Chapter 7Water and Atmospheric

Moisture

Geosystems 5eAn Introduction to Physical Geography

Robert W. ChristophersonCharlie Thomsen

OverviewAfter reading this lecture you should be able to:

1. Describe the origin of Earth's waters, define the quantity of water that exists today, and list the locations of Earth's freshwater supply.

2. Describe the heat properties of water and identify the traits of its three phases: solid, liquid, and gas.

3. Define humidity and the expressions of the relative humidity concept; explain dew-point temperature and saturated conditions in the atmosphere.

4. Define atmospheric stability and relate it to a parcel of air that is ascending or descending.

5. Illustrate three atmospheric conditions—unstable, conditionally unstable, and stable—with a simple graph that relates the environmental lapse rate to the dry adiabatic rate (DAR) and moist adiabatic rate (MAR).

6. Identify the requirements for cloud formation and explain the major cloud classes and types, including fog.

1. Approximately where and when did 1. Approximately where and when did Earth's waters originate?Earth's waters originate?

Most of the water on Earth was formed within the planet (what is another possible source?), reaching Earth's surface through a process called outgassing, by which water and water vapor emerge from layers deep within and below the crust, as much as 25 km or more below Earth's surface. Various geophysical factors explain the timing of this outgassing over the past 4 billion years. In the early atmosphere, massive quantities of outgassed water vapor condensed and fell in torrents, only to vaporize again because of high temperature at Earth's surface. For water to remain on Earth's surface, land temperatures had to drop below the boiling point of 100°C, something that occurred about 3.8 billion years ago.

2. If the quantity of water on Earth has been quite 2. If the quantity of water on Earth has been quite constant in volume for at least 2 billion years, how can sea constant in volume for at least 2 billion years, how can sea

level have fluctuated?level have fluctuated? Today, water is the most common compound on Earth, having achieved the present volume of 1.36 billion km3 approximately 2 billion years ago. This quantity has remained relatively constant, even though water is continuously being lost from the system, escaping to space or breaking down and forming new compounds with other elements. Lost water is replaced by pristine water not previously at the surface, water that emerges from within Earth. The net result of these inputs and outputs to water quantity is a steady-state equilibrium in Earth's hydrosphere. Despite this overall net balance in quantity, worldwide changes in sea level do occur and are called eustasyeustasy, which is specifically related to changes in volume of water and not movement of land. These changes are explained by glacio-eustatic factors. Glacio-eustatic factors are based on the amount of water stored on Earth as ice.

3. Describe the location of Earth's water, both oceanic 3. Describe the location of Earth's water, both oceanic and fresh. What is the largest repository of freshwater at and fresh. What is the largest repository of freshwater at

this time?this time?

The greatest single repository of surface freshwater is ice. Ice sheets and glaciers account for 77.78% of all freshwater on Earth. Add to this the subsurface groundwater, and that accounts for 99.36% of all freshwater. The remaining freshwater, although very familiar to us, and present in seemingly huge amounts in lakes, rivers, and streams, actually represents but a small quantity, less than 1%. (See Figure 7.3)

Figure 7.3: Ocean and freshwater distribution on Earth. Pie diagrams show all water, freshwater only, and surface

water percentages.

4. What happens to the physical structure of water as it cools below 4°C (39°F)? What are some visible indications

of these physical changes?As water cools, it behaves like most compounds and contracts in volume, reaching its greatest density at 4°C. But below that temperature, water behaves very differently from most compounds, and begins to expand as more hydrogen bonds form among the slower-moving molecules, creating the hexagonal structures of ice (See below). This expansion continues to a temperature of –29°C, with up to a 9% increase in volume possible. The expansion in volume that accompanies the freezing process results in a decrease in density. Specifically, ice has 0.91 times the density of water, and so it floats.

