Chapter 4 - Moisture and Atmospheric Stability

Question 1

Hydrologic Cycle

Answer 1

The continuous exchange of water among the oceans, the atmosphere, and the continents.

Question 2

Transpiration

Answer 2

The release of water vapour to the atmosphere by plants.

Question 3

Evapotranspiration

Answer 3

Because we cannot clearly distinguish between the amount of water that evaporates from land from that which is transpired from plants, the term evapotranspiration is often used to describe the common process.

Question 4

The quantity of water lost to evaporation over the oceans is not equaled by precipitation. Why then does sea level not drop?

Answer 4

Because the precipitation exceeds evaporation over the continents and this water flows back to the sea.

Question 5

Sketch and describe the movement of water through the hydrologic cycle.

Answer 5

1. Water from the oceans, and, to a lesser extend, from land areas evaporate into the atmosphere.
2. Winds transport the moisture-laden air, often over great distances, until conditions cause the moisture to condense into cloud droplets.
3. The process of cloud formation may result in precipitation. The precipitation that falls into the ocean has ended its cycle and is ready to begin another.
4. A portion of the water that falls on the land soaks into the ground, some of it moving downward and then laterally, where it eventually seeps into lakes and streams (infiltration).
5. Much of the water that soaks in or runs off returns to the atmosphere through evaporation.
6. In addition, some of the water that infiltrates the ground is absorbed by plants through their roots and they release it via tranpiration.

5 & 6 are evapotranspiration.

Question 6

What properties of water set it apart from most other substances?

Answer 6

1. Water is the only liquid found at Earth’s surface in large quantities.
2. Water is readily converted from one state of matter to another.
3. Water’s solid phase, ice, is less dense than liquid water.
4. Water has a high heat capacity - meaning it requires considerable energy to change its temperature.

Question 7

Hydrogen Bonds

Answer 7

The attractive forces that exist between hydrogen atoms in one water molecules and oxygen atoms of any other water molecule.

Occurs because oxygen atoms have a greater affinity for the bonding electrons than hydrogen atoms resulting in the oxygen end of a water molecule having a partial negative charge and the hydrogen side having a partial positive charge.

Question 8

Water’s solid phase, ice, is less dense than liquid water. Why is this unique property of water important?

Answer 8

Because ice is less dense than the liquid water beneath it, a water body freezes from the top down. When ice forms on a water body, it insulates the underlying liquid and slows the rate of freezing at depth. If a water body froze from the bottom, many lakes would freeze solid during the winter, killing the aquatic life. Also, deep bodies of water, such as the Arctic Ocean, would never be ice covered which would alter Earth’s heat budget, which in turn would modify global atmospheric and ocean circulations.

Question 9

Explain what happens as ice melts to become liquid water.

Answer 9

Hydrogen bonds are what hold molecules together to form ice. In ice, hydrogen bonds produce a rigid hexagonal network. The resulting molecular configuration is very open (lots of empty space). When ice is heated sufficiently, it melts. Melting cause some, but not all, of the hydrogen bonds to break. As a result, the water molecules in liquid water display a more compact arrangement. This accounts for the fact that water in its liquid phase is denser than it is in the solid phase.

Question 10

What property of water causes large water bodies to remain warmer than adjacent land masses in winter but cooler in summer?

Answer 10

When water is heated, some of the energy is used to break hydrogen bonds rather than to increase molecular motion (an increase in average molecular motion corresponds to an increase in temperature). Thus, under similar conditions, water heats up and cools down more slowly than most other common substances. As a result, large water bodies tend to moderate temperatures by remaining warmer than adjacent land masses in winter and cooler in summer.

Question 11

Calorie

Answer 11

The amount of heat required to raise the temperature of 1 gram of water 1C (1.8F).

Question 12

Latent Heat of Melting

Answer 12

It requires 80 calories to melt 1 gram of ice.

Question 13

Latent Heat of Fusion

Answer 13

Freezing of water releases 80 calories per gram to the environment.

Question 14

Evaporation

Answer 14

The process of converting a liquid to a gas (vapour).

Absorbs energy,

Question 15

Latent Heat of Vaporization

Answer 15

The energy absorbed by water molecules during evaporation that is used to give them the motion needed to escape the surface of the liquid and become a gas.

Varies from ~600 calories per gram for water at 0C to 540 calories per gram at 100C.

Question 16

Explain why evaporation is called a cooling process.

Answer 16

During the process of evaporation, the higher-temperature (faster-moving) molecules escape the surface. As a result, the average molecular motion (temperature of the remaining water) is lowered - hence the expression “evaporation is a cooling process.” In the case of your body, the energy used to evaporate water comes from your skin - chance, you feel cool.

Question 17

Condensation

Answer 17

Occurs when water vapour changes to the liquid state.

Results in fog and clouds and dew.

Releases energy,

Question 18

Latent Heat of Condensation

Answer 18

Water vapour molecules release energy in an amount equivalent to what was absorbed during evaporation.

The energy released when water vapour changes to the liquid state.

