Lab 5 Atmospheric Moisture and Humidity

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Kent State University *

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Geography

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Oct 30, 2023

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Lab 5: Atmospheric Moisture and Humidity Instructions Watch the lectures on atmospheric moisture, winds, adiabatic processes, and the videos on how to use a sling psychrometer, and the processes of adiabatic cooling and heating. After you have reviewed the lectures, notes, and the videos, use this document to work through your lab. Once you have finished answering all questions in this lab, submit your answers online in the link titled “Lab 5 – Atmospheric Humidity”. After the due date when your lab is graded, you will be able to review your lab and answers. When you first submit your lab, your score may appear low. Keep in mind that your lab instructor will need to grade your lab, especially the written responses. The computer can automatically grade and score multiple choice, matching, and true/false questions. Any written or essay responses will need to be graded by your instructor. Once your lab instructor grades those questions, your lab grade will be updated to the correct grade. Goals • Understand relative humidity and dew point to measure atmospheric moisture. • Understand adiabatic heating and cooling. • Explain how air density, pressure, and temperature changes in a rising parcel of air. • Determine the condensation level and temperature on a mountain using correct lapse rates given base temperature and elevation. • Determine the condensation level and its influence on cloud development. Key Terms / Concepts Adiabatic cooling Adiabatic heating Sling psychrometer Dew Point Albedo Condensation Relative humidity Dry Adiabatic Lapse Rate (DALR) Wet Adiabatic Lapse Rate (WALR) Once you have completed questions below, fill in your answers in the assessment link online titled “Lab 5 – Atmospheric Moisture and Humidity Assessment”.

Atmospheric Moisture The amount of water present in the air, whether it is in the solid, liquid, or gaseous state, plays an important role in the weather experienced on Earth. Atmospheric moisture refers to water vapor and precipitation in the Earth’s atmosphere. This amount of water in the atmosphere varies considerably from place to place and, therefore, it is measured and reported. The amount of water vapor in the atmosphere is called the humidity. Specific humidity measures the mass of water vapor in a given mass of air. Relative humidity (RH), a more common measure of atmospheric moisture, is a ratio between the amount of water vapor in the air of a given temperature and the maximum amount that air could hold at that temperature. Relative humidity is expressed as a percentage. So, if the air holds half the moisture possible at the present temperature, the RH is 50%. When the humidity is 100%, the air holds the maximum amount possible. When the air is cooled, the capacity is reduced and has a higher percentage of total capacity. So, temperature and relative humidity are inversely related. As temperature increases, the amount of water vapor a parcel of air can hold increases; therefore, as the temperature of an air parcel increases, the relative humidity (or degree of saturation) decreases. Conversely, relative humidity increases in a cooling parcel of air. Figure 1 shows the relationship between temperature and relative humidity at 5 A.M., 11 A.M., and 5 P.M. The temperature at 5 A.M. is the coolest temperature of the day, so this temperature has less capacity to hold water vapor (in blue). The RH is 100%. During the day, the water vapor content does not change much, however the temperature does. So, the warmer temperature has a greater capacity to hold water vapor and the relative humidity will be less. Figure 1: Relationship between temperature and humidity. Image Source: http://uidahophysicalgeography.weebly.com/module-4-page-3.html

