1.6: Lab 6 - Atmospheric Humidity

Learning Objectives

• Identify the processes that occur when water changes phases.
• Calculate the energy required to change the phase of water.
• Calculate relative humidity and the dew point temperature.
• Use a sling psychrometer to collect field data and interpret psychrometric tables to determine relative humidity and dew point temperature.
• Describe the atmospheric lifting mechanisms.
• Use the adiabatic rates to calculate how air temperatures and relative humidity change when an air parcel goes over an orographic barrier.

Introduction

Water is an amazing substance! It is made of the two most abundant elements in the universe, two hydrogen atoms and an oxygen atom, forming H₂O. More than seventy percent of the Earth’s surface is covered with it. We are constantly surrounded with water, in its gaseous form, in the air around us.It can also be found in its liquid form in the ground under our feet as soil moisture and groundwater. Serve yourself a glass of ice water (Figure 6.1). You’ll have the only substance on Earth found in all three phases (solid, liquid, and gas) simultaneously.

Figure 6.1: Water Can be Found in Three Phases Simultaneously. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

Water is the only substance that is less dense in its solid state than in its liquid state. Ice floats on water. Because of this, life can continue to thrive in our lakes, rivers, and streams that freeze over each winter. Water is a universal solvent. Given enough time, everything will dissolve in water. This not only allows our blood to carry nutrients throughout our body, but water also functions in nature, bringing nutrients to the leaves of plants, transporting minerals across the landscape, and acting as a major erosional process. Water is the basis of life on Earth.

Further, water conserves heat energy much more efficiently than other natural substances such as soil. It requires a much greater amount of energy to change the temperature of water. This is referred to as the specific heat capacity. With our world being 70% covered with water, our oceans have an extraordinary capacity to store and distribute energy.

Water releases a huge amount of energy when it condenses or freezes. Inversely, water absorbs that same, huge amount of energy when it evaporates and melts. This is referred to as latent heat. You will learn much more about latent heat below.

Part A. The Many Forms of Water

As mentioned previously, water is found naturally in three different phases. You will be exploring the processes of phase change and the energy that is related to those phase changes. You are probably already familiar with most of the six phase changes of water:

➢ Freezing occurs when liquid water transforms into ice.

➢ Melting occurs when ice transforms into liquid water.

➢ Evaporation occurs when liquid water transforms into water vapor.

➢ Condensation occurs when water vapor transforms into liquid water.

➢ Deposition occurs when water vapor transforms into ice.

➢ Sublimation occurs when ice transforms into water vapor.

Exercise \(\PageIndex{1}\)

1. Label the six phase changes of water on the diagram below (Figure 6.2).

Figure 6.2: Phase Changes of Water. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

What is required to change the phase of water? For each of these steps, there is a significant transfer of energy between the water and the surrounding environment. Let’s think of a few examples:

➢ In the morning you take a warm shower. Some of the warm water evaporated in the bathroom. When you get out of the shower, you find that the cold glass of the mirror has small water droplets all over it. The cold glass absorbed some energy causing water to condense on the surface. Condensation releases energy and warms the environment (in this case, the glass).

➢ Later that day, you injure your knee in a soccer accident. Your coach pulls out a plastic bag, fills it with ice and tells you to sit down and “ice” it. What happens? As the ice begins to melt (changing from a solid to a liquid) it absorbs heat. It makes your knee feel cold because that energy is being absorbed from your knee. So, the process of melting absorbs energy and cools the environment.

➢ After practice, you head to the nearest river, lake, or beach to cool off. It’s several degrees cooler near the water than the soccer field. Evaporation absorbs energy and cools the environment.

While these are some common examples, this absorption and release of heat energy by the phase changes of water help regulate temperature and distribute energy from equatorial areas to polar regions.

This transfer of energy between water and the surrounding environment is referred to as latent heat. Each phase change of water will either absorb or release latent heat energy. Figure 6.3 outlines the amount of energy required to change the phase of one gram (1 gram of water is equivalent to 1 milliliter or 1 cubic centimeter) of water. Between ice and liquid water, 80 calories are absorbed during melting or released during freezing at 0°C. It takes 100 calories to raise or lower the temperature of water between 0°C and 100°C. Between liquid water and water vapor, 540 calories are absorbed during evaporation or released during condensation.