Question 19

Sublimation

Answer 19

The conversion of a solid directly to a gas, without passing through the liquid state.

Absorbs energy.

Question 20

Deposition

Answer 20

The conversion of a vapour directly to a solid, without passing through the liquid state.

Releases energy.

Question 21

Humidity

Answer 21

The general term used to describe the amount of water vapour in the air.

Question 22

Absolute Humidity

Answer 22

The mass of water vapour in a given volume of air (usually as grams per meter):

Absolute humidity = (Mass of Water Vapour (g))/(Volume of Air (cubic meters))

Question 23

Mixing Ratio

Answer 23

The mass of water vapour in a unit of air compared to the remaining mass of dry air:

Mixing Ratio = (Mass of Water Vapour (g))/(Mass of Dry Air (kg))

Question 24

Why do meteorologists prefer to use mixing ratio over absolute humidity?

Answer 24

As air moves from one place to another, changes in pressure and temperature cause changes in its volume. When such volume changes occur, the absolute humidity also changes, even is no water vapour is added or removed. Consequently, it is difficult to monitor water vapour content of a moving mass of air when using the absolute humidity index.

Question 25

Vapor Pressure

Answer 25

The part of the total atmospheric pressure that is attributable to its water vapour content.

Question 26

Saturation

Answer 26

The maximum possible quantity of water vapour that the air can hold at any given temperature and pressure.

Water molecules returning to the surface equals the number leaving.

Question 27

Saturation Vapor Pressure

Answer 27

The vapour pressure, at a given temperature, wherein the water vapour is in equilibrium with a surface of pure water or ice.

The pressure exerted by the motion of water vapour molecules when air is saturated.

Question 28

What determines whether the rate of evaporation exceeds the rate of condensation (net evaporation) or vice versa?

Answer 28

1. The temperature of the surface water, which in turn determines how much motion (kinetic energy) the water molecules possess. At higher temperatures the molecules have more energy and can more readily escape. Thus, under otherwise similar conditions, because hot energy has more energy, it evaporates faster than cold water.
2. The vapour pressure in the air around the liquid. Vapour pressure determines the rate at which the water molecules return to the surface (condense). when the air is dry (low vapour pressure), the rate at which water molecules return to the liquid phase is low. However, when the air around a liquid reaches the saturation vapour pressure, the rate of condensation will be equal to the rate of evaporation. Therefore, all else being equal, net evaporation is greater when the air is dry (low vapour pressure) than when the air is humid (high vapour pressure).

Question 29

Relative Humidity

Answer 29

A ratio of the air’s actual water vapour content compared with the amount of water vapour required for saturation at that temperature (and pressure).

Question 30

How can relative humidity change?

Answer 30

1. By water vapour being added to or removed from the atmosphere.
2. Because the amount of moisture required for saturation is a function of air temperature, relative humidity varies with temperature; a decrease in temperature results in an increase in relative humidity and vice versa.

Question 31

List three ways relative humidity changes in nature.

Answer 31

1. Daily changes in temperature (daylight versus night time temperature).
2. Temperature changes that result as air moves horizontally from one location to another.
3. Temperature changes caused as air moves vertically in the atmosphere.

Question 32

How do clouds affect humidity?

Answer 32

When air aloft is cooled below its saturation level, some of the water vapour condenses to form clouds. Since clouds are made of liquid droplets, this moisture is no longer part of the water vapour content of the air,

Question 33

Dew-Point Temperature aka Dew Point

Answer 33

The temperature to which air needs to be cooled to reach saturation.

In nature, cooling below the dew point causes water vapour to condense, typically as dew, fog, or clouds.

A measure of the actual moisture content of a parcel of air.

Question 34

Hygrometer

Answer 34

An instrument designed to measure relative humidity.

Question 35

Adiabatic Temperature Changes

Answer 35

The cooling or warming of air caused when air is allowed to expand or is compressed, not because heat is added or subtracted.

When air is compressed it warms and when air expands it cools.

Question 36

Parcel

Answer 36

An imaginary volume of air enclosed in a thin elastic cover. Typically it is considered to be a few hundred cubic meters in volume and assumed to act independently of the surrounding air.

Question 37

Entrainment

Answer 37

The infiltration of surrounding air into a vertically moving air column. For example, the influx of cool, dry air into row downdraft of a cumulonimbus cloud; a process that acts to intensify the downdraft.

Question 38

Dry Adiabatic Rate

Answer 38

Unsaturated air cools at a constant rate of 10C for every 1000 meters of ascent and when unsaturated air descends and comes under increasing pressure it is compressed and heated 10C for every 1000 meters of descent.

Question 39

Lifting Condensation Level

Answer 39

The altitude at which a parcel of air reaches saturation and cloud formation begins.

Question 40

Why does the adiabatic rate of cooling change when condensation begins? What is the wet adiabatic rate?

Answer 40

The latent heat that was absorbed by the water vapour when it evaporated is released as sensible heat. Although the parcel will continue to cool adiabatically, the release of latent heat slows the rate of cooling. In other words, when a parcel of air ascends above the lifting condensation level, the rate at which is cools is reduced. This slower rate of cooling is called the wet adiabatic rate.

