2. Record both the wet and dry bulb temperatures.

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1 Name: LAB: Dewpoint and Relative Humidity Date: Introduction: (adapted from L. Wood, 2003) Evaporation of water is happening all the time. As long as there is water vapor molecules above the liquid, there is condensation going on. It s important to know that evaporating molecules take heat from the environment, leaving the environment they left cooler, while condensing molecules release heat, making the environment warmer. This is why as water evaporates from your skin, it makes your skin cooler. Adding heat to the environment (raising the temperature) will make energy available for evaporation, and cause the amount of evaporation to increase. Taking heat away (cooling things off) will favor condensation because a cooler environment will take heat from the water vapor molecules more readily. The amount of water vapor in the air has an effect on evaporation. As more water molecules enter the air, more of them will condense. At the dewpoint, the rate of condensation equals the rate of evaporation. At this point, the air is said to be saturated with water vapor. Evaporation seems to stop because for every molecule that evaporates, another one condenses. At this point, you would not cool off from being sweaty. So, if the air is dry (has few water vapor molecules), evaporation will greatly exceed condensation and cooling will exceed heating. As more water vapor enters the air and humidity increases, more and more condensation occurs until the air is saturated. Therefore, when using a sling psychrometer, the temperature of the wet bulb is an indication of the humidity of the air around it. The more the temperature of the wet cloth is below the air temperature, the more evaporation from it exceeds condensation onto it, and the LESS humid the air is. It is the DIFFERENCE in temperature that is the telling number. A sling psychrometer measures this temperature difference to help us determine humidity. Materials: Sling psychrometers Procedures: 1. Wet the cloth on the wet bulb of the sling psychrometer. And sling around for 30 seconds. Check the temperature on the wet bulb thermometer. Sling for 10 more seconds. Check the wet bulb again. If it moved, keep slinging in increments of 10 seconds until the wet bulb thermomter stops moving. 2. Record both the wet and dry bulb temperatures. 3. Use you Earth Science Reference Tables page 12 to determine both the dewpoint and relative humidity.

2 Use your readings from class, the information below, and your Earth Science Reference Tables to fill in the blanks below. Use the completed table to answer the discussion questions that follow. ºF ºC ºF ºC Room Dry Bulb 6. Dry Bulb Wet Bulb Wet Bulb Difference Difference 5 Dewpoint Dewpoint 4 1. Dry Bulb Dry Bulb Wet Bulb 10 Wet Bulb 16 Difference Difference 4 2. Dry Bulb Dry Bulb Wet Bulb 15 Wet Bulb Difference Difference 7 Rel. Hum. % Rel. Hum. 40 % 3. Dry Bulb 0 9. Dry Bulb 18 Wet Bulb Wet Bulb 18 Difference 2 Difference 4. Dry Bulb Dry Bulb Wet Bulb -8 Wet Bulb Difference Difference Dewpoint Dewpoint 8 Rel. Hum. % Rel. Hum. 40 % 5. Dry Bulb 4 Wet Bulb 0 Difference Dewpoint Rel. Hum. %

3 Discussion Questions: 1. What happens to humidity if moisture is added to a room? 2. What would happen to the rate at which water evaporates if moisture is added to the room? 3. How should the temperature of a wet bulb thermometer change as water vapor in the air increases? 4. What is the difference between absolute humidity and relative humidity? 5. What factors affect or determine the relative humidity of the air? 6. What is the dewpoint temperature? What can be said about air that is at the dewpoint? 7. Determine the Dewpoint and Relative Humidity of air when: a. Dry Bulb = 20ºC Wet Bulb = 12ºC b. Dry Bulb = 10ºC Wet Bulb = 7ºC 8. If all other factors remain unchanged, what happens to the relative humidity of an air mass when the temperature rises? Explain. 9. If all other factors remain unchanged, what happens to the relative humidity of an air mass as more water evaporates into the air? Explain. 10. Imagine an air mass, temperature 12ºC, dewpoint 8ºC. What is the relative humidity of that air mass? Explain how you got your answer. 11. What would happen to the air mass above in question #7b. if the air were cooled to 7ºC? What would the relative humidity be?

4 Reading Comprehension - Answer the following questions in complete sentences based on the reading below on humidity. How Humidity is Measured By Jack Williams, USATODAY.com 05/20/ Updated 11:05 AM ET The humidity is one of the important measurements that weather observers make because the amount of water vapor in the air determines whether clouds or fog are likely and whether it's going to rain or snow. Humidity refers to the amount of invisible water vapor in the air and various measurements are used to define it. One is called the dewpoint, or the temperature at which condensation occurs. A hygrometer is a common way to measure the humidity. Such an instrument has a mirror that exposed to the open air, and which can be cooled as precise track is kept of the mirror's temperature. As the mirror cools, dew (or frost if the temperature is below freezing) will form on it. The temperature at which this happens is the air's dew point (or frost point in below-freezing air). An observer could watch the mirror to see when dew or frost forms, but a more precise way is to use photoelectric cells to detect when dew fogs the mirror. Other kinds of hygrometers use materials with electrical resistance that varies with the amount of moisture absorbed. With such hygrometers, a measurement of electrical resistance can be calibrated as a humidity measurement. Human (or animal) hair turns out to be a pretty good way to measure the humidity, as anyone who's ever complained about a "bad hair day," can tell you. The graphic below shows how this works. Source: USA TODAY research by Chad Palmer The length of a strand of human hair changes with different relative humidities. As the relative humidity increases, hair becomes longer, and as the humidity drops it becomes shorter. On very humid days, your hair actually becomes longer and this extra length causes the frizziness that gives us bad hair days.

5 An instrument that uses hair to measure humidity is known as a hair hygrometer. This instrument uses strands of human or horse hair with the oils removed attached to levers that magnify a small change in hair length. An ink pen and rotating cylinder, known as a hygrograph, can provide a record of how relative humidity varies throughout the day. The disadvantages of the hair hygrometer and hygrograph are that they are not as accurate as other kinds of hygrometers such as the sling psychrometer. Also, a hair hygrometer needs frequent adjustment and calibration. A hair hygrometer also tends to have large errors at very high and very low relative humidities. 1. Describe the two types of Hygrometers (in your own words). 2. Why is a hair hygrometer not as accurate as other types of hygrometers? 3. Why would a hair hygrometer be better to use on a day below 0ºC than a sling psychrometer (that uses water)?