of water that is at a temperature of 60° F.

§ However air at 60° F “feels” comfortable because the rate of heat transfer from the body to air is slow since air is a poor heat conductor.

n In the atmosphere, air can be warmed or cooled by contacting a warm or a cold ground surface via conduction.

Energy Transfer via Convection

n Convection is the transfer of heat by actual movement of mass within a fluid. Convection is a very important means of energy transport in the atmosphere. Convection only occurs in fluids (liquids and gases), not in solids. Two types of convection are important in the atmosphere:

o Dry Convection is simply warm air rising and cold air sinking. This commonly takes place on warm, sunny days.

§ Take a look at figure 2.6 from textbook, which shows the development of “thermals”, which are small bubbles of warm, rising air and the corresponding sinking of cooler air.

o Moist Convection accounts for energy removed due to evaporation of water, liquid à gas (usually from near the ground surface). This energy is later delivered where the water vapor (gas) condenses, gas à liquid (typically during cloud formation). The net effect is that energy is removed from near the ground surface and later released high up in the atmosphere.

§ Draw a figure to help explain this process.

§ Because so much energy is involved in phase changes of water between the liquid and gaseous phases, a tremendous amount of energy is transferred from near the ground up into the atmosphere via moist convection. In Earth’s climate, over 3 times more energy is transferred from the surface to the atmosphere due to moist convection than is transferred via dry convection and conduction.

Overview of the Hydrological Cycle on Earth (Chapter 4)

n Put up figure 4.1 from textbook.

n The cycling of water from the oceans to the atmosphere to precipitation on land and eventually running back to the oceans is absolutely essential for life on land to exist.

A closer look at the physical processes of evaporation and condensation

n In order to describe the physics of these processes, we need some way to specify the water vapor content of the atmosphere. There are many ways to do this and we will use a few of them in this class. The first is something called vapor pressure.

n Vapor pressure is the pressure (force/area) exerted by water vapor molecules alone. The higher the concentration of water vapor molecules (number density), the higher the vapor pressure. The average air pressure at sea level is about 1013 millibars (mb). If the total air pressure is 1013 mb and water vapor makes up 1% of the air molecules, then the vapor pressure is 1% of 1013 mb or 10.13 mb. Water vapor is a trace gas in the atmosphere of Earth -- the maximum vapor pressure is never more than about 40 mb. For now the important concept is that vapor pressure is one way to keep track of the amount of the gas water vapor. The higher the vapor pressure, the greater the amount of water vapor in the air.

n We will use an in-class handout as a visual aid to assist in understanding. We will also need the following definitions and background material:

o The rate of evaporation is the number of water molecules that change phase from liquid to gas per second. This rate depends mainly on the temperature of the liquid water surface … the higher the temperature, the faster the rate.

o The rate of condensation is the number of water molecules that change phase from gas to liquid per second. This rate depends mainly on the vapor pressure … the higher the vapor pressure, the faster the rate.

o The processes of evaporation and condensation go on simultaneously. They also occur over a wide range of temperatures. The latent heat diagram that I drew earlier is a bit misleading in that it can lead you to think that in order for water to evaporate, we need to heat the liquid water up to 100° C. But this is not true. Water is evaporating all the time at lower temperatures. You can convince yourself of this by setting out a glass of water. Eventually it will evaporate even though it was never heated to its boiling point.

o Use handout to describe how water behaves. We can make the following points:

§ In a closed system (like figure 4.5 in the handout), the air above a liquid water surface will become saturated with water vapor. We can measure the saturation vapor pressure at various temperatures in a lab.

§ Saturation is the maximum amount of water vapor that can exist in the air (i.e., the capacity for water vapor). As air temperature increases, the saturation vapor pressure increases sharply.

· Warm air can hold more water vapor than cold air

§ Look at and explain figure 4.5 from textbook

· Briefly describe what it means for water to boil. Water is said to boil when the saturation vapor pressure of the water is equal to the surrounding air pressure. Using figure 4.5 and applying what we know about air pressure (it decreases with increasing altitude), we can understand why water will boil at a lower temperature at high elevations, e.g., on top of a mountain.

n Evaporation and Condensation in the atmosphere

o Near the Earth's surface, the vapor pressure is usually less than the saturation vapor pressure. (NOTE: the actual vapor pressure of the air can vary from zero up to the saturation vapor pressure). Therefore, the rate of evaporation is greater than the rate of condensation, i.e., liquid water near the Earth's surface is continually evaporating (changing phase from liquid to gas). You can easily convince yourself that this is true by leaving out a glass of water. All of the liquid will eventually evaporate. The reason the air near the ground does not reach saturation is because after the water evaporates, the water vapor is able to move away from the surface. It is not trapped as in the closed experiment described above.

o Once water has evaporated it becomes part of the gases that make up the atmosphere. When air rises upward, it cools (the reason rising air cools will be explained later). As air cools, its saturation vapor pressure decreases, in other words, the maximum amount of water vapor that the air can hold decreases. If the air rises high enough and cools sufficiently, it will not be able to hold all the water vapor it contains. When this happens, water vapor must condense back to liquid water. This is how clouds form. Clouds are composed of tiny droplets of liquid water (and possibly ice). Water vapor is an invisible gas and cannot ever be seen. If you see it, e.g., clouds, steam, your breath on a cold day, it must be liquid water droplets. We will talk more about clouds soon.