Dew point is the temperature at which the air is saturated (relative humidity is 100%), water vapor then changes from water vapor into precipitation, clouds form at this point. Condensation is the result of the air reaching saturation and relative humidity is 100%. For any given temperature, the dew point can be calculated. Note: If the dew point temperature and the air temperature are the same, the RH will be 100%. The lower the dew point temperature compared to the air temperature, the RH will be less. A sling psychrometer is a tool designed to allow you to determine dew point temperature and relative humidity. This is an instrument consisting of two thermometers mounted together on a plate (Figure 2). One of the thermometers has a cloth wick tied around the end of the thermometer. This is called the wet bulb thermometer. The wick is saturated with water, and then the instrument is swung through the air. As it swings through the air, the water evaporates from the wick and cools the wet-bulb thermometer (evaporational cooling due to the latent heat of vaporization). The thermometer without the wick is called the dry bulb thermometer. The air temperature and the amount of moisture in the air will influence how quickly water will evaporate or change into water vapor and enter the air. As water evaporates, it removes heat to make this change of state and this heat is stored in the water vapor in the air. This heat will be released when the water vapor returns to its liquid or its solid state by the processes of condensation or sublimation respectively. The sling psychrometer instrument makes use of these principles. The wick is dampened and as water evaporates from this cloth, heat is taken with it. The wet-bulb thermometer will register the wet-bulb temperature, which is usually lower than the air temperature (the dry-bulb temperature), registered by the dry-bulb thermometer where no evaporation has taken place. This difference in temperature is called the wet bulb depression. The dry-bulb temperature is always higher than the wet-bulb temperature except for when the air is saturated or the relative humidity is 100%. In this situation, as water evaporates from the wet bulb, an equal amount of condensation is returning the heat which was lost. Therefore, the two temperatures will be the same. A person would usually do about 5 to 10 trials until both thermometers stay consistent in their temperatures. Once the final temperature is determined, both thermometers, in either Celsius or Fahrenheit are recorded. After both thermometers have registered their temperatures and the depression has been determined, two tables, one in Celsius or Fahrenheit can be used to find relative humidity. Figure 2 : Sling Psychrometer Image Source: http://www.buzzle.com/articles/how-does-a-sling-psychrometer-work.html

Again, the amount of cooling (wet-bulb temperature depression) that takes place is directly proportional to the amount of water in the air, i.e. the drier the air, the more the cooling. Therefore, the larger the difference between the temperature of the dry-bulb and the wet-bulb thermometers, the lower the relative humidity in the air. If the air is saturated, no evaporation can occur, and the two thermometers will have the same reading. To find the wet bulb depression, you would subtract the wet bulb temperature from the dry bulb temperature (dry bulb – wet bulb). Once you determine the wet bulb depression, if your value is negative, take the absolute value and make it positive to find the RH and dew point in the tables below. To find the relative humidity, you will use Table 1 , relative humidity, in percent (%) in Celsius, or Table 2 , relative humidity, in percent (%) in Fahrenheit. To understand how to determine relative humidity from the wet bulb and dry bulb temperatures, you will refer to Table 1, RH in Celsius. Below is an example of temperature readings taken from a sling psychrometer during the spring months in the state of Texas. After a series of trials using the instrument, the dry bulb reading is 22 ° C and the wet-bulb temperature is 18 ° C. The first thing you should do is find the wet-bulb depression. You find this reading by subtracting the wet bulb reading from the dry bulb reading. In this case below, 22 ° C minus 18 ° C is 4 ° C. So the wet bulb depression is 4 ° C. After you obtain this information, you can now find the relative humidity (RH) using Table 1. Table 1 shows air temperature down the left side of the table and the depression of the wet-bulb thermometer across the top of the table. To find RH, you cross reference the dry-bulb temperature, (which is the air temperature) with the wet-bulb depression (in yellow highlight). You will see that the RH is 68%. To find the dew point (see Table 3), you cross reference the air temperature with the wet-bulb depression (in yellow highlight). The dew point is 20 ° C. Dry Bulb Temperature Wet Bulb Temperature Wet Bulb Depression (Dry Bulb – Wet Bulb) Relative Humidity (RH) 22°C 18°C 4°C 68% Dry Bulb Temperature Wet Bulb Temperature Wet Bulb Depression (Dry Bulb – Wet Bulb) Dew Point 22°C 18°C 4°C 20°C

Fill in the table below using Tables 1 through 3 to determine dew point ( °C) and RH (%). You will notice that the blank cells have numbers encased in parentheses. The numbers will correspond to the lab assessment when you are ready to submit your answers online. Credit will not be given if proper units are not used when submitting your answers online. Each response given in the table below in parentheses is worth 0.5 points for a total of 9 points. Air Temperature (°C) Wet Bulb Temperature (°C) Wet Bulb Depression (°C) Dew Point Temperature (°C) Relative Humidity (%) 14 10 (1) (2) (3) 6 2 (4) (5) (6) 18 13 (7) (8) (9) 4 -1 (10) (11) (12) 12 11 (13) (14) (15) 0 -1 (16) (17) (18) 19. Write a paragraph below and compare changes in relative humidity as temperature increases. How does a change in relative humidity with no change in water vapor between 6 AM to 6 PM on a typical day result, for example, for a mid-latitude location in the United States during the warm months? (1.5 points)

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