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LAB MODULE 7: ATMOSPHERIC MOISTURE

Note: Please refer to the GETTING STARTED lab module to learn tips on how to set up and maneuver through the Google Earth () component of this lab.

KEY TERMS

The following is a list of important words and concepts used in this lab module:

Adiabatic processes

Frontal uplift

Physical states of water

Cirrus clouds

Hydrologic cycle

Relative humidity

Condensation level

Maximum humidity

Specific humidity

Convectional uplift

Orographic uplift

Stratus clouds

Cumulus clouds

Precipitation

Wet (and dry) bulb temperature

LAB LEARNING OBJECTIVES

After successfully completing this module, you should be able to:

● Describe and explain the hydrologic cycle

● Identify different cloud types

● Explain the adiabatic process

● Compare and contrast different uplift mechanisms

● Compare and contrast different types of humidity

● Explain how precipitation occurs

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INTRODUCTION

In this lab module you will examine some fundamental concepts and principles related to atmospheric moisture. Topics include physical states of water, humidity, adiabatic processes, cloud classification and precipitation. While these topics may appear to be disparate, you will learn how they are inherently related.

The modules start with four opening topics, or vignettes, which are found in the accompanying Google Earth file. These vignettes introduce basic concepts related to atmospheric moisture. Some of the vignettes have animations, videos, or short articles that will provide another perspective or visual explanation for the topic at hand. After reading the vignette and associated links, answer the following questions. Please note that some components of this lab may take a while to download or open, especially if you have a slow internet connection.

Expand the ATMOSPHERIC MOISTURE folder and then expand the INTRODUCTION folder.

Read Topic 1: The Physical States of Water.

Question 1: Explain how this statement is false: Heat is temperature.

A. Temperature is energy, while heat is a measure of temperature

B. Heat is energy, while temperature is a measure of heat

C. Heat is energy, while temperature is the transfer of energy from one state to another

D. Temperature is energy, while heat is the transfer of energy from one state to another

Question 2: Is evaporation the absorption or release of latent heat?

A. Absorption

B. Release

C. Both

D. Neither

Read Topic 2: The Hydrologic Cycle

Question 3: According to the video, what is the common length of storage time for most atmospheric water (rainfall, snowfall) that fall onto land?

A. Only a few hours

B. Several days

C. Weeks or more

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D. It is unknown

Read Topic 3: Atmospheric Humidity

Question 4: How can you have a specific humidity that is low in the high latitudes of the northern hemisphere (as shown by the prominence of blue in first video) and yet have a high relative humidity (as shown by the prominence of red in the second video)?

A. Function of temperature – low temps have a low specific humidity but a low maximum humidity and thereby high relative humidity

B. Function of location – high altitudes (near the poles) have more humidity than low altitudes (near the Equator) and thereby high relative humidity

C. Function of climate – low temperatures have low specific humidity but a high maximum humidity and thereby a high relative humidity

D. Function of humidity – the specific humidity is high and therefore the relatively humidity must also be high

Read Topic 4: Human Interaction

Question 5: What is the primary coarse aerosol in the Atlantic Ocean, between Africa and South America? (Hint: Look to where the potential origin lies and what is found in that location)

A. Sea salts from the Indian Ocean

B. Smoke from fires in Africa

C. Nitrates from coastal populations

D. Dust (sand) from the Sahara Desert

GLOBAL PERSPECTIVE

In this module you will learn about factors influencing precipitation and that precipitation varies spatially and temporally. This section will introduce you to some of these patterns.

Expand GLOBAL PERSPECTIVE and then select June Precipitation.

This map shows total precipitation for the month of June 2011. Precipitation is the condensation of atmospheric water vapor into various forms of water, including rain, sleet, snow, and hail. The amount of precipitation for any given area is measured in millimeters (mm).

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Double-click and select Location A.

Question 6: What is the approximate latitude and longitude (degrees only) for this location?

A. 28 N 82 W

B. 28 S 82 E

C. 28 N 82 E

D. 29 S 82 W

Question 7: Estimate the precipitation for this location.

A. Approximately 1 mm

B. Approximately 100 mm

C. Approximately 200 mm

D. Approximately 2000 mm

Double-click and select Location B.

Question 8: What is the approximate latitude and longitude (degrees only) for this location?

A. 28 N 114 E

B. 28 N 114 W

C. 28 S 114 W

D. 28 S 114 E

Question 9: Estimate the precipitation for this location.