Exercise \(\PageIndex{2}\)

Figure 6.3: Latent Heat Related to Phase Changes of Water. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

2. Using Figure 6.3 above, complete Table 6.1 below. Provide the energy required for one milliliter of water.

Phase Change	Heat Calories Exchanged	Latent energy released or absorbed?	Warming or cooling the surrounding environment?
Solid (0°C) → Liquid (0°C)
Liquid (0°C) → Solid (0°C)
Liquid (0°C) → Gas (100°C)
Gas (100°C) → Liquid (0°C)
Solid (0°C) → Gas (100°C)
Gas (100°C) → Solid (0°C)

Pin It! Latent Heat

Consider the huge amount of energy absorbed and released through evaporation and condensation! This is a major means of storing and transferring energy across our Earth. At the equator, water evaporates and then condenses as it rises. Further, the condensation that takes place in some storms actually causes an additional uplift in the storm. This is particularly true in tropical storms like hurricanes, which you will learn about later in the course.

Part B. Atmospheric Humidity

When we discuss the “humidity of the air,” we are typically talking about a calculated value referred to as relative humidity. There are two main variables that are considered when calculating relative humidity. These include specific humidity and the maximum water vapor of the air.

Specific humidity refers to the actual, measurable amount of water vapor in the air. It is the amount of water (in the form of a gas) that would be found if we were able to isolate each of the different types of gasses in the air. The value is usually reported in grams of water vapor per kilogram of air (g H₂O/kg air).

The maximum water vapor of the air parcel must also be considered. Think of an air parcel as a small box of the atmosphere that is distinct from the atmosphere surrounding it in terms of temperature and humidity. How much water can this air parcel hold? The ability of an air parcel to hold water vapor is a function of the temperature of the air parcel. Warmer air has a greater capacity for holding water vapor, while colder air has a lower capacity. These values are shown in the table on the left in Figure 6.4 below. The maximum water vapor is also reported in grams of water vapor per kilogram of air (g H₂O/kg air).

The more commonly used relative humidity is simply the ratio (expressed as a percentage) of the amount of water in an air parcel (the specific humidity), compared to the amount of water an air parcel can hold (the maximum water vapor). The equation used to calculate the relative humidity is as follows:

Relative Humidity = (Specific Humidity / Maximum Water Vapor) X 100

Guided Practice: Calculating Relative Humidity

Watch a tutorial video on how to calculate relative humidity. (Video length is 4:55).

Exercise \(\PageIndex{3}\)

3. Based on the data table provided on the left-side of Figure 6.4, plot the maximum water vapor of an air parcel in the blank graph on the right. Connect the points with a solid line.

Figure 6.4: Data and Table for Maximum Water Vapor based on Temperature. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

4. Using the data provided and your completed Figure 6.4, answer the following questions:

1. Just before sunrise, the temperature is 15°C (59°F) with a specific humidity of 8.5 kg H₂O/kg air. Calculate the relative humidity. Show your work.

2. By 10 am, the temperature has risen to 25°C (77°F). Assume that the specific humidity remains at 8.5 kg H₂O/kg air. Calculate the relative humidity. Show your work.

3. When the heat of the day strikes at 2 p.m., the temperature has risen to 35°C (95°F). Assume that the specific humidity remains at 8.5 kg H₂O/kg air. Calculate the relative humidity. Show your work.

4. Santa Barbara has a temperature of 22°C (70°F) and a relative humidity of 60%. What is the specific humidity of the air?

Table 6.2 contains temperature and relative humidity data over the course of three days.

5. Using a red pencil in the blank graph provided below (Figure 6.5), plot the temperature over the three days and connect your plotted points with a solid red line. Repeat the process using a blue pencil for the relative humidity.