5. What is latent heat? How is it involved in the phase changes of water?

For water to change from one state to another, heat energy must be added to it or released from it. The amount of heat energy must be sufficient to affect the hydrogen bonds between the molecules. For ice to melt, heat energy must increase the motion of the water molecules to break some of the hydrogen bonds. Despite the fact that there is no change in sensible temperature between ice at 0°C and water at 0°C, 80 calories are required for the phase change of 1 gram of ice to 1 gram of water. This heat, called latent heat, is hidden within the water and is liberated whenever a gram of water freezes. To accomplish the phase change of liquid to vapor at boiling, under normal sea-level pressure, 540 calories must be added to 1 gram of boiling water to achieve a phase change to water vapor. Those calories are the latent heat of vaporization

6. Take 1 g of water at 0°C and follow it through to 1 g of water 6. Take 1 g of water at 0°C and follow it through to 1 g of water vapor at 100°C, describing what happens along the way. What vapor at 100°C, describing what happens along the way. What amounts of energy are involved in the changes that take place?amounts of energy are involved in the changes that take place?

If we take a gram of water at O°C, it will require 100 calories to raise the temperature of water to 100°C, one calorie for each degree of change in temperature. At 100°C, water is at its boiling point. To change water's phase from liquid to vapor, will require another 540 calories. This energy is called the latent heat of vaporization. The total amount of energy absorbed by water in this process was 640 calories, 100 cal + 540 cal = 640 cal. To take a gram of ice at 0°C and turn it into vapor at 100°C, would require 80 additional calories, this energy absorbed by the water is called the latent heat of melting, or the latent heat of fusion. Thus, to transform ice at 0°C to vapor at 100°C would require a total of 720 calories, 80 calories to change from ice at 0°C to water at 0°C, plus 100 calories to heat water from 0°C to 100°C, and finally the additional 540 calories required to heat water at 100°C to vapor at 100°C, 80 cal + 100 cal + 540 cal = 720 cal.

7. What is humidity? How is it related to the energy 7. What is humidity? How is it related to the energy present in the atmosphere? To our personal comfort and present in the atmosphere? To our personal comfort and

how we perceive apparent temperatures.how we perceive apparent temperatures.Humidity is the vapor content of air. The vapor content of air changes according to the temperature of air and the temperature of water vapor. Warm air has a greater capacity to hold water than does cold air. Warm air stores a greater amount of energy in the atmosphere as the latent heat of evaporation, than cold air. Humidity affects the apparent temperature that we experience. For example, air with very little humidity, while it may be hot, is much more comfortable for humans. The warmth of the air enables the atmosphere to hold more moisture, and as we perspire, our bodies are cooled by this heat transfer. If we live in areas that are hot and humid, this means that the air is close to being saturated, and may not be able to hold more moisture. Our perspiration then may not be absorbed by the atmosphere, making the apparent heat seem much higher than actual air temperature.

8. Define relative humidity. What does the concept 8. Define relative humidity. What does the concept represent? What is meant by the terms saturation and represent? What is meant by the terms saturation and

dew-point temperature?dew-point temperature?The water vapor content of air is termed humidityhumidity. The capacity of air to hold water vapor is primarily a function of temperature: warmer air has a greater capacity for water vapor, whereas cooler air has a lesser capacity. Relative humidityRelative humidity is not a direct measurement of water vapor; rather, it is expressed as a percentage of the amount of water vapor that is actually in the air (content), compared with the maximum water vapor the air could hold at a given temperature (capacity). Air is said to be saturated, or full, if it is holding all the water vapor that it can hold at a given temperature; under such conditions, the net transfer of water molecules between surface and air achieves a saturation equilibrium. Saturation indicates that any further addition of water vapor (change in content) or any decrease in temperature (change in capacity) will result in active condensation.The temperature at which a given mass of air becomes saturated is termed the dew-point temperaturedew-point temperature. In other words, air is saturated when the dew-point temperature and the air temperature are the same.

9. How do the two instruments described in this 9. How do the two instruments described in this chapter measure relative humidity?chapter measure relative humidity?

The hair hygrometerhair hygrometer uses the principle that human hair changes as much as 4% in length between 0 and 100% relative humidity. The instrument connects a standardized bundle of human hair through a mechanism to a gauge and a graph to indicate relative humidity. Another instrument used to measure relative humidity is a sling psychrometer,sling psychrometer, it has two thermometers mounted side-by-side on a metal holder. One is called the dry-bulb thermometer; it simply records the ambient air temperature. The other thermometer is called the wet-bulb thermometer; it is set lower in the holder and has a cloth wick over the bulb, which is moistened with distilled water. The psychrometer is then spun by its handle for a minute or two. The readings on the two thermometers can then be checked on a psychrometric table to determine relative humidity. The greater the difference between the two thermometers, the lower the relative humidity.