A. Approximately 1 mm

B. Approximately 100 mm

C. Approximately 200 mm

D. Approximately 2000 mm

Question 10: Does latitude play a prominent role in precipitation differences in these two examples in June?

A. Yes, latitude is a main reason for precipitation differences between Locations A and B

B. No, there are other geographic factors that account for the differences between Locations A and B

Select December Precipitation, and then double-click again on Location A.

Question 11: Estimate the precipitation for Location A.

A. Approximately 1 mm

B. Approximately 10 mm

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C. Approximately 200 mm

D. Approximately 2000 mm

Question 12: Does Location A have both a wet season and a dry season?

A. Very likely – there is more precipitation in winter than summer

B. Very likely – there is more precipitation in summer than winter

C. Not likely – there seems to be only a wet season (above 60mm) year-round

D. Not likely – there seems to be only a dry season (below 60 mm) year-round

Double-click and select Location C.

Question 13: What is the latitude (degrees only) for this location?

A. 4 N 114 E

B. 4 S 114 W

C. 4 N 114 W

D. 4 S 114 E

Toggle between June Precipitation and December Precipitation.

Question 14: Does Location C have both a wet season and a dry season?

A. Very likely – there is more precipitation in winter than summer

B. Very likely – there is more precipitation in summer than winter

C. Not likely – there seems to be only a wet season (above 60mm) year-round

D. Not likely – there seems to be only a dry season (below 60 mm) year-round

Question 15: Does latitude play a prominent role in precipitation? (Hint: look at the overall precipitation trend across the Earth at this approximate latitude)

A. Yes, latitude is a main reason for the precipitation pattern of Location C

B. No, there are other geographic factors that account for the precipitation at Locations C

Collapse and uncheck GLOBAL PERSPECTIVE.

HUMIDITY

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We learned from Topic 3 in the Introduction section the three types of humidity: maximum, specific and relative humidity. When we speak colloquially about humidity, we are usually referring to relative humidity. For example, on some hot summer days, the air may feel sticky and we say the (relative) humidity is high. Conversely, on cold winter days, the air may feel dry and we say the (relative) humidity is low.

We can use a simple device called sling psychrometer to measure the dry bulb temperature and the wet bulb temperature. The dry bulb temperature is the ambient air temperature, and is measured using a regular thermometer. The wet bulb temperature, however, is the temperature measured by covering the end of a thermometer in a wet cotton sleeve and then whirling it around to evaporate some water from the sleeve. Since evaporation is a cooling process, the wet bulb thermometer will record a lower reading than the dry bulb thermometer as long as the surrounding air is not saturated. By comparing the temperature between the two thermometer readings, and then looking up the values in Table 1, we can determine (sometimes by way of interpolation) the relative humidity.

For example:

1. Assume that the dry bulb temperature is 26°C, and the wet bulb temperature is 16°C.

2. With these two temperatures, use the following formula to calculate the wet bulb depression by subtracting the wet bulb temperature from the dry bulb temperature: 26°C – 16°C = 10°C

3. Refer to Table 1 to determine the relative humidity; in this case, the relative humidity (RH) is 34 percent (34%).

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Table 1. Table 1. Relative Humidity, Wet-Dry Bulb Method (Source: Adapted from the NOAA Relative Humidity and Dew Point table).

Expand the HUMIDITY folder.

Watch the videos under HUMIDITY and in conjunction with Table 1, determine the relative humidity for the following locations.

Click Mariposa Grove and record the wet and dry bulb temperatures.

Question 16: What is the relative humidity at Mariposa Grove?

Dry Bulb (˚C)

Wet Bulb (˚C)

Wet Bulb Depression (Dry-Wet), (˚C)

Relative Humidity (%)

Note to Editor: Use drop-down choices for each box. Choices as follows:

List of potential answers for Dry Bulb: 23.5°C, 16.5°C, 20°C, 27°C,

List of potential answers for Wet Bulb: 19°C, 19.5°C, 10°C, 15°C,

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List of potential answers for Wet Bulb Depression: 10°C, 8°C, 4°C, 1.5°C

List of potential answers for Wet Bulb Depression: 24%, 19%, 27%, 20%

Click California Central Valley and record the wet and dry bulb temperatures.

Question 17: What is the relative humidity just outside of Fresno?