Table 6.2: Temperature and Relative Humidity, April 20-22

Date and Time	Temperature (°F)	Relative Humidity (%)
Apr 20, 00:00	65.7	58
Apr 20, 04:00	61.6	47
Apr 20, 08:00	60.3	48
Apr 20, 12:00	74.4	33
Apr 20, 18:00	82.2	19
Apr 20, 20:00	72.9	43
Apr 21, 00:00	60.1	58
Apr 21, 04:00	54	72
Apr 21, 08:00	59.9	62
Apr 21, 12:00	77.3	35
Apr 21, 18:00	86.8	23
Apr 21, 20:00	79.6	30
Apr 22, 00:00	64.3	59
Apr 22, 04:00	58.4	69
Apr 22, 08:00	63.4	55
Apr 22, 12:00	79.4	34
Apr 22, 16:00	87.5	18
Apr 22, 20:00	80	21

Figure 6.5: Plotted Temperature and Relative Humidity. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

6. Based on the plotted data on Figure 6.5, what time of day would you see the highest relative humidity readings: early morning or late afternoon? Why is this? Explain your response in one to two sentences.

7. What observations can you make regarding the relationship of temperature and relative humidity?

Dew Point Temperature

So far, you have learned about the relationship between temperature and relative humidity. You have noted that as temperature increases, relative humidity decreases. You have discovered that early in the morning, when the temperatures are the lowest, relative humidity is typically at its highest. What would happen if night time temperatures dropped even colder? The relative humidity would increase, but by how much?

Relative humidity can only reach a maximum of 100%. At this point, the air parcel can no longer hold the same amount of water vapor (an invisible gas) and so water droplets begin to form (suspended water droplets in the form of fog or clouds are created by condensation). The temperature at which relative humidity reaches 100% is called the dew point temperature.

In order to calculate the dew point temperature, you will still need to know the specific humidity of the air parcel, but your goal is to determine at what temperature the maximum water vapor will be the same as the specific humidity. You will continue to use the plotted data in Figure 6.4 above, but you may find you are using it in the opposite order from your previous use.

Exercise \(\PageIndex{4}\)

Let’s return to some of the scenarios you were working with before.

8. If the air parcel has a specific humidity at 8.5 kg H₂O/kg air, what is the dew point temperature? Show your work.

9. Santa Barbara has a temperature of 22°C (70°F) and a relative humidity of 60%. What is the dew point temperature of the air? Show your work.

10. Apply What You Learned: If you wake up to a foggy morning, what is the relative humidity? Explain your response in one to two sentences.

Psychrometric Tables and the Sling Psychrometer

One instrument used to measure humidity is called a psychrometer. A sling psychrometer has two thermometers mounted together in a manner that they can be swung around or twirled. The two thermometers include one that is simply mounted to the apparatus. This is referred to as the dry bulb thermometer, and is used to measure the ambient (surrounding) air temperature. The second thermometer has a wick that is moistened with water at the ambient air temperature. This is referred to as the wet bulb thermometer. Moisture evaporates from the wick as air is intentionally circulated over it by twirling the psychrometer (the so-called “sling psychrometer”). The latent heat of evaporation from the wick lowers the wet bulb temperature. If the air is dry, there will be a lot of evaporation, lowering the wet bulb temperature. If the air is very humid, there will be very little evaporation and the temperature will not drop far on the wet bulb. The difference between the dry bulb and wet bulb temperatures is called the wet bulb depression.

Table 6.3: Relative Humidity Psychrometric Table

Table 6.4: Dew Point Temperature Psychrometric Table

With the help of a psychrometer, data has been collected around California. Table 6.5 below contains the dry bulb and wet bulb temperatures for locations around California.

Exercise \(\PageIndex{5}\)

11. Complete Table 6.5. Tip: the wet bulb depression is simply the difference between the dry bulb temperature and the wet bulb temperature.
    1. Use the psychrometric tables above to determine the relative humidity and the dew point temperature for each location. Along the left axis, find the temperature closest to the dry bulb temperature. Along the top, find the closest wet bulb depression value. If your exact depression value is not listed, you may need to estimate your value between two that are listed.