15. What are the forces acting on 15. What are the forces acting on a vertically moving parcel of air?a vertically moving parcel of air?

There are two opposing forces which act on a vertically moving parcel of air. These forces are an upward buoyant force and a downward gravitational force. Temperature and density characteristics of the air mass determine which force will be more influential. If an air mass is higher in temperature than the surrounding atmosphere or if an air mass is less dense than the surrounding air, it will continue to rise vertically. Similar to any gas, as it rises in the air, the air mass begins to expand due to decreasing pressure in higher altitudes of the atmosphere. An unstable air mass will continue to rise until the surrounding air has similar density and temperature characteristics. An air mass will become dominated by the downward pull of gravity as it cools in the atmosphere. The temperature and density of the air mass may be lower than the surrounding air. This will cause the air mass to descend. As the parcel falls, external pressure in the atmosphere increases causing the air mass to compress (see figures).

16. What would atmospheric temperature and moisture 16. What would atmospheric temperature and moisture conditions be on a day when the weather is unstable? conditions be on a day when the weather is unstable?

When it is stable?When it is stable? On a day when the weather is unstable, the atmosphere is dominated by warm air that may absorb available moisture and reach the level of saturation. These warm air parcels may rise vertically causing them to cool, condense and precipitate. You would experience evaporation during the warm period of the day, perhaps beginning the morning with a cloudless sky and seeing clouds form throughout the day as the air warms and the humid parcels of air rise in the atmosphere, cooled by the environmental lapse rate. As the parcels cool in the atmosphere, or after 4 P.M., when the atmosphere begins to cool, precipitation may occur.On a day when the weather is stable, there is no vertical movement of air. The atmosphere remains cool and dry, due to the lower capacity of cool air to hold moisture. You would experience a cloudless sky, or perhaps a greenhouse effect caused by pollution or clouds which may cool the lower atmosphere.

CLOUDSCLOUDSCloud TypesCloud Types

Common Cloud ClassificationsCommon Cloud Classifications

17. Specifically, what is a cloud? 17. Specifically, what is a cloud? Describe the droplets that form a cloud.Describe the droplets that form a cloud.Clouds are not initially composed of raindrops. Instead, they are made up of a multitude of moisture droplets, each individually invisible to the human eye without magnification. An average raindrop, at 2000 µm diameter (0.2 cm), is made up of a million or more moisture droplets, each approximately 20 µm diameter (0.002 cm).

Cloud NamingCloud NamingAncient people wove tales into the sky and for hundreds of years it was thought that the continuous parade of clouds across the heavens represented an endless number of forms too numerous to name. In 1803, a British chemist named Luke HowardLuke Howard changed all that. Based on years of keen observation, he introduced a cloud-naming scheme employing Latin words that is still in use today. Howard claimed that there was a fixed number of cloud types that could be easily identified based on visual characteristics. 

This process of cloud naming was introduced to North America in the first Encyclopedia Americana published in 1830. During the remainder of the 19th Century, Howard’s cloud names were reworked and reorganized by his contemporaries, culminating in the “International Year of the Clouds” in 1896 when the first atlas of clouds was published. Howard’s cloud classification system was expanded to include three different cloud levels: high, mid and low level clouds. The combinations result in 30 distinct cloud types.

Howard’s four basic Latin named cloud types include:

CIRRUSCIRRUS: meaning “fiber” or “hair” in Latin

CUMULUSCUMULUS: “heaped” or “piled”

STRATUSSTRATUS: “sheet” or “layer”

NIMBUSNIMBUS: “rainmaking”

18. What are the basic forms of clouds? Describe how the basic cloud forms vary with altitude.

Clouds usually are classified by altitude and by shape. They come in three basic forms–flat, puffy, and wispy—which are found in four primary classes and ten basic types. Clouds that are developed horizontally and are flat and layered, are called stratiform cloudsstratiform clouds. Those that are developed vertically and are puffy and globular are termed cumuliformcumuliform. Wispy clouds usually are quite high, composed of ice crystals, and are labeled cirroformcirroform. These three basic forms occur in four altitudinal classes: low, middle, high, and those that are vertically developed occur across all altitudes.