Dry Bulb (˚C)

Wet Bulb (˚C)

Wet Bulb Depression (Dry-Wet), (˚C)

Relative Humidity (%)

Note to Editor: Use drop-down choices for each box. Choices as follows:

List of potential answers for Dry Bulb: 23.5°C, 16.5°C, 20°C, 27°C

List of potential answers for Wet Bulb: 19°C, 19.5°C, 10°C, 15°C

List of potential answers for Wet Bulb Depression: 10°C, 8°C, 4°C, 1.5°C

List of potential answers for Wet Bulb Depression: 46%, 42%, 53%, 45%

Click Redwood Forest and record the wet and dry bulb temperatures.

Question 18: What is the relative humidity in the redwood forest?

Dry Bulb (˚C)

Wet Bulb (˚C)

Wet Bulb Depression (Dry-Wet), (˚C)

Relative Humidity (%)

Note to Editor: Use drop-down choices for each box. Choices as follows:

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List of potential answers for Dry Bulb: 23.5°C, 16.5°C, 20°C, 27°C

List of potential answers for Wet Bulb: 19°C, 19.5°C, 10°C, 15°C

List of potential answers for Wet Bulb Depression: 10°C, 8°C, 4°C, 1.5°C

List of potential answers for Wet Bulb Depression: 60.5%, 68%, 68.75%, 69.5%,

Click Monterey Bay, CA and record the wet and dry bulb temperatures.

Question 19: What is the relative humidity at the beach at Monterey Bay?

Dry Bulb (˚C)

Wet Bulb (˚C)

Wet Bulb Depression (Dry-Wet), (˚C)

Relative Humidity (%)

Note to Editor: Use drop-down choices for each box. Choices as follows:

List of potential answers for Dry Bulb: 23.5°C, 16.5°C, 20°C, 27°C, 15°C

List of potential answers for Dry Bulb: 19°C, 19.5°C, 10°C, 15°C, 27°C

List of potential answers for Wet Bulb Depression: 10°C, 8°C, 5.5°C, 4°C, 1.5°C

List of potential answers for Wet Bulb Depression: 80%, 81.75%, 84.5%, 85.25%, 91%

Collapse and uncheck HUMIDITY.

ADIABATIC PROCESS

As a parcel of air (also known as a thermal) rises, the pressure decreases (the parcel expands) and it cools. This process is known as adiabatic cooling.

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Conversely, as a parcel of air descends, its pressure increases (the parcel compresses) and it warms. This process is known as adiabatic warming. These changes in temperature are a result of changes in pressure within the air parcel itself, with an expanding parcel promoting a decrease in temperature (cooling) and a compressing air parcel promoting an increase in temperature (warming).

When the relative humidity (RH) of a rising parcel of air is less than 100% (meaning it is not saturated), the parcel cools at the dry adiabatic rate (DAR), which is approximately 1°C/100m. Likewise, a descending air parcel that is not saturated warms at the same DAR.

For example, imagine a rising parcel of air with a temperature of 15˚C and an RH of 60%. If the parcel rises 400 meters in elevation, its temperature will be 11˚C. In other words, the air parcel cools 1˚C for every 100m increase in elevation, thereby cooling 4˚C.

Thing are different, however, if the RH of an air parcel is 100% (i.e. the air parcel is saturated). When the RH is 100%, the air parcel cools at the wet adiabatic rate (WAR), which is approximately 0.5°C/100m. The WAR is not as great as the DAR because latent heat of condensation (the energy when water vapor condenses to a liquid) is released.

For example, the temperature of a rising saturated parcel of air is 18°C. If this parcel continues to rise another 1000 meters in elevation, its temperature will be 13°C. In other words, the air parcel cools 0.5˚C for every 100m increase in elevation, thereby cooling 5˚C.

Click ADIABATIC PROCESSES to watch the video.

For the following questions, use the following air parcel conditions:

An unsaturated parcel of air with a temperature of 20˚C rises 1200m to the condensation level and then continues to rise saturated for another 600m.

Question 20: What is the temperature of the parcel when it becomes saturated?

A. 32˚C

B. 8˚C

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C. 5˚C

D. 14˚C

Question 21: What is the temperature of the parcel when it stops rising?

A. 32˚C

B. 8˚C

C. 5˚C

D. 14˚C

The unsaturated air parcel then descends 1800m back to its original elevation.

Question 22: What is the temperature of the parcel once it has descended to its original elevation?

A. 40˚C

B. 23˚C

C. 14˚C

D. 20˚C

Question 23: When the air parcel completes its decent at its original elevation, how does this ending temperature compare to the starting temperature?

A. Warmer

B. Cooler

C. Same

D. Variable (warmer or cooler)

Uncheck ADIABATIC PROCESSES.

CLOUD CLASSIFICATION

Scientists classify clouds according to their form and altitude. There are three cloud classes based on form: cirrus, cumulus and stratus.

