GEOG 1112L Lab 3

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University Of Georgia *

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1112L

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Geography

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Apr 3, 2024

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pdf

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9

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Name: Lab day: GEOG 1112L Lab 3: Atmospheric Moisture This lab will introduce you to methods and terms used in Meteorology to measure and quantify moisture in the atmosphere. The topics covered in this lab are: ● Phase changes of water ● Saturation ● Measuring and quantifying atmospheric moisture ● Instrumentation used when measuring atmospheric moisture Water and its phase changes : On Earth, water is a unique substance in that it exists in three different states, or phases. The three phases are solid (ice), liquid (water), and gas (water vapour). Water changes phase through six different processes: evaporation, condensation, freezing, melting, deposition, and sublimation. As water changes phase, there is an energy transfer that occurs. This transfer of energy is known as Latent Heat . As water changes phase from a solid to liquid to gas, water absorbs latent heat, and the air around the water cools. As water changes phase from a gas to a liquid to a solid, water releases latent heat and the air around it warms. The two phase-change processes that you are probably the least familiar with are deposition and sublimation. Deposition is when water changes phase from gas to a solid, so it changes directly from water vapor to ice. An example of this is the formation of frost. This occurs when the ground itself is below freezing, but an airmass with a temperature above freezing moves over the ground. Because of the difference in temperature, as the air comes into contact with the ground, the water vapor in it cools rapidly and changes directly from vapor to tiny ice pellets, or frost. Sublimation is when water changes phase from a solid to gas, again skipping the liquid water phase. Think of dry ice when you think of this process. Dry ice is carbon dioxide frozen into blocks. As the ice particles within the dry ice come into contact with a warmer surface or warmer air, the energy transfer to the ice is so rapid and so large that it changes phase directly to a gas. 1) On the diagram on the top of the next page, please label the six phase changes of water. (1.5pts) 2) In which processes would latent heat be absorbed? (0.5pts) 3) In which processes would latent heat be released? (0.5pts)
Moisture in the atmosphere : moisture in the atmosphere comes from two main sources. ● Evaporation of liquid water: this water can come from oceans, lakes, puddles, etc. ● Transpiration from plants: this is a process in which plants convert liquid water to water vapor and release it into the atmosphere Moisture Terminology : there are several terms that we use to quantify moisture in the atmosphere. The four that are commonly used are Relative Humidity, Vapor Pressure, Mixing Ratio, and Dewpoint temperature. 4) Use your textbook or your lecture notes to define these four terms. What are the units for each term? (2pts) Relative Humidity:
Vapor Pressure: Mixing Ratio: Dewpoint: Saturation : An important term used frequently when discussing atmospheric moisture is saturation. This occurs when the maximum amount of water vapor that could be in the atmosphere is in the atmosphere. Once saturation is reached, there is an equilibrium between the rate of evaporation and condensation. The point when this occurs is dependent on the amount of energy available to change liquid water into water vapor, and this amount of energy depends on the temperature of the air. Because warmer air particles have more energy for evaporation, warmer air has a higher water vapor capacity. A term related to saturation is saturation vapor pressure . This is the value of vapor pressure when the air has reached saturation, meaning that this is the highest amount of pressure that water vapor can exert given the temperature . Because the moisture capacity of the atmosphere is dependent on temperature, saturation vapor pressure is also dependent on temperature. However, the relationship between temperature and saturation vapor pressure is not linear. Because the water vapor content of the atmosphere is dependent on the amount of energy available for evaporation, saturation vapor pressure increases nonlinearly, or exponentially, with temperature. This relationship can be depicted using the Clausius-Clapeyron Curve , shown in the figure on the top of Page 4. Any air masses with values falling on the line would be saturated. We can use this curve to solve for a variety of moisture related variables. If you have the temperature of an air mass, you can find the temperature on the graph and find the
corresponding saturation vapor pressure (on the curve), and vice versa. If you have the air mass’s vapor pressure, you can find that value on the graph and find the corresponding dew point temperature (also on the curve). Please circle the correct answer for the following questions: 5) Moisture capacity increases / decreases with increasing temperature. (0.5pts) 6) Saturation vapor pressure increases linearly / exponentially as temperature increases. (0.5pts) 7) Water molecules in a warmer air mass have more / less energy. Because of this, evaporation rates are higher / lower in warmer air masses. (0.5pts) Measuring Atmospheric Moisture : Because atmospheric moisture is dependent on temperature, we can use various temperature measurements to evaluate the current value of moisture. We can do this by using an instrument called a Sling Psychrometer . This instrument is comprised of two thermometers. One is covered in a gauze pad, and the other is uncovered. These thermometers measure two different temperatures: The Dry bulb temperature and the Wet bulb temperature. The Dry bulb temperature is the temperature of the air. This is the value you
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