HIGH CLOUDSHIGH CLOUDS Generally these clouds form above 20,000 feet and usually appear white except at

sunrise or sunset when they can ignite in brilliant reds and oranges. Clouds at this height are usually thin and are almost exclusively composed of ice crystals.

1.1. CIRRUSCIRRUS - Wispy clouds. Considered one of the most beautiful cloud types.

2.2. CIRROSTRATUSCIRROSTRATUS - These clouds form a thin sheet covering the entire sky. The sun or moon is often still visible. When ice crystals that form this cloud type are properly aligned, a “rainbow ring” or halo can form around the sun or moon, often signaling precipitation in the next 24 to 48 hours.

3.3. CIRROCUMULUSCIRROCUMULUS - Patches of high clouds featuring small heaps making the sky cover appear like fish scales—sometimes called a “mackerel sky.”

MID CLOUDSMID CLOUDSThe base of these clouds are located between 6,500-23,000 feet and are

composed of water droplets and when temperatures are low enough, some ice crystals.

1.1. ALTOSTRATUSALTOSTRATUS - This sheet of mid-level clouds usually covers the entire sky and is primarily made of water droplets. This deck of clouds is usually 1,500-3,000 feet thick. Occasionally the disk of the sun or moon is dimly visible. A corona—a colorful ring with blue on the inside— may form around the sun or moon when this sheet of clouds is present.

2.2. ALTOCUMULUSALTOCUMULUS - Mid-level clouds with larger heaps than cirrocumulus. This cloud type is very pleasing to view and may cover the entire sky or appear in smaller bands.

LOW CLOUDSLOW CLOUDSThe base of low clouds lie below 6,500 feet and are most always

composed of water droplets, but in cold weather, these clouds may also contain ice particles and snow.

1.1. STRATUSSTRATUS - These clouds form a low hanging gray, featureless sheet that covers the entire sky and has no defined cloud base or top. Ground fog is simply a layer of stratus clouds touching the ground.

2.2. STRATOCUMULSSTRATOCUMULS - These low hanging layered clouds have cotton ball-like edges and are located at the same altitude as fair weather cumulus (next slide).

3.3. NIMBOSTRATUSNIMBOSTRATUS - This thick mass of low clouds has no well-defined base and covers the entire sky with dark gray jagged bases, producing continuous rain or snow.

CLOUDS WITH HEIGHTCLOUDS WITH HEIGHTThe cumulus cloud family can build vertically to varying heights in the sky from a few thousand feet to 65,000 feet

—even higher than commercial aircraft!1.1. CUMULUSCUMULUS - These fair weather, low-

level clouds usually bubble up late morning through mid afternoon with plenty of blue sky in between and little vertical growth above their flat bases that form between 1,500-4,000 feet.

2.2. CUMULUS CONGESTUSCUMULUS CONGESTUS - These swelling cumulus clouds are marked by flat bases between 3,000-6,000 feet and growing towers that appear like cauliflower with rounded sharp edges that can climb to 33,000 feet.

3.3. CUMULONIMBUSCUMULONIMBUS - These mature thunderstorm clouds produce heavy rainfall, lightning and occasionally hail. The top of these clouds can climb to 65,000 feet, penetrating into the stratosphere and are marked by a flat lid of wispy clouds called an anvil.

SPECIALTY CLOUDSSPECIALTY CLOUDSThere are some clouds that look like flying saucers and others that

appear like writing on the sky. 1.1. LENTICULAR CLOUDSLENTICULAR CLOUDS - These

clouds are formed by air currents moving over a mountain peak or ridge. They have smooth edges making them appear like a stack of “flapjacks” or even flying saucers! Their name literally means “lens shaped.”

2.2. CONTRAILSCONTRAILS - These narrow clouds criss-cross the sky when the air aloft is moist. They are formed in the wake of jet aircraft from moisture released during the burning of fuel. When the air is dry, they fail to form or disappear quickly. When the air is moist, wind and gravity spread them out over time.