● Cirrus clouds are wispy, thin clouds comprised of ice crystals.

● Cumulus clouds have distinct puffy shapes with flat bases formed at the condensation level.

● Stratus clouds are gray sheet like clouds covering most of the sky.

Clouds are further classified according to their altitude.

● High clouds are found over 6km (20,000 ft.) in the atmosphere

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● Middle clouds are between 2.5km and 6km (6,500 ft. to 20,000 ft.).

● Low clouds are those less than 2.5km.

Click CLOUD CLASSIFICATION.

Use the animation to identify characteristics of cloud types and to complete the table below. The first one has been done for you as an example

Cloud type

Form

(choose from wispy, puffy, patchy, or sheet)

Altitude

(choose from high, middle or low)

Rain

(choose yes or no)

Altostratus

Sheet

Middle

No

24. Altocumulus

25. Cirrocumulus

26. Cirrus

27. Cumulonimbus

28. Cumulus

29. Stratocumulus

30. Stratus

Note to Editor: Q24-Q30 above should be drop-down choices for each box. Choices are located under Form, Altitude, and Rain

Uncheck CLOUD CLASSIFICATION.

PRECIPITATION PROCESSES

Introduction

When water vapor in the air is cooled to its saturation point, water droplets or ice crystals form. Once the water droplets or ice crystals become large enough to fall under the force of gravity, precipitation occurs. In order for this occur, air must rise such that sufficient condensation takes place. This required lifting of an air parcel commonly happens in one of many ways, including convectional uplift, orographic uplift, frontal uplift, and convergent (cyclonic) uplift. We will cover the first two in this module as they showcase the processes associated with adiabatic cooling. To note, geography plays an important role in precipitation (or lack thereof), as certain geographic areas are more inclined to produce a particular type of uplift.

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Convectional uplift

Convectional uplift occurs when a parcel of air within a larger stable air mass is heated by the hot ground and rises. When this parcel rises above the condensation level, cumulus clouds tend to form. In many cases these clouds will drift along with the wind and eventually dissipate, producing no rain. But in some cases the air is unstable and strong convectional uplift occurs. Cumulonimbus clouds often form, producing rainfall and in more severe cases, thunderstorms develop. While convectional uplift and associated precipitation can occur almost anywhere over land, there are certain places where this uplift is more common. For example, the Great Plains region in the United States commonly experiences this type of uplift during the summer months, which produces rainstorms in the afternoon. Equatorial regions where solar insolation is intense are subjected to precipitation from conventional uplift.

Click Convectional Rainfall and watch the video.

Question 31: True or False: Convectional uplift goes through the process of adiabatic cooling.

A. True

B. False

Orographic uplift

Orographic uplift is caused by mountains which force an air parcel upwards as the air flows. As the parcel rises, the air pressure decreases, causing the parcel to expand and the air temperature to decrease. When the parcel reaches the condensation level, clouds form, and in some cases, precipitation occurs. After the parcel has cleared the mountains, it descends and the air is compressed, leading to an increase in temperature. This drier, warmer parcel creates a rainshadow on the leeward side of mountain ranges. This type of rainfall is common along the mountain ranges near the Pacific Ocean as well as oceanic islands such as Hawai’i and New Zealand.

Click Orographic Processes and watch the video on Orographic Uplift. After watching the video, explain the following scenarios:

Question

Initial (Start) Temperatur

Final (End)

Temperature

Did it Rain?

(yes or no)

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e

32. Scenario 2

33. Scenario 7

34. Scenario 9

Question 35: How does a rain event change the final temperature from the initial temperature?

A. Increases

B. Decreases

C. Stays the same

D. Variable (increases or decreases)

Question 36: Does is rain on the windward or leeward side?

A. Windward

B. Leeward

Question 37: What is the relative humidity when it rains?

A. 0 percent

B. 50 percent

C. 100 percent

D. Variable

Double-click and select Location D.

Question 38: This location is on the ________ of the Cascade Mountains.

A. Windward side

B. Leeward side

C. Convergent side

D. Frontal side

Double-click and select Location E.

Question 39: What is another name for the dry area found around Location E?

A. Windward

B. Rainshadow

C. Orographic

D. Convective

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Double-click and select Location F.

Question 40: What type of adiabatic uplift would lead to precipitation at Location F?

A. Divergent

B. Frontal

C. Orographic

D. Convectional

Collapse and uncheck the PRECIPITATION PROCESSES folder. You have completed Lab Module 7.