Table 6.5: Psychrometric Data Across California. Data collected on March 19, 2020 at 13:00 from The Weather Channel

Location	Dry Bulb Temp. (°C)	Wet Bulb Temp. (°C)	Wet Bulb Depression	Relative Humidity (%)	Dew Point Temperature (°C)
Eureka	12	10
Redding	14	8
San Francisco	13	9
Bishop	8	2.5
Bakersfield	14	9
Sacramento	12	9.5
Death Valley	17	9
Avalon	14	10
Los Angeles	14	9.5
San Diego	17	12

12. Using the internet or an atlas, find each of the locations shown in Table 6.5. Mark and label each point on the map below (Figure 6.6).

Figure 6.6: Blank Map of California. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

13. Which locations had the lowest ambient temperatures (dry bulb temperatures)? Why do you think these locations had the lowest temperatures? Explain your response in one to two sentences.

14. Which locations had the highest relative humidity? Why do you think that is the case? Explain your response in one to two sentences.

15. Which locations had the lowest relative humidity? Why do you think that is the case? Explain your response in one to two sentences.

16. Which of the locations on the map is closest to your college? What is the relative humidity at this location?

17. Use Your Critical Thinking Skills: Based on your spatial and data analysis, what conclusions can you draw regarding the relationship of location and relative humidity? Explain your response in two to three sentences.

Data Collection with a Sling Psychrometer

If you have access to a sling psychrometer, take some readings! Your professor may have some suggested or required locations. Instructions for using a sling psychrometer:

Step 1

Use clean, distilled water at the ambient air temperature to moisten the wet bulb’s wick.

Step 2

Avoid handling the bulbs themselves when recording temperature because this can change the temperature. Also, for the same reason, try to avoid taking readings from the psychrometer in direct sunlight unless directed to do so at a particular site.

Step 3

Twirl the psychrometer for a minimum of 60 seconds. Read the values quickly (always read the wet bulb thermometer’s value first). Twirl the thermometers again for a minimum of 20 seconds, and read the results quickly again. Repeat this procedure until you get two consecutive readings that are identical. This may take three or more repetitions at each site.

Record only the final wet bulb and dry bulb temperatures.

Step 4

Complete the following data collection forms (one for each site). Use the psychrometric tables provided above to determine the relative humidity and dew point temperature.

Variable	Your Data
Date:
Time:
Description of location (surrounding vegetation, proximity to buildings, water, landscaping, etc.):
Dry Bulb Temperature:
Wet Bulb Temperature:
Relative Humidity:
Dew Point Temperature:
Notes:

Variable	Your Data
Date:
Time:
Description of location (surrounding vegetation, proximity to buildings, water, landscaping, etc.):
Dry Bulb Temperature:
Wet Bulb Temperature:
Relative Humidity:
Dew Point Temperature:
Notes:

Variable	Your Data
Date:
Time:
Description of location (surrounding vegetation, proximity to buildings, water, landscaping, etc.):
Dry Bulb Temperature:
Wet Bulb Temperature:
Relative Humidity:
Dew Point Temperature:
Notes:

Variable	Your Data
Date:
Time:
Description of location (surrounding vegetation, proximity to buildings, water, landscaping, etc.):
Dry Bulb Temperature:
Wet Bulb Temperature:
Relative Humidity:
Dew Point Temperature:
Notes:

18. What patterns did you observe across your different data collection sites? Explain your response in one to two sentences.

19. Were there any results that surprised you or that you were not expecting? Why do you suspect you had these findings? Explain your response to one to two sentences.

20. Were there any other noteworthy findings or observations that you found in your data collection? Explain your response to one to two sentences.

Part C. Lifting Mechanisms

You have learned that temperature has a significant impact on relative humidity. Temperature patterns vary based on time of day as well as physical location. One other significant impact on temperature is the vertical movement of air. As an air parcel rises, the less dense surrounding air gives room for the air parcel to expand. As it expands, the temperature decreases. If an air parcel rises and cools down to the dew point temperature, clouds begin to form. This may lead to precipitation.