3.3. MAMMATUS CLOUDSMAMMATUS CLOUDS - These pouch like clouds typically form on the underside of a cumulonimbus cloud close to the end of its life cycle.

Table 7.2. Summary: Cloud Classes and TypesTable 7.2. Summary: Cloud Classes and Types

19. Explain how clouds might be used as indicators of the 19. Explain how clouds might be used as indicators of the conditions of the atmosphere and of expected weather?conditions of the atmosphere and of expected weather?Clouds can be used to indicate conditions of the atmosphere and expected weather, due to their ability to demonstrate the atmospheric stability, atmospheric temperature and moisture level or relative humidity. The altitude at which cloud develop are good indicators of atmospheric temperatures. Clouds, low in elevation, such as stratus or cumulus clouds, reflect a cool atmosphere, which allow clouds to form at low altitudes in the troposphere. This may lead to condensation and precipitation of a saturated air mass. Cirrus clouds are a good example of this. As they thicken and sink to low elevations, they are often associated with oncoming storms. Vertically developed clouds, such as cumulus or cumulonimbus clouds, demonstrate air masses laden with moisture, and one could expect to see precipitation depending on their height and mass. Other cloud formations illustrate scarcity of moisture in the atmosphere, or a warm atmosphere that limits condensation in the lower atmosphere. Good examples of these clouds could be stratocumulus clouds often associated with clearing weather and wispy cirrus clouds high in the upper troposphere.

20. What type of cloud is fog? List and define the 20. What type of cloud is fog? List and define the principal types of fog.principal types of fog.

A cloud in contact with the ground is commonly referred to as fog. The presence of fog tells us that the air temperature and the dew-point temperature at ground level are nearly identical, producing saturated conditions. Generally, fog is capped by an inversion layer, with as much as 30 C° difference in air temperature between the ground under the fog and the clear, sunny skies above. By international agreement, fog is officially described as a cloud layer on the By international agreement, fog is officially described as a cloud layer on the ground, with visibility restricted to less than 1 km.ground, with visibility restricted to less than 1 km. As the name implies, advection fog advection fog (see next slide) forms when air in one place migrates to another place where saturated conditions exist. Valley fog Valley fog (see 2nd slide) is another form of advection fog- because cool air is denser than warm air, it settles in low-lying areas like valleys. Another type of fog forms when cold air flows over the warm water of a lake, ocean surface, or even a swimming pool. An evaporation fogevaporation fog, or steam fogsteam fog, may form as the water molecules evaporate from the water surface into the cold overlying air. A type of advection fog, involving the movement of air, forms when moist air is forced to higher elevations along a hill or mountain. This upslope lifting leads to cooling by expansion as the air rises. Radiation fog Radiation fog (see 3rd slide) forms when radiative cooling of a surface chills the air layer directly above that surface to the dew-point temperature, creating saturated conditions and fog. This fog occurs especially on clear nights over moist ground.

Advection Fog

Figure 7.24

Valley Fog

Figure 7.25

Radiation Fog

Figure 7.26

21. Examples of occurrences of fog in the United 21. Examples of occurrences of fog in the United States and Canada. Where are the regions of States and Canada. Where are the regions of

highest incidence?highest incidence? According to Figure 7-27 (next slide), fog occurs in greatest frequency along the coastlines of the Pacific, the North Atlantic and the Labrador Sea, with over 80, 80-180, and 90-150 days of fog per year in these respective locations. This is due to cold air temperatures causing evaporation fog in the Atlantic and Labrador regions. While advection fog, the movement of warm air along a cold body of water, occurs along the Pacific, due to the cool temperature of the California Current. Advection fog also occurs along the Great Lakes, as warm air from the tropics travels over the Great Lakes giving regions, such as Montreal more than 60 days of fog. Other inland locations receive a high number of days with fog, mostly associated with orographic or upslope fog where the warm, moist air is forced upslope and cools to create fog. Good examples of this are in the Cascades causing 100-200 days of fog in Oregon and Washington states, and causing more than 80 days of fog along the Appalachian Mountains.

Figure 7.27:Figure 7.27:Fog Fog

Incidence Incidence Map.Map.

End of Chapter 7

Geosystems 5eAn Introduction to Physical Geography

Robert W. ChristophersonCharlie Thomsen