Because the amount of latent heat released depends on the quantity of moisture present in the air (generally between 0 and 4%), the wet adiabatic rate varies from 5C/1000m for air with a high moisture content to 9C/1000m for air with a low moisture content.

Question 41

What four mechanisms cause air to rise?

Answer 41

1. Orographic lifting, in which air is forced to rise over a mountainous barrier.
2. Frontal wedging, in which warmer, less dense air is forced over cooler, denser air.
3. Convergence, which is a pileup of horizontal air flow that results in upward movement.
4. Localized convective lifting, in which unequal surface heating causes localized pockets of air to rise because of their buoyancy.

Question 42

Localized Convective Lifting

Answer 42

Unequal surface heating causes localized pockets of air to rise because of their buoyancy.

Question 43

Orographic Lifting

Answer 43

The process in which mountains or highlands act as barriers to the flow of air and force the air to ascend. The air cools adiabatically, and clouds an precipitation may result.

As air ascends a mountain slope, adiabatic cooling often generates clouds and copious precipitation. By the time the air reaches the leeward side of a mountain, much of its moisture has been lost. If the air descends, it warms adiabatically, making condensation and precipitation even less likely.

Question 44

Rainshadow Desert

Answer 44

A dry area on the lee side of a mountain range.

Question 45

Front

Answer 45

A boundary (discontinuity) separating air masses of different densities, one warmer and often higher in moisture content than the other.

Question 46

Frontal Wedging

Answer 46

The lifting of air resulting when cool air acts as a barrier over which warmer, lighter air will rise.

Question 47

Convergence

Answer 47

The condition that exists when the wind distribution within a given region results in a net horizontal inflow of air into the area. Because convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favourable to cloud formation and precipitation.

Question 48

Sun Showers

Answer 48

Short, widely scattered rain that can accompany localized convective lifting.

Question 49

Stable Air

Answer 49

Air that resists vertical displacement because it is cooler than the surrounding environment (more dense).

If it is lifted, adiabatic cooling will cause its temperature to be lower than the surrounding environment.

Question 50

Unstable Air

Answer 50

Air that does not resist vertical displacement because it is warmer than the surrounding environment (less dense).

If it is lifted, its temperature will not cool as rapidly as the surrounding environment, and so it will continue to rise on its own.

Question 51

Absolute Stability

Answer 51

When the environmental lapse rate is less than the wet adiabatic rate.

The rising parcel of air is always cooler and heavier than the surrounding air, producing stability.

Question 52

Absolute Instability

Answer 52

When the environmental lapse rate is greater than the dry adiabatic rate.

The ascending parcel of air is always warmer than its environment and will continue to rise because of its own buoyancy.

Occurs most often during the warmest months and on clear days, when solar heating causes the lowermost layer of the atmosphere to be warmed to a higher temperature than the air aloft. The result is a steep environmental lapse rate that renders the atmosphere unstable.

Question 53

Conditional Stability

Answer 53

When moist air has an environmental lapse rate between the dry and wet adiabatic rates (between 5* and 10*C/1000m).

The atmosphere is said to be conditionally unstable when it is stable with respect to an unsaturated parcel of air but unstable with respect to a saturated parcel of air.

Question 54

Explain the difference between the environmental lapse rate and adiabatic cooling.

Answer 54

The environmental lapse rate is the actual temperature of the atmosphere, as determined from observations made by radiosondes and aircraft. Adiabatic temperature changes, on the other hand, are the changes in temperature that a parcel of air experiences as it moves vertically through the atmosphere.

Question 55

How does stability change?

Answer 55

Any factor that causes the air near the surface to become warmed in relation to the air aloft increases instability. In contrast, any factor that cause the surface air to be chilled results in the air becoming more stable.

In general, any factor that increases the environmental lapse rate renders the air more unstable, whereas any factor that reduces the environmental lapse rate increases the air’s stability.

Question 56

Give examples of conditions that enhance instability.

Answer 56

1. Intense solar heating warming the lowermost layer of the atmosphere.
2. The heating of an air mass from below as it passes over a warm surface (produces fog).
3. General upward movement of air caused by processes such as orographic lifting, frontal wedging and convergence.
4. Radiation cooling from cloud tops.

Question 57

Give examples of conditions that enhance stability.

Answer 57

1. Radiation cooling of Earth’s surface after sunset.
2. The cooling of an air mass from below as it traverses a cold surface.
3. General subsidence within an air column.

Question 58

Subsidence

Answer 58

An extensive sinking motion of air, most frequently occurring in anticyclones. The subsiding air is warmed by compression and becomes more stable.

Question 59

How does stability affect daily weather?

Answer 59

The air’s stability, or lack of it, determines to a large degree whether clouds develop and produce precipitation and whether that precipitation will come as a gentle shower or a violent downpour. In general, when stable air is forced aloft, the associated clouds have very little vertical thickness, and precipitation, if any, is light. In contrast, clouds associated with unstable air are towering and are frequently accompanied by heavy precipitation.