There are four major lifting mechanisms in which air is forced upwards:

➢ Convergent Lifting: Air flowing from different directions “crashes” into one another, causing the air to be forced upward.

➢ Convectional Lifting: Over a warm surface, the heated air is forced upwards.

➢ Orographic Lifting: Air forced up and over a mountain.

➢ Frontal Lifting: As two different air masses intersect, the warm, humid air is forced upward.

Exercise \(\PageIndex{6}\)

21. Label the following lifting mechanism diagrams based on their descriptions provided above.
    1. ________________________________________________________:

Figure 6.7: Lifting Mechanism Sample 1. Figures by Scott Crosier is licensed under CC BY-NC-SA 4.0

2. ________________________________________________________:

Figure 6.8: Lifting Mechanism Sample 2. Figures by Scott Crosier is licensed under CC BY-NC-SA 4.0

3. ________________________________________________________:

Figure 6.9: Lifting Mechanism Sample 3. Figures by Scott Crosier is licensed under CC BY-NC-SA 4.0

4. ________________________________________________________:

Figure 6.10: Lifting Mechanism Sample 4. Figures by Scott Crosier is licensed under CC BY-NC-SA 4.0

Pin It! Rising Air

These lifting mechanisms cause the air to rise, cool, and eventually form clouds. If conditions are right, this could lead to precipitation. In fact, whenever there is precipitation, it began with a lifting mechanism. The spatial patterns of precipitation are determined by the distribution of these lifting mechanisms!

Part D. Adiabatic Rates

As an air parcel rises, there is less pressure surrounding the outside of that air parcel and it is allowed to expand. As it expands, its temperature decreases. It is said to cool by expansion. Inversely, as an air parcel subsides or is forced downward, it is heated by compression. The warming and cooling rates for a parcel of expanding or compressing air are termed adiabatic rates. Figure 6.11 shows adiabatic cooling as an air parcel rises and adiabatic warming as it subsides.

Exercise \(\PageIndex{7}\)

Figure 6.11: Temperature Changes as Air Rises or Subsides. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

22. Label the windward (the side where the wind begins to rise) and the leeward (the side where the wind subsides) sides of the mountain on Figure 6.11.

Depending on the relative humidity of an air parcel, the adiabatic rate will be different. In a dry air parcel, which means the air parcel has a relative humidity that is less than 100%, the rate of change is the dry adiabatic rate (DAR) of +/- 10°C per 1,000 meters (5.5°F per 1,000 feet). This is the rate of change that occurs solely through the process of expansion or compression.

However, once the relative humidity reaches 100%, the air is saturated, and if it continues to cool, water vapor condenses into water droplets. These suspended water droplets are clouds. The air parcel will continue to cool by expansion, but it is also being heated through latent heat. The change of temperature for an air parcel with a relative humidity of 100% is the saturated adiabatic rate (SAR) of -6°C per 1,000 meters (-3.3°F per 1,000 feet). The saturated adiabatic rate is sometimes referred to the saturated adiabatic lapse rate, the moist adiabatic rate, or the wet adiabatic rate.

Exercise \(\PageIndex{8}\)

23. In one to two sentences, explain why the saturated adiabatic rate is lower than the dry adiabatic rate. Hint: consider whether or not latent heat is released when condensation occurs.

Guided Practice: Using the Adiabatic Rates

Watch a tutorial video on how to use the adiabatic rates to understand how an air parcel warms or cools according to how its altitude changes. (Video length is 5:35).

Exercise \(\PageIndex{9}\)

In this section, you will explore the change of temperature and relative humidity as an air parcel is forced up and over the Sierra Nevada mountain range. Using the adiabatic rates provided, answer the following questions. You may find it helpful to work on diagramming the scenario (Figure 6.12) as you proceed.

24. Assume an unsaturated air parcel located at sea level (0 meters) in the Sacramento Valley has an initial air temperature of 17°C and a specific humidity of 4kg H₂O/kg air. The prevailing westerlies are forcing the air parcel eastward towards the western slope of the Sierras.

1. Calculate the relative humidity and the dew point temperature of the air parcel at 0 meters. Show your work.

As the air rises and cools, the relative humidity becomes greater and greater. Eventually, the air has a relative humidity of 100%. This altitude is referred to as the lifting condensation level (LCL). This altitude, at which clouds will start to form, is the base of clouds.

LCL = (Air Temperature - Dew Point Temperature)

|DAR| (in ‘000s of m)

2. At what elevation will clouds start to form? Calculate the LCL for this air parcel using the preceding formula. Show your work.

LCL = _______ meters

The temperature is always equal to the dew point temperature at the lifting condensation level. As the equation above suggests, the lifting condensation level (LCL) is defined by the air parcel rising and cooling at the dry adiabatic rate until it reaches its dew point temperature.

As the air rises above the lifting condensation level, the air continues to cool by expansion; however, the development of clouds (water vapor to water droplets) releases latent heat. The air parcel continues to cool, but not as quickly. The air parcel will cool at the saturated adiabatic rate.

3. Calculate the expected temperature of the air parcel at the ridge of the Sierras (4,000 meters). Don’t forget to calculate the DAR of the air before it reaches the LCL and the SAR as it continues to rise to the peak. Show your work.

The following formula might be of assistance:

Change in Temp = Rate (DAR or SAR) x Change in Elevation

Temp at the LCL =______°C

Temp at 4,000 m =______°C

As the air descends down the leeward side of the Sierra Nevada mountain range, the air parcel is heated by compression.

4. If temperature rises, does relative humidity increase or decrease?

5. Based on your answer above, will the sinking air warm at the DAR or SAR? Why? Explain your response in one to two sentences.

6. What is the temperature of the air parcel once it reaches Reno at 1,500 meters? Show your work.

Temp at Reno=______°C

7. Based on your calculations above, use the profile below (Figure 6.12) and fill in any missing information about the labeled locations.
8. Draw arrows indicating the movement of air over the range and label the regions where the air will warm or cool based on the DAR or the SAR. Also, label the windward and leeward sides of the mountain.

Figure 6.12: Temperature and Relative Humidity over the Sierra Nevada Mountain Range. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

Part E. Wrap-Up

Consult with your geography lab instructor to find out which of the following wrap-up questions you should complete. Attach additional pages to answer the questions as needed.

25. What is the most important idea that you learned in this lab? In two to three sentences, explain the concept and why it is important to know.

26. What was the most challenging part of this lab? In two to three sentences, explain why it was challenging. If nothing challenged you in the lab, write about what you think challenged your classmates.

27. What is one question that you have about what you learned in this lab? Write your question along with one to two sentences explaining why you think your question is important to ask.

28. Review the learning objectives on page 1 of this lab. How would you rate your understanding or ability for each learning objective? Write one sentence that addresses each learning objective.

29. Sketch a concept map that includes the key ideas from this lab. Include at least five of the terms shown in bold-faced type.

30. Create an advertisement to educate your peers on the important information that you learned in this lab. Include at least three key terms in your advertisement. The advertisement should be about half a page in size (about 4 inches by 6 inches).

31. One way to think of physical geography is that it is the study of the relationships among variables that impact the Earth's surface. Select two variables discussed in this lab and describe how they are related.

32. How does what you learned in this lab relate to your everyday life? In two to three sentences, explain a concept that you learned in this lab and how it relates to your day-to-day actions.

33. How does what you learned in this lab relate to current events?
    1. Write the title, source, and date of a news item that relates to this lab.
    2. In two to three sentences, discuss how the news item relates to what you have learned in this lab.
    3. In one to two sentences, discuss whether or not the news item accurately represents the science that you learned. Tip: consider whether or not the news item includes the complexity of the topic.

34. Search O*NET OnLine to find an occupation that is relevant to the topics presented in today's lab. Your lab instructor may provide you with possible keywords to type in the Occupation Quick Search field on the O*NET website.
    1. What is the name of occupation that you found?
    2. Write two to three sentences that summarize the most important information that you learned about this occupation.
    3. What is one question that you would want to ask a person with this occupation?